E1: Perovskite Manganites
Session Chairs
Monday PM, November 30, 2009
Room 103 (Hynes)
9:30 AM - **E1.1
Sr2FeMoO6 — A Fascinating Compound.
D. D. Sarma 1
1 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka, India
Show AbstractSr2FeMoO6 is a magnetic metal with an unusually high Curie temperature (~ 420 K) and belongs to the double perovskite family of compounds. It shot in to fame a few years ago due to its remarkable magnetoresistive properties. [1] We discuss the origin of ferromagnetism in this and related compounds based on a mechanism driven by the kinetic energy, [2-4] establishing these as members of a new class of magnetic materials; this mechanism also explains [4,5] the ferromagnetic coupling of transition metal ions in dilute magnetic semiconductors, such as Mn-doped GaAs and in pyrochlore, Tl2Mn2O7. [6] We also show [7] that the magnetoresistance in Sr2FeMoO6 arises from a magnetically triggered nearly-resonant tunnelling condition in contrast to other mechanisms discussed so far in the context of CMR and GMR materials. Finally, I shall present our very recent and unpublished results that are relevant to the nature of disorder in this material, which has drastic influence on not only magnetic but also electronic structure of these compounds [8], providing a critical test-bed for Altshuler-Aronov theory.This work has been contributed by S. Ray, D. Topwal, P. Mahadevan, T. Saha-Dasgupta, M. De Raychaudhury, A. Kumar, P. Sanyal, H.R. Krishnamurthy, C. Dasgupta, R. Cimino, S. Turchini, S. Zennaro and N. Zema, A. Zunger, K. Tanaka, M. Kobayashi, A. Fujimori, H. Kato, Y. Tokura, M. Avignon, C. Meneghini, F. Liscio, F. Bardelli, and S. Mobilio.References:1. K. I. Kobayashi et al., Nature (London) 395, 677 (1998).2. D.D. Sarma et al., Phys. Rev. Lett. 85, 2549 (2000).3. S. Ray et al., Phys. Rev. Lett. 87, 097204 (2001).4. D.D. Sarma, Current Opinion in Solid State & Material Sciences 5, 261 (2001).5. P. Mahadevan, A. Zunger and D.D. Sarma, Phys. Rev. Lett. 93, 177201 (2004).6. T. Saha-Dasgupta, M. De Raychaudhury and D. D. Sarma, Phys. Rev. Lett. 96, 087205 (2006).7. D.D. Sarma et al., Phys. Rev. Lett. 98, 157205 (2007).8. M. Kobayashi, K. Tanaka, A. Fujimori, S. Ray, and D.D. Sarma, Phys. Rev. Lett. 98, 246401 (2007).
10:00 AM - E1.2
Controlled Fabrication of Epitaxial Ferromagnetic Oxide Artificial Nano-Constriction Structures and their Giant Magnetoresistive Properties at Room Temperature.
Hidekazu Tanaka 1 , Kazuya Goto 1 , Tomoji Kawai 1
1 ISIR-Sanken, Osaka University, Osaka Japan
Show AbstractEpitaxial (Fe,Mn)3O4 (FMO) ferromagnetic oxide artificial nanowire (NW) structures were deposited across Pt/Cr bi-layer electrodes with a high controllability on their shape and positioning by using Atomic Force Microscope (AFM) lithography with Molybdenum (Mo) lift-off in combination with a Pulsed Laser Deposition (PLD) technique [1]. The oxide wire widths were systematically controlled from 5µm down to 100nm by controlling AFM lithography current for Mo-mask. The resistivity of FMO-NW structures was increased at below around 400nm in width. Magnetic Force Microscope (MFM) revealed that FMO-NW with 120nm showed single line of aligned ferromagnetic domains which caused the resistivity increase [2]. As further development, we have fabricated ferromagnetic oxide FMO nano-constriction structures within the artificial NW-structures by using two different steps of AFM lithography and PLD technique. The sizes of three FMO nano-constriction structures were 150nm, 100nm and 50nm. The I-V characteristic of the 50nm width FMO constriction structure was dramatically changed from linear to non-linear property after making the nano-constriction structure. The MR ratio was also dramatically increased from -0.2% to 150% at room temperature. Because the length of spin domain wall and the size of nano-constriction structure become comparable, it is considered that large spin polarization [3, 4] of (Fe,Mn)3O4 and compression of domain wall bring huge MR at room temperature. We also discuss this mechanism.Our results offer well-defined epitaxial transition metal oxide nanostructures with a remarkable flexibility toward nano-scale oxide spintronics.References H. Tanaka et al, [1] Adv. Mater, 20 (2008) 909-913, [2] Nano Lett., 9 (2009) 1962–1966, [3] Phys. Rev. B, 76 (2007) 205108, [4] Solid State Commun., 147 (2008) 397-400
10:15 AM - E1.3
(S)Tem-Eels Investigation of Composition Profiles in (001) and (110) LCMO Layers as a Function of Layer Thickness.
Sonia Estrade 1 , Josep Rebled 1 , Ingrid Infante 2 , Gervasi Herranz 2 , Florencio Sanchez 2 , Josep Fontcuberta 2 , Rosa Cordoba 3 , Francisco de la Pena 4 , Michael Walls 4 , Christian Colliex 4 , Jordi Arbiol 1 , Francesca Peiro 1
1 MIND-IN2UB, Dept. Electrònica, Universitat de Barcelona, Barcelona, Catalunya, Spain, 2 , Institut de Ciència de Materials de Barcelona-CSIC, Barcelona, Catalunya, Spain, 3 , INA: Instituto Universitario de Investigación en Nanociencia de Aragón, Zaragoza Spain, 4 , Laboratoire de Physique des Solides, UMR CNRS, Orsay France
Show AbstractAlthough the microstructure, chemistry and electronic structure of mixed-valence ferromagnetic manganite films, as LCMO (La2/3Ca1/3MnO3), have been the object of very detailed (S)TEM-EELS studies in recent years [1,2], little attention has been paid, so far, to (110) LCMO films, reported to display enhanced magnetic properties when compared to their (001) counterparts [3]. Besides, very thin LCMO films seem to present an anomalous behaviour when compared to thicker films [4], and, thus, a systematic comparative characterization of (001) and (110) LCMO films is of the utmost importance to understand the overall behaviour of these films.In the present work, we address the detailed (S)TEM-EELS characterization of (001) and (110) LCMO / STO (SrTiO3) thin films of a wide rage of thicknesses. Microstructural and chemical characterization of moderately thin LCMO layers grown on STO reveals that different stress relieving mechanisms exist for the two considered orientations, with defects found in more relaxed (110) films, and a Ca2+ ion migration towards free surface, and concomitant Mn oxidation state variation, in more stressed (001) films. Yet, for very thin films (thickness below some 20 nm), a La3+ ion migration towards free surface, and concomitant Mn oxidation state variation, is found, irrespective of layer orientation. In this thickness range, cation incorporation rate instead of elastic energy seems to determine the chemistry of the layers.References:[1] Varela, M., Oxley, M. P., Luo, W., Tao, J., Watanabe, M., Lupini, A. R., Pantelides, A. R., Pennycook, S. J., Physical Review B, 79, 085117 (2009)[2] Simon, J., Walther, T., Mader, W., Klein, J., Reisinger, D., Alff, L., Gross, R., Appl. Phys. Lett. 84, 3882 (2004)[3] I. C. Infante, F. Sánchez, J. Fontcuberta, M. Wojcik, E. Jedryka, S. Estradé, F. Peiró, J. Arbiol, V. Laukhin, and J. P. Espinós, Phys. Rev. B 76, 224415 (2007)[4] YL Qin, HW Zandbergen, ZQ Yang, J Aarts, Phil. Mag., 85, 4465 (2005)
10:30 AM - E1.4
Tailoring Anisotropy in Half Metallic La0.7Sr0.3MnO3 Thin Films.
Paolo Perna 1 2 , Francisco Teran 1 , Erika Jimenez 1 , Julio Camarero 1 , Laurence Mechin 2 , Rodolfo Miranda 1
1 , IMDEA Nanociencia, Madrid, Madrid, Spain, 2 , GREYC - ENSICAEN, Caen France
Show AbstractIn the past decades many improvements in the fabrication of artificial magnetic nanostructures, thin films [1], superlattices have been made tailoring the properties of a large class of materials exploiting to advanced techniques of patterning and stress relaxation mechanisms. Planar devices making use of magnetic domain walls (DW) could represent a valuable alternative to the common read heads composed of multilayered ferromagnetic heterostructures exploiting the out-of-plane uniaxial magnetic anisotropy originating from the interfaces. An interesting class a materials based on the mixed-valence manganese oxide exhibiting a metal-insulator transition accompanied by so-called colossal magnetoresistance (CMR) effects have attracted high interest in the condensed matter physics community. Thanks to their almost 100 % spin polarization [3], they appear as potential candidates for spintronics applications such as read-heads for magnetic hard disks and non-volatile magnetic memories. The manganite of composition La0.7Sr0.3MnO3 (LSMO) showing a Curie temperature above 300 K is of particular interest since it can potentially lead to devices operated at room temperature.We present the high-resolution angle resolved vectorial Kerr magnetometry characterizations at room temperature of epitaxial La0.7Sr0.3MnO3 thin films grown by pulsed laser deposition onto SrTiO3 in different crystallographic directions and on vicinal substrates. The symmetry breaking effects due to the strain induced by the substrate is depicted within the framework of the current theory involving four-fold and two-fold magnetic anisotropy competition [4]. La0.7Sr0.3MnO3 films grown on vicinal (001) SrTiO3 show a pure uniaxial anisotropy with the easy axis lying along the steps. In films grown onto (110) SrTiO3 and onto (001) SrTiO3 a more complicated picture can be described in term of biaxial and uniaxial anisotropy contributions, related to the magnetocrystalline and substrate induced strain effects, respectively. In particular, in vicinal films the symmetry breaking at atomic steps is at the origin of the in-plane uniaxial magnetic anisotropy contribution, whereas in flat (001) and (110) films the biaxial magnetocrystalline anisotropy is balanced by the contribution of the uniaxial anisotropy due to the strain induced by the substrate, resulting in a butterfly-like behavior of the remanence magnetization polar plot rather than a four-leaves one that is expected for a system with cubic symmetry [4]. [1] A. Ruotolo, et al., Appl. Phys. Lett. 91, 132502 (2007)[2] M. Mathews, et al., Appl. Phys. Lett. 87, 242507 (2005)[3] M. Bowen, et al., Appl. Phys. Lett. 82 (2), 233 (2003)[4] J. Camarero et al., Phys. Rev. B 77, 024426 (2008)
10:45 AM - E1.5
Substrate Effects on Magnetotransport Properties of the Pr0.5Ca0.5MnO3 Thin Films.
Margo Staruch 1 , Joseph Budnick 1 , Menka Jain 1 2 , Haiyan Wang 3
1 Physics , University of Connecticut, Storrs, Connecticut, United States, 2 Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 3 Electrical and Computer Engineering, Texas A&M University, College Station, Texas, United States
Show AbstractThin films of Pr0.5Ca0.5MnO3 were deposited on (001) SrLaAlO4, (001) LaAlO3, (001) NdGaO3, and (001) SrTiO3 substrates using a chemical solution deposition technique. X-ray diffraction and surface area diffraction patterns confirmed that the films were c-axis oriented. Films were also strained as expected due to lattice mismatch between film and substrate. The extent and type of strain varied with thickness of the film and substrate, respectively. The zero field resistivity of the films showed semiconducting behavior. The magnetotransport behavior of these films has been compared with the bulk behavior, which exhibits charge ordering and very high melting magnetic fields (27T). For films of different thickness on SrTiO3 substrate, it was found that the thinnest film had maximum strain and maximum magnetic field was required to melt the charge ordering in the film. With the increase in the thickness of the film the strain is reduced but the melting field drops considerably leading to colossal magentoresistance at low temperatures. The change in melting field due to the thickness of the film is most likely related to the defects produced by the relaxation of the stress in the film. More details of the structural, microstructural, and magnetotransport properties of the Pr0.5Ca0.5MnO3 films with both compressive and tensile strains will be presented.
11:30 AM - **E1.6
Half-metallic Manganite Surfaces.
Neil Mathur 1 , Lee Phillips 1 , Xavier Moya 1 , Alessandro Potenza 2 , Stuart Cavill 2 , Sarnjeet Dhesi 2
1 Materials Science, University of Cambridge, Cambridge United Kingdom, 2 , Diamond Light Source, Didcot United Kingdom
Show AbstractCarbon nanotubes are good for spin transport as the carriers move quickly and depolarize slowly [1]. Half-metallic manganites make suitable electrodes for these non-magnetic channels, but there is scope for optimizing the manganite surface, cf. the optimization of buried oxide interfaces in magnetic tunnel junctions [2]. I will discuss epitaxial manganite films that were grown by pulsed laser deposition, and studied at various temperatures on beamline I06 of the Diamond Light Source using PhotoEmission Electron Microscopy (PEEM) and X-Ray Magnetic Circular Dichroism (XMCD). [1] LE Hueso, JM Pruneda, V Ferrari, G Burnell, JP Valdés-Herrera, BD Simons, PB Littlewood, E Artacho, A Fert and ND Mathur, Nature 445 (2007) 410. [2] Y Ishii, H Yamada, H Sato and H Akoh, Appl. Phys. Lett. 89 (2006) 042509
12:00 PM - E1.7
The Origin of Spin-valve-type MR (SVMR) in Sr2FeMoO6.
Sugata Ray 1 2 , Srimanta Middey 2 , Somnath Jana 2 , Alok Banerjee 3 , Rajeev Rawat 3 , Prabuddha Sanyal 4 , Dipankar Das Sarma 5
1 Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India, 2 Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India, 3 , UGC-DAE CSR, Indore, Madhya Pradesh, India, 4 , S. N. Bose National Centre for Basic Sciences, Kolkata, West Bengal, India, 5 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka, India
Show AbstractUnderstanding the spin-dependent tunneling of conduction electrons through an insulating barrier, placed between two ferromagnetic metallic electrodes, as a result of the application of external magnetic field is becoming a key requirement in the emerging field of spintronics [1]. This exciting electronic process leads to drastic changes in electrical conductivity of a material as a function of applied magnetic field and is termed as tunneling magnetoresistance (TMR). The simplistic theoretical model of TMR predicts that the electrical resistance should be lowest in case of perfect parallel spin alignment of the neighboring ferro-/ferrimagnetic grains, enabling maximum electron tunneling, while the highest resistive state of the material, at a given temperature, should occur at the zero magnetization point (the magnetic coercivity, Hc) [2]. This prediction has been proved to be true for most of the TMR materials, while only recently an exception was observed [3] in cold-pressed polycrystalline pellets of a room temperature TMR system, namely Sr2FeMoO6 [4]. This unusual phenomenon immediately opened up an avenue of interest with its valve-like control on TMR, and was duly named as spin-valve-type MR (SVMR). However, given the difficulty, the real mechanism behind SVMR was not experimentally investigated in the beginning, although an appealing theoretical picture was proposed [3]. Consequently, we tried to probe SVMR using magnetic measurement techniques like ac and dc susceptibilities as well as magnetoresistance measurements, in great detail, which has revealed the true origin of SVMR. Our experimental investigation proves the spontaneous formation of a spin-glass (SG) like surface layer around each soft ferrimagnetic (FiM) grain of Sr2FeMoO6 and also the existence of a very strong exchange coupling between the glassy skin and the soft ferrimagnetic core, generating the well-known ‘exchange bias’ effect [5]. Our results reveal that the pinned ferrimagnetic spins at the FiM/SG interface act as the ‘valve’, which is the central point of the SVMR effect. The concurrence of the temperature at which SVMR appears and the Tg of the glassy surface provides the most revealing and conclusive proof for the proposed mechanism. In support of these experimental results, detailed simulation studies, based on model Hamiltonian, are currently underway on an extended version of our earlier proposed model.[1] S. A. Wolf et al., Science 294, 1488 (2001).[2] S. Perkin, H. Yang, S.-H. Yang, and M. Hayashi, Handbook of Magnetism and Advanced Magnetic Materials, edited by H. Kronmüller and S. Perkin (Wiley 2007).[3] D. D. Sarma, S. Ray, K. Tanaka, M. Kobayashi, A. Fujimori, P.Sanyal, H.R. Krishnamurthy, and C. Dasgupta, Phys. Rev. Lett. 98, 157205 (2007).[4] K. -I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, and Y. Tokura, Nature 395, 677 (1998).[5] M. Ali et al., Nature Materials 6, 70 (2007).
12:15 PM - E1.8
Growth and Structure-Property Correlations in La0.67Sr0.33MnO3/CeO2/YSZ/Si(001) Heterostructures.
Ravi Aggarwal 1 , Sudhakar Nori 1 , Punam Pant 1 , Roger Narayan 1 2 , Jagdish Narayan 1
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Joint Department of Biomedical Engineering, University of North Carolina, Chapel Hill, North Carolina, United States
Show AbstractThe half-metallic perovskite La0.67Sr0.33MnO3 (LSMO) is an ideal candidate for multifunctional epitaxial heterostructures which have potential applications in spintronics and other memory devices [1]. The important physical phenomena exhibited by manganites, such as metal-insulator transition (MIT), colossal magnetoresistance (CMR) and para to ferromagnetic phase transition are very sensitive to the choice of substrate material, growth conditions and buffer layers used [2]. Growth of high quality epitaxial films of LSMO on Si(001) substrates is important for technological applications of LSMO. In the present work, we report growth and characterization of epitaxial LSMO/CeO2/YSZ/Si(001) heterostructures deposited by pulsed laser deposition. The growth of LSMO on CeO2 buffer layer in the present study is in contrast to the earlier reports in the literature where a Bi4Ti3O12 (BTO) buffer layer (along with YSZ buffer layer) has been used to integrate LSMO with Si(001). The detailed x-ray diffraction characterization confirmed that LSMO films were (001) oriented and epitaxial. The crystalline quality of LSMO films was assessed by rocking curve measurements. Further structural characterization was done with high resolution transmission electron microscopy which showed that interfaces in LSMO/CeO2/YSZ/Si(001) heterostructure were sharp. The electrical resistivity measurements showed that the resistivity of LSMO films peaks at ~260 K. Isothermal magnetization was measured at different temperatures in the range 10 - 350 K, with magnetic field strengths of up to 20 kOe. It was observed that below 320 K the LSMO films showed perfect ferromagnetic (FM) behavior, with paramagnetic (PM) behavior becoming prominent at higher temperatures. The temperature dependent magnetization data also corroborated well with M(H) data and indicated a FM-PM transition at ~340 K. It was found that Bloch’s T3/2 law fits fairly well to the saturation magnetization data. References:1. A. Tiwari, C. Jin, D. Kumar and J. Narayan, Rectifying electrical characteristics of La0.7Sr0.3MnO3/ZnO heterostructure, Applied Physics Letters 2003;83:1773.2. V. Bhosle, J.T. Prater and J. Narayan, Anisotropic magnetic properties in [110] oriented epitaxial La0.7Sr0.3MnO3 films on (0001) sapphire, Journal of Applied Physics 2007;102.
E2: Spin Transport, Magnetoresistance
Session Chairs
Monday PM, November 30, 2009
Room 103 (Hynes)
2:30 PM - **E2.1
Electric-field Manipulation of Magnetization Vector.
Hideo Ohno 1
1 Laboratory for Nanoelectronics and Spintronics Research Institute of Electrical Communication, Tohoku University, Sendai Japan
Show AbstractFerromagnetism and magnetization in Mn-doped III-V semiconductors can be manipulated by various means; for example, by changing its carrier concentration by electric fields or by spin-current flowing along with the electric current. In addition, because of the anisotropic spin-orbit split valence bands, one can expect to see modulation of the magnetic anisotropy potential landscape by changing the carrier concentration. Here, we report electrical control of magnetization direction through manipulating magnetic anisotropy by electric-fields [1]. In order to measure the magnetic anisotropies under a gate that applies the electric-field to the channel, we measured the planar Hall effect as a function of the angle of in-plane external magnetic-fields. Analyses showed that there are biaxial as well as uniaxial anisotropies. As the sheet carrier concentration is reduced by applying electric-field to the channel, the uniaxial anisotropy field reduced its magnitude and eventually changed its sign, whereas no significant change was apparent in the biaxial anisotropy field. From the electric-field dependent anisotropy fields, one can show that the angle of the magnetization direction in the absence of magnetic fields can be modulated by electric-fields. This opens up a unique route for switching magnetization direction solely by electronic means, not resorting to magnetic-field, spin-current, mechanical stress, nor multiferroics. References:[1]D. Chiba, M. Sawicki, Y. Nishitani, Y. Nakatani, F. Matsukura, and H. Ohno, Nature, 455 (2008).Acknowledgement:We acknowledge D. Chiba, F. Matsukura, M. Sawicki, Y. Nishitani, Y. Nakatani and T. Dietl for fruitful collaboration and discussion. This work was supported in part by the "Research and Development for Next-Generation Information Technology" program from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
3:00 PM - E2.2
Multiferroic Tunnel Junctions: Prediction of Four Resistance States from First-principles.
Julian Velev 1 , Chun-Gang Duan 3 , J. Burton 2 , Alexander Smogunov 4 , Erio Tosatti 4 , Sitaram Jaswal 2 , Evgeny Tsymbal 2
1 Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 3 Key Laboratory of Polarized Materials and Devices, East China Normal University, Shanghai China, 2 Department of Physics, University of Nebraska, Lincoln, Nebraska, United States, 4 , International Centre for Theoretical Physics International Centre for Theoretical Physics , Trieste Italy
Show AbstractElectron tunneling and ferroelectricity have had long but separate histories. In the past decade both attracted significant interest due to application in electronic devices such as magnetic tunnel junctions (tunneling) and ferroelectric capacitors (ferroelectricity) relevant to non-volatile random-access memories. Recently, driven by demonstrations of ferroelectricity in ultrathin films, it was proposed to combine these two phenomena in a multiferroic tunnel junction (MFTJ) utilizing a ferroelectric barrier between two magnetic electrodes [1]. Due to sensitivity of the conductance to both the magnetization alignment of the electrodes (magnetoresistance) and orientation of the polarization in the ferroelectric barrier (electroresistance), this junction can serve as a four-state resistance device. Here based on first-principles calculations we demonstrate the existence of the four resistance states in SrRuO3/BaTiO3/SrRuO3 MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal exciting prospects of MFTJs for application in multifunctional electronic devices.
1. E. Y. Tsymbal and H. Kohlstedt, “Tunneling across a ferroelectric”, Science 313, 181 (2006).
3:15 PM - E2.3
Modification of The Interface Between Ferromagnetic Metal Electrodes and Organic Substances for Improvement of Spin-valve Characteristics.
Ryuta Adachi 1 , Ryota Sasaki 1 , Hirokazu Tada 1
1 Division of Materials Physics, Osaka University, Toyonaka Japan
Show AbstractConsiderable attention has recently been paid to the study of spin dependent carrier tarnsport in sandwich structures composed of organic materials (OMs) and ferromagnetic (FM) metal electrodes. It has been reported, in the research field of semiconductor-based spintronics, that the conductivity mismatch between FM metals and semiconductors prevents spins from the efficient injection. It seems also the case for organic spintronic devices. Most works reported so far on organic spintronics used half metallic La0.67Sr0.33MnO3 (LSMO) as an electrode of sandwich structures. Clear spin-valve characteristics were not demonstrated for the devices composed of OMs and ordinal FM metals such as Co, Fe and NiFe. In the present study, we modified the FM metal-organic interface in layered sandwich structures by inserting thin oxide layers, and nanocomosites to improve the spin injection properties.The devices were prepared on thermally grown SiO2 surfaces. The bottom electrodes with a dimension of 1 mm x 5mm x 30 nm were fabricated on SiO2 by photolithogaraphy and lift-off techniques. The electrodes were coated with Al2O3 or nanocomposites of pentacene/Co. Al2O3 was prepared by oxidation of thin aluminum films deposited on the NiFe elecrodes. Nanocomposite layers were prepared by vacuum evaporation of Co metals onto thin pentacene layers. Organic films with the thickness of approximately 200 nm were deposited on the modified electrodes. The upper electrodes were prepared by evaporation of Co, on which a thin aluminum layer was prepared to protect the Co layer from the oxidation. Spin-valve characteristics with a mageto-resistance ratio of 2 – 10 % were observed for the devices with modified electrodes in the temperature range from 5 K to 100 K. It was found that the modification of the FM metals-organic interfaces improved the spin injection characteristics.
3:30 PM - E2.4
Growth and Carrier Dynamics of Molecular Superlattice Toward Molecular Spintronics Device.
Nobuya Hiroshiba 1 2 , Ryoma Hayakawa 1 , Petit Matthieu 1 , Toyohiro Chikyow 1 , Kiyoto Matsuishi 2 , Yutaka Wakayama 1
1 Advanced Electronic Materials Center, NIMS, Tsukuba Japan, 2 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba Japan
Show AbstractThe aims of this study are to clarify the carrier dynamics properties such as charge transfer (CT) and optical carrier generation, recombination at hetero molecular interface and to explore the possibility of application of the molecular superlattice for spintronics. Organic molecules are one of major candidate materials for spintronics because they have weak spin-orbital (SO) interactions, long spin relaxation time and a variety of choice of materials. Moreover, the organic molecules have various functions related with spin degree of freedom (SDF) such as ferromagnetism, magnetic-resistance (MR) and superconductivity. These functions related with SDF are mainly caused by CT and/or optical carrier doping at the hetero molecular interface in superlattice structure [1]. However, the device applications using these functions have not been successful so far and challenges still remain for basic studies of molecular single-crystals. To solve these issues, we propose molecular superlattice as a thin-film device, where p- and n-type semiconducting molecules are alternately stacked with well-defined order. In this study, the molecular superlattice consisting of N,N’-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) and quaterrylene (QT) layers were prepared by using an ultra-slow deposition technique [2,3]. The morphology of the films and the orientation of the molecules in each layer were analyzed by atomic force microscopy (AFM) and an x-ray reflection (XRR) method. To examine the carrier transport, the transistor properties of hetero molecular devices were measured. In addition, the displacement current measurements (DCM) were carried out to clarify the process of carrier injection and accumulation. Photoluminescence (PL) was measured to investigate optical properties and carrier dynamics.Ambipolar properties were observed in transistor properties, indicating that the hole and electron carriers were transported in QT and PTCDI-C8 respective layers without annihilation by scattering at the hetero interface. This result clearly demonstrates that the molecular superlattice has a long mean free path, which is advantageous for spin carrier transport. On the other hand, we observed unique photoluminescence excitation (PLE) spectra, which are different from those of single-component molecular films. The result shows the possibility of CT between QT and PTCDI-C8 layers.These results demonstrate that the molecular superlattice possesses a potential for controlling spin carrier injection and transport.[1] N. Toyota, M. Lang and J. Müller, Low-Dimensional Molecular Metals, Springer Series in Solid-State Sciences vol. 154, Springer-Verlag, Berlin Heidelberg (2007).[2] H. Sasaki et. al., Appl. Phys. Lett. 88 (2006), 081907[3] N. Hiroshiba et. al., Org. Electronics, (in press).
3:45 PM - E2.5
Bias Dependence of Spin Filter Tunneling in Magnetic Insulators.
Martina Mueller 1 2 , Guo-Xing Miao 2 , Jagadeesh Moodera 2 , Claus Schneider 1
1 Institute of Solid State Research, Research Centre Juelich, Juelich Germany, 2 Francis Bitter Magnet Lab, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractSpin filtering (SF) is an efficient route to produce highly spin-polarized currents which are essential for spintronics applications. SF relies on electron tunneling through a magnetic tunnel barrier whose exchange-split conduction band leads to different tunneling probabilities for each spin type. The SF efficiency is directly connected to the magnetic and transport properties of the barrier, while its quantification is usually conducted by indirect techniques, i.e. magnetoresistance measurements or using superconducting counterelectrodes.We present a study of the characteristic features of spin-filter tunneling in Europium Oxide (EuO)-based tunnel junctions without using magnetic electrodes or external magnetic fields for spin detection. Current-voltage (I-V) curves show a pronounced bias voltage dependence which is caused by the superposition of direct- and Fowler-Nordheim (FN) tunneling paths. Direct evidence for spin filtering is given by successive onsets of FN tunneling at high bias voltages. From temperature-dependent I-V curves we extracted the variation of effective tunnel barrier height below the Curie temperature, which systematically correlates with the spontaneous magnetization of EuO. We deduced an exchange splitting of EuO of 0.48 eV and thus could determine the resulting spin filter efficiency of the magnetic tunnel barrier by fully electrical means.
4:30 PM - **E2.6
Magnetic Field Effects from Excited States in Organic Semiconductor Devices.
Bin Hu 1
1 , University of Tennessee, Knoxville, Tennessee, United States
Show AbstractIt has been found that non-magnetic organic semiconductors can exhibit magnetic responses in electrical current, electroluminescence, photoluminescence, and photocurrent when an external magnetic field is applied. These intrinsic magnetic responses have been named as magnetic field effects. Especially, the magnetic field effects indicate that an external magnetic field can be used as a non-contact approach to control the light-emitting, photovoltaic, and charge transport processes in organic semiconductors. Furthermore, the magnetic field effects can be used as effective tools to visualize spin-dependent excited states and charge transport in electroluminescence, photoluminescence, photocurrent, and electric current in organic semiconductors. In particular, the magnetic field effects have opened a new direction to develop organic spintronics by using all non-magnetic organic semiconductors.In general, the magnetic field effects can be naturally attributed to (i) magnetic field-dependent singlet/triplet ratio and (ii) singlet/triplet ratio-dependent excited processes . There are two critical issues: tuning spin-orbital coupling and controlling spin-spin interaction, that can determine the amplitude and sign of magnetic field effects. Specifically, tuning spin-orbital coupling can modify the dependence of singlet/triplet ratio on magnetic field and consequently changes the amplitude of magnetic field effects. On the other hand, controlling spin-spin interaction can directly change singlet and triplet involvements in spin-dependent excited processes, leading to positive and negative tuning on magnetic field effects. Based on recent research progress, this presentation will discuss (i) how spin-orbital coupling can be tuned by using inter-molecular interaction in controlling the amplitude of magnetic field effects , (ii) how singlets and triplets are involved in magnetic field effects in understanding the mechanism of magnetic field effects , and (iii) how the spin-spin interaction can be changed by controlling electron-hole pairing distance in tuning magnetic field effects with positive and negative signs towards the development of magnetic field effects-based organic spintronics.
5:00 PM - E2.7
Hysteretic Magnetoresistance in Organic Diodes.
Sayani Majumdar 1 2 , Himadri Majumdar 1 , Harri Aarnio 1 , Ronald Osterbacka 1
1 Dept. of Physics and Center for Functional Materials, Åbo Akademi University, Åbo, Åbo, Finland, 2 Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, Turku, Turku, Finland
Show AbstractOver the past two decades, the growth of organic electronics has been phenomenal. All research has been predominantly focused towards the understanding of charge transport in organic semiconductors (OS) small molecules and pi-conjugated polymers (PCP)s and its implications in devices. Study of spin in organics is a relatively unexplored territory. It provides a whole new scope of physics along with immense potential for application. Major challenge in the field of spintronics is the optimization of electron spin lifetimes across relevant length scales and hetero-interfaces. In this respect, OS is envisioned to be better than their inorganic counterparts. OS comprises of light molecules like carbon, hydrogen etc. and possess low spin-orbit interaction and hyperfine interaction; leading to longer spin-correlation length. PCPs are especially the better choice as their conjugation length is much higher than the small molecules and oligomers, leading to better transport properties and their solution processability lead to simpler fabrication process. Magnetic field effects on OS were demonstrated already in the early ‘90s. However, demonstration of sizable room temperature magnetoresistance under small applied magnetic fields in organic diodes [1] (OMAR) caused resurgence of interest. To explain this effect several models have been proposed but none of them could fully explain all the experimental observations. Earlier, we reported that for electron-hole pair formation is important for observation of OMAR in PCP diodes [2]. Here, we report another important aspect of this effect - hysteretic OMAR in polymeric diodes. We found that magnitude and lineshape of OMAR depends strongly on the scan speed of the magnetic field and on the time delay between two successive measurements. The width and magnitude of OMAR varied with the polymeric material. These observations in organic diodes are in addition to the weak ferromagnetic ordering in the PCP materials in the ground state [3]. The ferromagnetism could not be explained by the impurity content in the material alone. All these experimental observations will be discussed during the presentation and signature of similarity of magnetotransport in other inorganic materials will be provided. References:[1]T. L. Francis, Ö. Mermer, G. Veeraraghavan and M. Wohlgenannt, New J. Phys. 6, 185 (2004).[2]S. Majumdar, H. S. Majumdar, H. Aarnio and R. Österbacka, Phys. Rev. B 79, 201202 (2009).[3]S. Majumdar, H. S. Majumdar, J. O. Lill, J. Rajander, R. Laiho and R. Österbacka, Synth. Met. (submitted, arXiv:0905.2021).
5:15 PM - E2.8
Magnetic Tunnel Junctions with Co-based Perpendicular Magnetic Anisotropy Multilayers.
Zeenath Tadisina 1 2 , Anusha Natarajarathinam 2 , Subhadra Gupta 1 2 , Tim Mewes 2 , Patrick LeClair 2 , Eugene Chen 3 , Shengyuan Wang 3
1 Metallurgical & Materials Engineering, The University of Alabama, Tuscaloosa, Alabama, United States, 2 MINT Center, The University of Alabama, Tuscaloosa, Alabama, United States, 3 , Grandis, Inc.,, Fremont, California, United States
Show AbstractCoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy (PMA) free and reference layers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effect of Co thickness, Pd thickness and number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]*n multilayer films have been studied for both free and reference layers. The best results were obtained for [Co(0.2nm)/Pd(1nm)]*9 bilayers for a reference layer with a coercivity of 5000 Oe after annealing, and [Co(0.3nm/Pd(1nm)]*4 bilayers for the free layer with a coercivity of 1200 Oe after annealing. The magnetic behavior of these PMA systems was studied by alternating gradient magnetometry. A Pd/Co ratio of 5 resulted in high perpendicular anisotropy and perfectly square loops, whereas decreasing the Pd/Co ratio to 1 at a Co thickness of 0.5 nm resulted in bow-tie shaped out-of-plane anisotropy loops indicating a multi-domain structure. X-ray diffraction, transmission electron microscopy (TEM) and local electrode atom probe (LEAP) studies were carried out to investigate the structure of the multilayers, interface smoothness, and growth of (111) texture as a function of deposition conditions and post-deposition annealing. CoFeB/MgO/CoFeB trilayers sandwiched between the PMA multilayer material systems were studied as a function of CoFeB thickness and MgO thickness. The resistance-area (RA) product and tunneling magnetoresistance (TMR) of the unpatterned MTJ stacks were tested by current-in-plane tunneling (CIPT) measurements to optimize the MgO barrier and PMA stacks prior to actual device fabrication. After initial matrix experiments were carried out to determine the experimental parameter space, a statistical Design of Experiments (DOE) was conducted to optimize the film structure that would maintain stable perpendicular anisotropy for a reasonable thickness of CoFeB and MgO. The CoFeB thickness was varied from 0.4 nm to 2 nm and the MgO thickness was varied between 1 to 2 nm. The transport properties of the patterned MTJ stacks were measured in a PPMS system from 10K to 400 K. A TMR of 10% at 10K was obtained for these perpendicular MTJ’s (pMTJ) even before optimization of the MgO and CoFeB with a combination of in-situ lamp annealing and post-deposition furnace annealing and field cooling.
5:30 PM - **E2.9
Interfacial Magnetoelectric Coupling for Electrically Controllable Spin-based Properties.
Vincent Garcia 1 2 , Manuel Bibes 1 , Stephane Fusil 1 , Karim Bouzehouane 1 , Shaima Enouz-Vedrenne 4 , Cyril Deranlot 1 , Laura Bocher 3 , Dominique Imhoff 3 , Neil Mathur 2 , Agnes Barthelemy 1
1 , Unité Mixte de Physique CNRS/Thales, Palaiseau France, 2 Dept. Materials Science, University of Cambridge, Cambridge United Kingdom, 4 , Thales Research & Technology, Palaiseau France, 3 Laboratoire de Physique des Solides, Université Paris Sud, Orsay France
Show Abstract Spintronics focuses on physical effects that take advantage of the electron spin in addition to its charge [1]. A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics that relies on zero-current, gate-controlled operations. Efforts have been made to control spins with electric fields [2], for instance by varying the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. Direct or strain-mediated magnetoelectric coupling may be used to manipulate magnetization [3], and thus possibly the spintronic response of spin-valves or magnetic tunnel junctions [4]. Often, the key magnitude controlling the spintronic response is not the magnetization but the carrier spin polarization. Motivated by this observation we have defined and studied magnetic tunnel junctions integrating a ferroelectric barrier. At room temperature, we use piezoresponse force microscopy to show robust ferroelectricity down to 1 nm in highly strained BaTiO3 films, and conductive atomic force microscopy to demonstrate the resistive readout of the polarization state via its influence on the tunnel current. The resulting electroresistance effect scales exponentially with the ferroelectric film thickness, reaching ~75000 % at 3 nm [5]. Our approach exploits the otherwise undesirable leakage current - dominated by tunneling at these very low thicknesses - to read the polarization state without destroying it.We also find a reversible variation of the tunnel magnetoresistance depending on the polarization direction in the ferroelectric. This experimentally demonstrates the predicted local, large and non-volatile control of spin polarization using a ferroelectric [6]. Our results may be viewed as the signature of a giant interfacial magnetoelectric coupling and a step towards the ferroelectric control of other spin-based properties like magnetic anisotropy and exchange interactions. More practically, they suggest a new low-power approach for spin-based information control. [1] C. Chappert et al, Nature Mater. 6, 813 (2007).[2] H. Ohno, et al, NATURE 408, 944 (2000)[3] Y.-H. Chu, et al. Nature Mat. 7, 478 (2008) ; W. Eerenstein et al, Nature Mater. 6, 348 (2007)[4] M. Bibes & A. Barthélémy, Nature Mater. 7, 425 (2008) ; C. Binek & B. Doudin, J. Phys.: Condens. Matter. 17, L39 (2005)[5] V. Garcia et al.; Nature doi : 10.1038/nature08128[6] C.-G. Duan et al, Phys. Rev. Lett. 97, 047201 (2006)
E3: Magnetoresistance and X-ray Spectroscopy in Organic and Molecular Systems
Session Chairs
Tuesday AM, December 01, 2009
Room 103 (Hynes)
9:30 AM - **E3.1
Carbon Nanotubes, Graphene, Molecules : Promising Materials for Spintronics.
Albert Fert 1 , Jean-Marie George 1 , Luis, E. Hueso 2 , Henri Jaffres 1 , Neil, D. Mathur 2 , Richard Mattana 1 , Michael Tran 1
1 CNRS/Thales, Universite Paris-Sud, Palaiseau France, 2 Department of Materials Science, University of Cambridge, Cambridge United Kingdom
Show AbstractThe usual materials of classical spintronics are magnetic and nonmagnetic metals, magnetic and nonmagnetic semiconductors and, for tunnel junctions, insulating materials like MgO or alumina. However, nowadays, promising results begin to be obtained with a new family of materials which includes carbon nanotubes, graphene and several types of magnetic or non-magnetic molecules. The general advantage of carbon-based materials is mainly their long spin lifetime related to the small spin-orbit coupling of carbon, but, as we will see, the very high electron velocity of some of them is also of great interest for spintronics.The first part of the talk will be an introduction on classical spintronics and a review of what can be done with molecular materials for TMR, spin transport in lateral structures, magnetic switching or microwave generation by spin transfer. In the second part of the lecture I will focus on the general problem of spin transport in anonmagnetic lateral channel between a spin-polarized source and a spin-polarized drain, a structure which is at the basis of several concepts of logic devices or spin transistors. The main difficulty is related to the transformation of the spin information – related to the magnetic configuration of the electrodes- into a large electrical signal, ideally ΔV/V ≈ 1 or larger, if V is the bias voltage and ΔV some voltage variation induced by a change of the magnetic configuration. In experiments on structures in which the lateral channel is a metal or a semiconductor, ΔV/V does not exceed a few 1% and the electrical signal ΔV is generally in the μV range [1]. In contrast, in the experiments on carbon nanotubes between ferromagnetic contacts we will present, high values of ΔV/V ( above 70%) and large ΔV (of the order of 100 mV) can be obtained [2]. After a description of the theoretical background, we will discuss the origin of the difficulties for semiconductors and explain why large values of ΔV/V and ΔV can be easily obtained with carbon nanotubes. We will emphasize the potential of carbon nanotubes, graphene and other molecules for spintronics, and conclude by presenting some next challenges for molecular spintronics.[1] Jonker, B.T. and Flatté, M.E.F. Electrical spin injection and transport in semiconductors, in Nanomagnetism (eds. Mills D.L. & Bland J.A.C.) (Elsevier, 2006).[2] Hueso, L. E., Pruneda J-M., Ferrari V., Burnell G., Valdés-Herrera J.P., Simons B.D., Littlewood P.B., Artacho E., Fert A. and Mathur N.D.. Transformation of spin information into large electrical signals via carbon nanotubes, Nature 445, 410 (2007).
10:00 AM - E3.2
Single Molecular Magnet - Gold Nanoparticle Composites.
Balaji Gopalan 1 , Gudrun Bovenkamp 1 , Vadim Palshi 1 , Challa Kumar 1
1 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States
Show Abstract The past decade has witnessed a great interest in single molecule magnets (SMM) especially those based on manganese such as Mn12-acetate due to their potential applications in ultrahigh density data storage and quantum computing devices. The focus so far has been in finding different derivatives of Mn or other complexes based on transition elements in order to raise the blocking temperature of single-molecule magnets (SMMs). Herein we propose a fundamentally new approach of generating new category of SMMs through formation of composite structures by binding SMMs on to nanoparticles. In this report, we present the synthesis and characterization of single molecular magnet - gold nanoparticle composite- Mn12-cysteinate functionalized gold nanoparticles. The synthesis involved first functionalization of Mn12-acetate using cysteine ligand to obtain Mn12-cysteinate. This is followed by a ligand exchange reaction between dodecanethiol capped Au nanoparticles and Mn12-cysteinate ligands resulting in Mn12-cysteinate-Gold nanoparticle composite. The composite was characterized using several methods, primarily synchrotron radiation-based X-ray absorption spectroscopy and high resolution transmission electron microscopy (HRTEM).
10:15 AM - **E3.3
A Site-specific Kondo Effect at Ambient Temperatures in Iron-based Molecules and the Modulation.
Hongjun Gao 1 , L. Gao 1 , Q. Liu 1 , Y. Zhang 1 , N. Jiang 1 , H. Zhang 1 , S. Du 1 , W. Hofer 2
1 , Institute of Physics, CAS, Beijing China, 2 , University of Liverpool, Liverpool United Kingdom
Show AbstractDue to their importance in the emerging field of molecular electronics, the transport properties of single molecules have been the focus of intensive research. Magnetic properties of transition metal atoms in a host molecule can be detected by a Kondo resonance in cryogenic scanning tunneling microscopes. These properties are important variables in the fabrication of single molecule devices. In this talk, I will present measurements of the Kondo effect of iron phthalocyanine (FePc) molecules on an Au(111) surface. Our results indicate a high Kondo temperature, well above room temperature, for the FePc molecule adsorbed on Au(111) surface. It is further revealed that a substantial change of the effect is with the adsorption configuration of the molecule. Furthermore, magnetic properties of an interface can be controlled at the molecular level by site-specific adsorption and modification of molecular structures. We verify by first-principles calculations that the molecular environment alters the hybridization of spin-polarized states of the molecule with states of the metal substrate, and that this alteration depends on the adsorption site of the molecule. The finding opens up the possibility to tailor magnetic properties of an organic interface to the desired specifications.
10:45 AM - E3.4
Experimental and Device Modeling Approaches to Understanding the Sign Change in Organic Magnetoresistance.
Francisco Bloom 1 , Martijn Kemerink 1 , Wiebe Wagemans 1 , Bert Koopmans 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractThe organic magnetoresistance effect (OMAR) has been a scientific puzzle since no traditional magnetoresistance mechanisms can explain a room temperature low magnetic field (~20mT) effect with such a large magnitude (up to 25%) without the use of any magnetic materials. Another puzzling feature of OMAR is that the sign of the magnetoresistance (MR) can be changed by varying the temperature or the applied voltage. This sign change has been the topic of much recent research and resolving the origin of the sign change should be a major step in understanding the microscopic origin of OMAR.Previously, we have explored the properties of the sign change experimentally with Alq3 based devices. These devices showed a strong correlation between the sign change and the onset of minority charge carrier (hole) injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign [1]. From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR [2]. To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons [3]. The models show, counterintuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. The models show that lowering the space charge in the device by increasing the recombination decreases the strength of this sign inversion, this is verified experimentally in Alq3 devices doped with recombination centers [4]. We show that these models describe the observed MR(V) behavior very well. If one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature.The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR. [1]F. L. Bloom, W. Wagemans, M. Kemerink, and B. Koopmans, Phys. Rev. Lett. 99 257201 (2008).[2]P. A. Bobbert, T. D. Nguyen, F. W. A. van Oost, B. Koopmans, and M. Wohlgenannt , Phys. Rev. Lett. 99, 216801 (2007).[3]F. L. Bloom,M. Kemerink, W. Wagemans, and B. Koopmans (submitted)[4]F. L. Bloom, J. M. Veerhoek, W. Wagemans, and B. Koopmans (in preparation)
11:30 AM - **E3.5
Controlling the Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Films.
Wolfgang Kuch 1
1 Institut für Experimentalphysik, Freie Universität Berlin, Berlin Germany
Show AbstractOrganic molecules as building blocks of surface-mounted nanoscale systems have reached tremendous impact in solid state physics. Metallo porphyrins display a quasi-planar geometry, allowing for a two-dimensional assembly as electronic circuits or devices, while the fourfold coordinated metal center can be accessed by two additional adsorption sites. This can be used to gain control on the metal center spin. Manipulating the size or direction of the spin of the metal ion by tuning the molecule–substrate interaction would be an important step towards the realization of a surface-mounted molecular spintronic device.X-ray magnetic circular dichroism (XMCD) spectra of paramagnetic Fe-octaethylporphyrin molecules adsorbed on ferromagnetic epitaxial ultrathin Ni and Co films on Cu(001) reveal a ferromagnetic coupling of the Fe atom to the Ni and Co films. From the temperature dependence of the Fe and substrate XMCD signals, the magnetic coupling strength can be estimated for the two magnetic substrates, being weaker in the case of Ni. Interestingly, the sign of the magnetic coupling can be reversed when placing oxygen atoms between the porphyrin molecules and the substrate. The insertion of oxygen is achieved by growing the Ni and Co films on a preoxidized Cu(100) surface. The oxygen atoms act as a surfactant for the growth of the ferromagnetic films, floating on top of the surface. This results in a well-characterized c(2×2) superstructure of 0.5 monolayer of atomic oxygen on top of the ferromagnetic film. XMCD reveals an antiferromagnetic coupling between the metal centers of the Fe-octaethylporphyrin molecules and the Ni and Co films, the coupling energy of which is comparable to the ferromagnetic interaction found without oxygen in the case of the Ni substrate, but weaker in the case of the Co substrate. The antiferromagnetic coupling resembles the situation in systems like, for example, transition metal monoxides, in which a super-exchange interaction across oxygen atoms is responsible for the antiferromagnetism,Results were obtained in collaboration with M. Bernien, H. Wende, J. Miguel, C. Weis, M. Piantek, X. Xu, Ph. Eckhold, J. Kurde, K. Baberschke, and B. Krumme.
12:00 PM - E3.6
Magnetic Couplings in Molecular Semiconductor Thin Films.
Sandrine Heutz 1 , Zhenlin Wu 1 , Salahud Din 1 , Marc Warner 2 , Gabriel Aeppli 2 , Tim Jones 3 , Wei Wu 2 , Andrew Fisher 2
1 Department of Materials and London Centre for Nanotechnology, Imperial College London, London United Kingdom, 2 Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, London United Kingdom, 3 Department of Chemistry, Warwick University, Warwick United Kingdom
Show AbstractMolecular thin films can be used as alternatives to inorganic semiconductors in optoelectronic applications, such as photovoltaics, organic light emitting devices and field-effect transistors, due to the delocalised π -electrons on the organic backbone. In addition, some macrocycles, such as phthalocyanines (Pcs) or porphyrins, can also contain transition metal ions with unpaired spins, as in the case of CuPc (S=1/2). Pcs crystallise in a variety of structures, and magnetic coupling in the metal backbones has been observed. However, to exploit the combination of magnetism and semiconducting properties that render those materials particularly interesting for spintronic applications, their thin film magnetic properties need to be assessed and controlled. In this contribution, we present the growth and magnetic characterisation of molecular thin films deposited using sublimation techniques in the high and low vacuum regime, namely organic molecular beam deposition (OMBD) and organic vapour phase deposition (OVPD). These techniques are compatible with current optoelectronic device fabrication and can produce a range of morphologies and structures in Pcs, all corresponding to stacked molecules, but with different molecular shifts. We show that in the case of MPc films, magnetic coupling is indeed observed, enabling the transfer of interesting single crystal properties onto flexible substrates [1, 2]. Crucially, this coupling depends on the structure of the films, and can be switched between ferro and antiferro-magnetic during polymorphic phase transition. This work is extended to structures containing a combination of polymorphs, which can be fabricated by controlled nucleation or self-assembly in OVPD, and highlight the versatility of molecular films for information storage and logic. [1] Heutz et al., Advanced Materials 19 (2007)3618.[2] J. van den Brink and A. F. Morpurgo, Nature 450 (2007) 177.
12:15 PM - E3.7
Laterally Resolved Picosecond Magnetodynamics in Spin-Valve-Type Thin Film Elements.
Claus Schneider 1 , Alexander Kaiser 1 , Carsten Wiemann 1 , Stefan Cramm 1 , Carsten Tieg 2
1 Institute of Solid State Research IFF-9, Research Center Juelich, Juelich Germany, 2 , ESRF, Grenoble France
Show AbstractExploring the ultimate time scales of magnetic switching processes is an important issue in spin electronics. In particular, in spin valves or magnetic tunneling junctions magnetocrystalline anisotropies and interlayer exchange coupling are going to change the magnetodynamic response of the entire system as compared to the dynamics of the individual layers. Understanding the role of these interactions is a key to the design of optimized devices.We have thus employed time-resolved x-ray photoemission microscopy to address the magnetization dynamics in spin-valve type model systems in the nano- and picosecond regime. In Co/Cr/Fe(001) single crystalline elements we find a strong influence of the magnetocrystalline anisotropy, which tends to suppress rotation processes. On the nanosecond time scale, the dynamic response of top and bottom magnetic layer is the same, whereas on the picosecond time scale there is an indication for a dynamic “decoupling” of the layers. In polycrystalline FeNi/Cr/FeCo trilayer elements, the interlayer coupling character is found to determine the dynamic response. In particular, the rotational processes in the FeNi and FeCo layers differ significantly in the precession amplitude, which can be related to the different coercivity of the individual layers. By contrast, the motion of domain walls in both layers closely agrees, which can be explained by an enhancement of the interlayer coupling due to the domain wall stray fields. Our examples demonstrate that the detailed magnetodynamics in coupled magnetic layers is quite complex and depends strongly on the time scale under consideration. This has consequences for the performance of GMR-based spintronics elements.
12:30 PM - E3.8
Magnetic Responses of Ferromagnetic Ultrathin Film Upon the Presence of Adjacent Organic Layer.
Yuet-Loy Chan 1 , Ya-Jyuan Hung 1 2 , Jia-Hao Wang 1 2 , Hsu-Ting Chang 1 2 , Ying-Chang Lin 1 , Chih-Hao Lee 2 , Yao-Jane Hsu 1 , Der-Hsin Wei 1
1 , National Synchrotron Radiation Research Center, Hsinchu Taiwan, 2 Department of Engineering and System Science, National Tsing Hua University, Hsinchu Taiwan
Show AbstractOne recent development in spintronics is the incorporation of organic materials to take advantage of its weak spin de-coherence process that low-Z materials generally possess. Intuitively, a hybrid structure consisted of an organic layer sandwiched by two ferromagnetic films should be able to enjoy less constraint during structure fabrication while maintains its magnetic properties comparable with the all solid-state ones. However, for spin to travel through a ferromagnet/organic semiconductor/ferromagnet (FM/OSC/FM) trilayers, it involves not only the spin transport within organic film but also the spin injection and reception across FM/OSC interfaces. Therefore, it is imperative to explore what chemical and magnetic properties that interfaces are exhibited so that improved understanding on the electron/spin transport within organic-inorganic hybrid structures could be established. In this study, we utilized the in-situ prepared Cobalt/Pentacence/Cobalt (Co/Pc/Co) trilayer to examine how a magnetic (organic) thin film would response to the presence of an adjacent organic (magnetic) layer at ultrathin film regime. Employing the X-ray photoelectron spectroscopy (XPS) at various deposition stages, we found the Co layer is chemically inert upon the addition of Pc, but becomes chemically reactive when depositing on Pc layer. Unexpected disparity was also found in the magneto-optic Kerr effect (MOKE) measurements between Co/Pc and Pc/Co bilayers, in which a relatively high Co coverage was needed to have the magnetic order become observable. Employing the X-ray photoemission electron microscope (XPEEM), we found the morphology of Pc layer had an apparent impact on the domain configuration of Co top layer.
12:45 PM - E3.9
Investigations on the MTJ Interface Fe-MgO using Electron Spectroscopy with X-ray Standing Waves.
Benjamin Balke 1 2 5 , Christian Papp 1 2 , See-Hun Yang 3 , Stuart S. P. Parkin 3 , Sven Doering 4 , Ulf Berges 4 , Carsten Westphal 4 , Charles Fadley 1 2
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Dept. of Physics, Univ. of California Davis, Davis, California, United States, 5 , Univ. of Mainz, Mainz Germany, 3 , IBM Almaden Research Center, San Jose, California, United States, 4 Physics Dept., Tech. Univ. of Dortmund, Dortmund Germany
Show AbstractMultilayer nanometer-scale structures are ubiquitousin current magnetic devices, and the detailed characteristics of the layersmaking them up, including the interfaces between layers, are often decisive asto ultimate functional properties. Buried layers and interfaces are thus crucialelements in such devices, as well as many other nanoscale structures ofpotential interest in technology, but characterizing them fully presents uniquechallenges.We have used x-ray standing wave excitation of photoelectrons to study buried layersand interfaces between Fe and MgO as used in MTJs at the ALS in Berkeley.The samples were grown on synthetic multilayer mirrors and the x-ray incidence anglewas tuned to the 1st order Bragg reflection. This technique permits scanning the resultant standing wave field through nm-scale structures and analyzing the depth distribution of their structural, chemical, electronic, and magnetic properties.We were able to investigate the structural interfaces in the MTJ stack as well as the magnetic Fe-MgO interface performing depth-resolved XMCD measurements. Furthermore, it was possible to extract the matrix-element weighted depth-resolved DOS from the measurements of the valence band using the standing wave technique.BB and CP gratefully acknowledge the Feodor-Lynen fellowship of the Humboldt foundation.
E4: Heusler Alloy and DMS
Session Chairs
Tuesday PM, December 01, 2009
Room 103 (Hynes)
2:30 PM - **E4.1
Nuclear Magnetic Resonance Studies of Highly Spinpolarized Heusler Compounds.
Sabine Wurmehl 1 , Jürgen Kohlhepp 2 , Henk Swagten 2 , Bert Koopmans 2 , Christian Blum 3 1 , Vadim Ksenofontov 3 , Horst Schneider 3 , Gerhard Jakob 3 , Daniel Ebke 4 , Günter Reiss 4 , Bernd Büchner 1
1 Institute of Solid State Research, IFW Dresden, Dresden Germany, 2 Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven Netherlands, 3 , Johannes Gutenberg - Universität, D-55128 Mainz Germany, 4 Thin Films and Nano Structures, Department of Physics, Bielefeld University, D-33501 Bielefeld Germany
Show AbstractA successful application of highly spin-polarized materials in spintronic devices requires a detailed knowledge of the interplay between the structure and the magnetic and electronic properties. This is achieved by gaining knowledge of the local structure by means of Nuclear Magnetic Resonance (NMR). NMR directly probes the local environments of the active atoms and is thus able to resolve neighboring shells providing a unique tool to study the (local) structural properties of spin polarized materials [1,2].This talk will focus on recent results of structural characterization of highly spin polarized Heusler compounds by means of NMR, in particular focusing on thin films of Co2FeSi. Co2FeSi is predicted to be a half-metallic ferromagnet with an extraordinary high magnetic moment and Curie temperature [3]. However, a low tunnel magneto-resistance ratio, a lower spin polarization than predicted, and a lower magnetic moment were experimentally observed in thin film samples. The NMR study shows the main resonance line corresponding to 59CoCo nuclei in the L21 environment but also additional resonance lines at the high frequency side of the main line with mutual spacing of 32 MHz. The additional resonance lines correspond to 59Co with more Fe next neighbours than expected for the L21 type ordering, which is interpreted as the formation of an off-stoichiometric film yielded by sputtering from a stoichiometric target. This off-stoichiometry might explain the observed deviations from the expected half-metallic behaviour [4].[1] S. Wurmehl et al., Appl. Phys. Lett. 91, 052506 (2007).[2] S. Wurmehl, J. T. Kohlhepp, Topical review in J. Phys. D: Appl. Phys. 41, 173002 (2008).[3] S. Wurmehl et al., Appl. Phys. Lett. 88, 032503 (2006).[4] S. Wurmehl et al., J. Phys. D: Appl. Phys. 42, 084017 (2009).
3:00 PM - E4.2
Doping the Half-Heusler Alloy NiMnSb With 3d-Metals.
Igor Abrikosov 1 , Bjoern Alling 1 , Marcus Ekholm 1 , Andrei Ruban 2
1 Department of Physics and Measurement Technology (IFM), Linkoping University, Linkoping Sweden, 2 Department of Material Science and Engineering, Royal Institute of Technology, Stockholm Sweden
Show AbstractThe crystal structure has a pronounced effect on the magnetic properties of materials, and therefore its modification allows one to tune the magnetic properties of the alloys. We illustrate this idea by results of a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d-metals Cr, Mn, Fe, Co, and Ni both with respect to energetics and magnetic properties [1,2]. The half-Heusler alloy NiMnSb is believed to be half-metallic at 0 K. This would make it ideal for spintronics applications. However, experiments have failed to reproduce full spin polarization at elevated temperatures. Starting from the formation energies we discuss the possibility of placing the 3d- dopants on different crystallografic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions as well as the density of states and outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn has the largest impact on magnetic interactions in the system while preserving the half metallic property. In particular, we predict that there is very large exchange coupling between the anti-site Mn and the atoms of the original Mn sublattice. Moreover, we have investigated the effect of replacing the Ni with Mn on the structural stability of this compound. Our calculations show that the disordered alloy can be stable at and above room temperature. Therefore, we suggest the possibility that Mn and Cr dopants increase the thermal stability of half-metallicity in NiMnSb with implications for its possible usage in spintronics applications. We also demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1−xCuxMnSb alloys as a case study [3]. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase the Curie temperature in Cu-rich samples.[1] B. Alling, S. Shallcross, and I. A. Abrikosov, Phys. Rev. B 73, 064418 (2006).[2] B. Alling, M. Ekholm, and I. A. Abrikosov, Phys. Rev. B 77, 144414 (2008).[3] B. Alling, A. V. Ruban, and I. A. Abrikosov, Phys. Rev. B 79, 134417 (2009).
3:15 PM - E4.3
Epitaxial growth of a Full-Heusler Alloy Co2FeSi on Silicon.for Si-based Semiconductor Spintronics
Shinya Yamada 1 , Yuji Enomoto 1 , Kenji Kasahara 1 , Kazutaka Yamane 1 , Kenji Yamamoto 1 , Yuichiro Ando 1 , Kohei Hamaya 1 2 , Masanobu Miyao 1
1 Department of Electronics, Kyushu University, Fukuoka Japan, 2 PRESTO, Japan Science and Technology Agency, Kawaguchi Japan
Show AbstractElectrical spin injection and detection in ferromagnet/semiconductor devices are key technologies for semiconductor spintronic applications. Silicon (Si) has been regarded as an ideal material because it has a long spin relaxation time and compatibilities with Si-ULSI technologies. Recently, we explored highly epitaxial growth of a ferromagnetic binary-Heusler alloy Fe3Si on Si by low-temperature molecular beam epitaxy (LT-MBE) and obtained high-quality heterointerfaces at the growth temperature (TG) of 130 oC [1]. Using the high-quality interfaces, we also demonstrated electrical spin injection and detection through Fe3Si/Si Schottky tunnel barriers [2]. However, the spin transport in Si has been limited in low temperature regime (< 273 K). To realize room-temperature demonstration of the spin transport in Si, we need to use half-metallic source-drain contacts which can realize high efficiency of the spin injection and detection. Considering these, we hope that full-Heusler alloys are promising materials for its half-metallic source-drain contacts. Expanding our LT-MBE techniques for Fe3Si/Si[1], we have explored highly epitaxial growth of a full-Heusler alloy Co2FeSi on Si(111) at various growth temperatures (TG). Co, Fe and Si were co-evaporated using Knudsen cells. During the growth, we observed in-situ reflection high energy electron diffraction (RHEED) patterns. At TG = 60 - 200 oC, the RHEED patterns exhibited symmetrical streaks, implying single crystal and good two-dimensional epitaxial growth of the Co2FeSi layers. To evaluate the Co2FeSi/Si interfaces, we measured cross-sectional transmission electron micrograph (TEM). For TG = 200 oC, ~ 4-nm-thick interlayers can be identified at the Co2FeSi/Si interface. Although the interlayer materials were decreased for TG = 130 oC, they can be seen yet. For TG = 60 oC, on the other hand, a very flat interface within the fluctuation of a few monolayers was observed. We also evaluated the presence of L21 ordered structures near the interface. Superlattice reflections of nano electron diffraction patterns were very weak for TG = 60 oC unfortunately, but we can enhance them by optimizing post-annealing conditions. For the Co2FeSi/Si layers annealed at 200 oC for 30 min, L21 ordered structures were observed with an abrupt heterointerface. Furthermore, magnetic properties of the Co2FeSi/Si(111) were similar to those of the high-quality Fe3Si/Ge(111) and Fe3Si/Si(111) which we observed [1,3]. From these facts, we made ready to demonstrate electrical spin injection and detection in silicon through a half-metallic Co2FeSi/Si(111) interface. [1] K. Hamaya et al ., Appl. Phys. Lett. 93, 132117 (2008). [2] Y. Ando et al ., Appl. Phys. Lett. 94, 182105 (2009).[3] Y. Ando et al ., J. Appl. Phys. 105, 07B102 (2009).
3:30 PM - **E4.4
Spin-resolved Unoccupied Density of States in Heusler Alloys.
Hans-Joachim Elmers 1 , Michael Kallmayer 1 , Peter Klaer 1 , Horst Schneider 1 , Elena Arbelo Jorge 1 , Christian Herbort 1 , Gerhard Jakob 1 , Martin Jourdan 1
1 , University Mainz, Mainz Germany
Show AbstractWithin the field of spintronics half-metallic ferromagnetism (HMF) plays a major role. HMF stands for a metallic character of e.g. the majority-spin states while the minority-spin states comprise an energy gap at the Fermi level. Thus, the electrical current is carried exclusively by majority-spin states making HMF materials very attractive for the fabrication of spintronic devices. Co-based Heusler alloys Co2YZ (transition metal Y and main group element Z) have attracted much attention in this field because ab-initio theory has predicted HMF and a high Curie temperature for many of these compounds. Moreover, in order to overcome the thermally induced suppression of high spin polarization in Heusler alloys a tailoring of the band structure through doping of the ordered alloys has been proposed. In contrast to the numerous theoretical predictions, only a few experimental verifications of the calculated band structures exist. A direct study of the band gap is however of particular importance for further improvement of these alloys with respect to spintronic applications. Although spin-resolved photoemission or scanning tunneling spectroscopy can directly probe the spin polarization at a half-metal surface, these methods have no access to the crucial buried interfaces in spintronic devices. We investigate the electronic properties of epitaxial Heusler films and polycrystalline bulk samples using circular dichroism in X-ray absorption spectroscopy (XMCD). Considering final state electron correlations, we are able to extract the spin-resolved partial density of states at the Co atom from XMCD data. Our experimental results corroborate the predicted half-metallic ferromagnetic properties of these alloys and reveal a compositional dependence of the Fermi energy position within the minority band gap.
4:30 PM - E4.5
Negative Magnetoresistance in Solid State Materials. The One Effect – The One Physical Model?
Sergei Obukhov 1
1 , A.F.Ioffe Institute of Physics and Technology, Saint-Petersburg Russian Federation
Show AbstractEffect of resistance decrease in magnetic field, Negative Magnetoresistance (NM), has been an important issue in semiconductors, ferromagnetic metals, superconductors and graphen studies over the past 50 years. But despite the considerable attention to the physical nature of this phenomenon and perspective to use NM in magneto-electronics the physical nature of NM still remains the matter of discussion. Giant Negative Magnetoresistance (ρB=0/ρB~105÷107) observed in Diluted Magnetic Semiconductors (DMS), for example in Hg1-xMnxTe and Magnetic Semiconductors (MS) such as Gd1-xvxS4 and EuSe, has always been explained in the framework of spin-polaron model [1,2]. In case of ferromagnetic metals a relatively small NM effect, (ρB=0/ρB)max< 1,2, was described as s-d scattering decrease in magnetic field. For NM effect explanation in materials which did not contain transition or rare earth metals ( nonmagnetic semiconductors, graphen, superconductors and etc.) the quantum correction [3] and weak localization models [4] have been used.Our experiments have demonstrated that gigantic value of NM (ρB=0/ρB=5T~104) can also be observed in nonmagnetic semiconductors - uniaxially stressed doped single crystals- p-Ge(Ga), p-InSb(Ge) and p-InSb(Mn) at low temperature T~1,2K and acceptor concentration Na within the critical concentration of Metal-Insulator Transition (MIT) Ncr =Na=NGa=NGe =NMn~1017cm-3. These facts support a new view on the nature of NM in solid state materials. We propose that the field of elastic stress around impurities or vacancies can restructure energy band and form antiferromagnetic spin-ordered sub-bands in solid state materials. These sub-bands are separated by energy gap Δ with the maximal value Δ~1meV in the case of InSb(Mn) [5] and by energy gap Δ in the range 1÷10meV in MS and DMS. External magnetic field aligns spins, closes the gap Δ between sub-bands and causes MIT, e.g. NM in semiconductors. In solid state materials with high impurity concentration like ferromagnetic metals and graphen these sub-bands could overlap in the absence of magnetic field. In the framework of above model external magnetic filed enhances sub-bands overlapping and thus conductivity increases. In superconductor cuprates the energy gap Δ could be associated with pseudogap reduced by magnetic field. References[1] J.K.Furdyna and J.Kossut. Diluted Magnetic Semiconductors , Vol. 25, Semiconductors and Semimetals, Academic Press, Inc. (1988).[2] E.L.Nagaev. Physics of Magnetic Semiconductors, Moscow, Mir. (1983). [3] B.L.Altshuler and A.G.Aronov, in Electron-Electron Interaction in Disordered Systems, edited by A.L.Efros and M.Pollak. North-Holland, Amsterdam. (1985).[4] X. Zhang, Q.Z. Xue, D.D. Zhu. Physics Letters A, 320, 471(2004).[5] S.A.Obukhov.Phys. Stat. Sol. (b) 242, No. 6, 1298–1306 (2005).
4:45 PM - E4.6
CeO2 based Diluted Magnetic Dielectrics for Spin Filter Application.
Paul Slusser 1 , Dhananjay Kumar 2 , Ashutosh Tiwari 1
1 Materials Science and Engineering, university of utah, Salt Lake City, Utah, United States, 2 Mechanical Engineering, NC A& T University, Greensborro, North Carolina, United States
Show AbstractThe ongoing search for room temperature spin filters has uncovered several candidate materials recently. Highly notable among these materials are the cerium oxide based Diluted Magnetic Dielectrics (DMDs). Few years ago our group showed that CeO2:Co films exhibit high temperature ferromagnetism with a giant magnetic moment. That report created a lot of excitement in the field. This was mostly because apart from being ferromagnetic CeO2:Co posses a cubic structure and is closely lattice matched with silicon. Moreover CeO2 has a high dielectric constant (26) and is transparent over the entire visible spectrum with a bandgap of 3.4 eV. Now in this report we are describing the synthesis and magnetic characterization of three new CeO2 based DMD systems. These systems are: copper, manganese, and zinc doped cerium oxide [Ce1-xCuxO2-δ, Ce1-xMnxO2-δ, and Ce1-xZnxO2-δ]. High quality epitaxial thin films of these materials were grown on LaAlO3(001) substrates by pulsed laser deposition (PLD) and were characterized using several state-of-the-art characterization techniques. Magnetization measurements showed that in the case of Ce1-xCuxO2-δ (x=0.03), films exhibit room temperature ferromagnetism with a saturated magnetization of ~1.0 μB/Cu atom. However, when additional copper ions were introduced into the system (x=0.15) superparamagnetic behavior was observed. In the case of Ce1-xMnxO2-δ films ferromagnetic behavior was observed in both the x=0.03 as well as x=0.15 samples. But, in the case of x=0.15 film evidence of phase separation was observed. In contrast to the above two cases, the Ce1-xZnxO2-δ and undoped CeO2 films showed a diamagnetic response. An in-depth structure-property correlation and analysis was performed to understand the observed magnetic behavior of the materials.
5:00 PM - E4.7
Samarium Oxide Thin Film Dilute Magnetic Dielectrics.
Nathan Gray 1 , Ashutosh Tiwari 1 , Dhananjay Kumar 2
1 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Mechanical Engineering, North Carolina A&T, Greensborro, North Carolina, United States
Show AbstractSpin tunnel filters are a very promising method of spin injection into semiconductors, which is required to achieve a functional spin transistor. EuSe, a magnetic insulating material, has been shown to achieve nearly 100% spin polarized injection current by Moodera. Dilute magnetic dielectrics also have very good potential for high efficiency spin injection as tunnel filters. In 2006 Co doped CeO2 was discovered to exhibit room temperature ferromagnetism with a giant magnetic moment. Motivated by these results, we have explored another dilute magnetic dielectric based on a rare earth oxide system, Co doped Sm2O3, and the type and origin of its magnetic properties.Samarium oxide is already under investigation as an optoelectronic material and a high-k dielectric replacement for SiO2 in complementary metal oxide semiconductors (CMOS). It has been shown to have a high dielectric constant (30.5 for epitaxial cubic phase). It is also chemically and thermodynamically stable with Si.We have doped Sm2O3 with varying amounts of Co, and fabricated thin films using pulsed laser deposition. Several deposition parameters were varied, and the effects on final crystal structure were determined. The films were extensively characterized to determine crystalline phase properties and magnetic characteristics. They show super paramagnetism with high magnetic moment per Co ion, and very large hysteretic response below blocking temperature. The origin of this magnetism is investigated, and the presence of secondary magnetic phases of cobalt and cobalt oxide are ruled out through x-ray diffractometry and high resolution transmission electron microscopy. A relationship between defect density and magnetic moment is found and discussed.
5:15 PM - E4.8
Intrinsic Exchange Bias in a Single Phase Ferrimagnet.
Kevin West 1 2 , Dao N. Nam 3 , Jiwei Lu 3 , Ryan Comes 2 , Stuart Wolf 3
1 Physics , University of California at San Diego, La Jolla, California, United States, 2 Engineering Physics, University of Virginia, Charlottesville , Virginia, United States, 3 Material Science and Engineering, University of Virginia , Charlottesville , Virginia, United States
Show AbstractExchange bias (EB) commonly presents itself as a shift of the hysteresis loop along the field axis in systems that have a ferromagnetic-antiferromagnetic interface. The EB phenomena has generally been considered an interface effect. Here we report on very peculiar magnetic hysteresis behavior of Ru0.25Cr0.75O2/TiO2 (001) thin films. We describe three previously unreported magnetic phenomenon inherent to this material; (i) intrinsic positive exchange bias in a single phase epitaxial thin film ferrimagnet with the absence of an exchange interface (ii) field modulation of the shifted M(H) loop along the field axis direction and (iii) inverted hysteresis (IH) loops in which the overall area of the loop is negative. We argue that some of these results can be understood within the context of a very unusual interface of the epitaxial film with the TiO2 (001) substrate.
5:30 PM - E4.9
Electronic and Magnetic Properties of Small Co-O Nanowires.
Liudmila Pozhar 1
1 Physics, University of Idaho, Moscow, Idaho, United States
Show AbstractElectron spin states of exchange-biased, core-shell quantum dots (QD) and wires (QW) satisfy major requirements applied to elements of quantum information processing and communication systems that are expected to support integrated information storage-processing, enable secure communications, and provide for a significant increase in efficacy and reliability of electronic devices and computational equipment of the future. In particular, typical electron spin relaxation time exceeding a nanosecond and reaching milliseconds has been experimentally observed for such systems, and a possibility to control single electron spins localized on an individual QD or QW has been demonstrated. In the work reported here electronic and magnetic properties of exchange-biased, core (Co)-shell (Co-O) QWs have been studied using the first principle, quantum many-body theoretical methods, including the Hartree-Fock (HF), restricted open shell HF, configuration interaction (CI) and multiconfiguration self-consistent field (CASSCF/MCSCF) approximations. The initial structure of the Co cores of the studied QWs have been derived from that of the corresponding symmetry elements of Co bulk lattices, and the minimum number of oxygen atoms has been added to the “surface” of the cores forming the corresponding shells to stabilize the molecules. The final geometry and composition of the studied QWs have been obtained by the total energy minimization procedure. In the case of the “quantum-confined” QWs the total energy has been minimized while applying spatial constraints to the center-of-mass positions of the QW atoms, to reflect QW synthesis in quantum confinement. The geometry of the corresponding “vacuum” QWs have been obtained by the unconditional total energy minimization, when the spatial constraints applied to the quantum-confined QWs were relaxed. The studies have been focused on the electronic energy level structure, direct optical transition energy (OTE), and charge and spin density distributions (CDD and SDD, respectively). The obtained results indicate that both vacuum and confined Co/C-O QWs are stable being molecules from a chemical standpoint, and possess significant SDDs that are about an order of magnitude larger than those of the previously studied semiconductor compound QDs. A typical ground state of these QWs is a HF triplet. The OTE of these QWs can be manipulated in a wide interval of values from below 1 eV to over 5 eV by adjusting the number of oxygen atoms in the shells, Co atoms in the cores, and the structure and composition of the quantum confinement. The obtained results closely correlate with experimental data available in literature and confirm that the studied QWs models can be used to guide nanomaterials development for integrated quantum information storage-processing.
5:45 PM - E4.10
Size-selected Magnetic MnAs Nanoparticles by Nanoparticle Conversion.
Michael Wolff 1 , Detlef Goerlitz 1 , Kornelius Nielsch 1 , Maria Messing 2 , Knut Deppert 2
1 Institute of Applied Physics, University Hamburg, Hamburg Germany, 2 Solid State Physics, University Lund, Lund Sweden
Show AbstractNanostructured manganese arsenide (MnAs) has been studied extensively in the past years as it is an attractive material for spintronic applications. Typically, MnAs nanoparticles are synthesized by post-growth annealing of GaAs:Mn structures at elevated temperatures or by self-organized growth. In both cases, control over particle diameter and number concentration is difficult to achieve, because the formation of MnAs nanoparticles depends on various parameters such as temperature and Mn content. We report on the synthesis of ferromagnetic manganese arsenide nanoparticles via conversion of primary Mn particles which are generated in an aerosol process using a spark discharge generator. After sintering and size-selection in an aerosol setup, the particles are deposited on GaAs(100)B and Si(111) substrates. Subsequent conversion to MnAs particles takes place in an annealing process under hydrogen atmosphere with an arsine background pressure. No protection against oxidation or other kinds of degradation is applied before or after the annealing process. The annealed MnAs particles exhibit hexagonal facets on GaAs(100)B substrates, whereas on Si(111) they remain spherical. Remanence curves with a Curie temperature slightly above room temperature and hysteretic behavior of the annealed particles are verified using a SQUID magnetometer. The in-plane and out-of-plane coercivities of the particles were found to be different for the two studied substrates. While the coercivities were found to be similar on Si substrates, particles deposited on GaAs showed an anisotropic ferromagnetic behavior with distinguished differences between the two directions, which can be attributed to an epitaxial relationship between the MnAs particles and the GaAs as illustrated already by the hexagonal facets. The saturation magnetization was found to be of the order of 150 to 300 emu/cm3. The lower value as compared to bulk may be attributed to oxygen in the particles. Our work shows that the concept of nanoparticle conversion can yield size-selected magnetic nanoparticles with the possibility to tune size, density and material independently. This opens up the creation of other materials such as MnN, MnP or MnSb.
E5: Spinel, Rutile, DMS
Session Chairs
Wednesday AM, December 02, 2009
Room 103 (Hynes)
9:30 AM - **E5.1
Spineltronics: Manipulating Spins in Spinel Compounds.
Brent Melot 1 , Daniel Shoemaker 1 , Ram Seshadri 1
1 Materials, University of California, Santa Barbara, California, United States
Show AbstractThe spinel crystal structure is seeing resurgent interest because of the possibility of exotic magnetic ground states associated with magnetic frustration and complex magnetic ordering. We describe work on spinel oxides with Cr and Mn on the B site, and magnetic tuning of the A site, from the dilute limit of no magnetism on the A site, to fully magnetic A sites. Starting with the parent compounds, CoCr2O4 and Mn3O4, both of which have conical ordering and hence multiferroic ground states, we show that magnetic tuning on the A site (substituting Co2+ and Mn2+ by Zn2+) yields rich magnetic phase behavior on the one hand, and intrinsic exchange bias on the other.
10:00 AM - E5.2
Atomic and Electronic Structure of Fe3O4(111)/MgO(111) and Fe3O4(111)/Al2O3(0001) Interfaces.
Vlado Lazarov 1 , Andras Kovacs 3 , Pedro Galindo 4 , Michael Weinert 2
1 Physics, University of York, York United Kingdom, 3 Materials, University of Oxford, Oxford United Kingdom, 4 Computer Science, University of Cadiz, Cadiz Spain, 2 Physics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States
Show AbstractMagnetite is half metal oxide that is attracting a lot of attention in spintronics due to its high Curie temperature, which is well above the room temperature (850 K). Recent efforts to incorporate magnetite in a device type of structures such as spin valves and magnetic tunnel junctions (MTJ) were not very successful. The reports on tunnelling magneto resistance ratio on Fe3O4(100)/MgO(100) based MTJs were less than 20 %. In addition questions about 100 % spin-polarization of the thin magnetite(100) films were raised in the light of the recent spin polarized XPS data, that is showing only 50-60% spin polarization of (100) oriented magnetite films. However subsequent experiments on (111) oriented magnetite thin films have shown that spin-polarization in these films can be as high as 85%. In this work we studied the atomic and electronic structure of Fe3O4(111)/MgO(111) and Fe3O4(111)/Al2O3(0001) interfaces and their effect on the spin transport from the half metal electrode through the oxide (barrier). The films were grown by Molecular Beam Epitaxy on single crystal MgO(111) and Al2O3(0001) substrates. Aberration corrected HRTEM coupled with multislice image simulations were used for determining the atomic structure of the interfaces. The best fit atomic model structures to experimental data were found to be as following: ../Mg/O/Fe/.., and ../Al/O/Fe/… In both type of the interfaces Fe is in octahedral position. Density Functional Theory (DFT) modelling confirmed that these structure have the lowest total energy. Moreover the DFT calculated electronic structure of the experimentally found structures for both interfaces is similar. This particular interface atomic arrangement creates interface states at the Fermi level which can potentially induce spin flipping as a result of spin-orbit coupling of the tunnelling electrons. In addition we consider several other atomic interface models between magnetite and MgO(Al2O3) and discuss how their electronic structure would influence the spin transport across them.
10:15 AM - E5.3
Electrodeposited Superlattices in the Magnetite/Zinc Ferrite System.
Jay Switzer 1 , Rakesh Gudavarthy 1 , Elizabeth Kulp 1 , Andrew Wessel 1 , Samantha Matthews 1
1 Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri, United States
Show Abstract In this work, both defect-chemistry and compositional superlattices in the Fe3O4/ZnFe2O4 system are electrodeposited. There is interest in depositing spintronic superlattices based on Fe3O4 because the ferrimagnetic material is purported to have 100% spin polarization at the Fermi level. Also, below the Verwey transition at 120 K, the material becomes ferroelectric. Hence, the material has multiferroic properties. In ZnFe2O4, the Zn(II) substitutes for Fe(II) producing a material that is antiferromagnetic below the Neél temperature. In the deposition of superlattices we use the fact that the departure from equilibrium is controlled through the applied overpotential. Films of Fe3O4 can be deposited with stoichiometries that depend on the applied potential. We prepare the films by electrochemical reduction of a Fe(III)-TEA (triethanolamine) complex at 80 oC in strongly alkaline solution. The deposition is believed to occur by an electrochemical-chemical (EC) mechanism. Because of the EC nature of the deposition reaction, it is possible to control the composition of the film through the applied potential. At low overpotentials at which j = 0, the surface concentration of Fe(TEA)3+ should be equal to the bulk concentration, whereas at high overpotential at which j = jL the surface concentration of Fe(TEA)3+ should approach zero. Hence the material should have an excess of Fe(III) at low overpotential and an excess of Fe(II) at high overpotential. Stoichiometric Fe3O4 should deposit at an applied potential of -1.065 V vs. Ag/AgCl. Superlattices of the material are electrodeposited by pulsing the applied potential between -1.01 and -1.065 V vs. Ag/AgCl. Compositional superlattices can be produced by adding Zn(II) to the deposition bath. Superlattices with modulation wavelengths ranging from 10 to 30 nm were produced. The superlattices were characterized by X-ray diffraction and focused ion beam (FIB) microscopy.Magnetoelectronic properties of the electrodeposited superlattices will be reported. A particularly interesting feature that the superlattices exhibit is resistance switching during perpendicular transport measurements.
10:30 AM - **E5.4
Prediction of False Ferromagnetism by LDA et al in Defected Oxides and 3d Impurities in Insulators.
Alex Zunger 1
1 , NREL, Golden, Colorado, United States
Show AbstractTheorists have been predicting ferromagnetism in either dilute 3d doped insulators or in defected insulators using various derivatives of Local Density Approximation methods. Experimentalists often cite such predictions in support of generally vague evidence of magnetism in such systems. I discuss here a number of ways in which LDA and related methods, as currently practiced by many, can lead to "false positive" for predicting magnetism. These misuses include: (1)Level placed by LDA in continuum instead of in band gap ( Ref 1,2): Cases where LDA and LDA +U erroneously predict that a 3d level resides inside the host conduction band ( having thus extended wavefunctions) wheras better approximations place it inside the host band gap ( having localized wavefunctions). In this case LDA leads to the illusion of long range magnetic interactions. We show how to fix this error via the " Nonlocal external potential" (NLEP) method. (2)Forgetting Percolation (Ref 3,4): Cases where theorists introduce vacancies in their supercell to examine their effect on magnetism, but miss the point that the concentration of such vacancies is related to the (calculable) vacancy formation energy. Once this is calculated, one often finds out that the vacancy concentration is lower by many orders of magnitude then what is required for percolation of the magnetic interactions. We show how this problem can be fixed by self consistently linking the vacancy concentration to the formation energies. This shows that to achieve percolation one will need to enhance the vacancy concentration by 3-5 orders of magnitude relative to what equilibrium grants. Brutal growth conditions would be needed to accomplish such deviations from equilibrium . This may explain why often “good" samples show no magnetism whereas "poor “ ones do. (3)Forgetting Polaronic localization of ligand orbitals (Ref 5): In many cases LDA calculations of cation vacancies predict the ensuing acceptor wavefunction to be distributed equally on all anion ligands (thus, leading to rather delocalized magnetic interactions), yet in reality the acceptor wavefunction has a (polaron-like) localization on just a subset of anion ligands, leading, in reality, to vanishing magnetic interactions. We show how the polaron can be correctly descirbed by extending LDA to include self interaction effects. This, however, reveals virtually no magnetic interaction between cation vacancies in common oxides because the hole now resides in localized ligand orbitals.(1) S. Lany, H. Raebiger, and A. Zunger, Phys. Rev. B Rapid Communication 77, 241201 (2008).(2) H. Raebiger, S. Lany, and A. Zunger, Nature 453, 763-766 (2008).(3) J. Osorio-Guillen, S. Lany, S. V. Barabash, and A. Zunger, Phys. Rev. B 75, 184,421 (2007).(4) J. M. Osorio, S. Barabash, S. Lany and A. Zunger, Phy.s Rev. Letters 96, 107203 (2006).(5) J. A. Chan, S. Lany, and Alex Zunger, Physical Review Letters (In Press, 2009).* Work done in collaboration with S.Lany, J. Osorio, H. Raebiger and J.Chan and supported by DOE BES.
11:30 AM - E5.5
Robust Magnetism of (110) CrO2 Films.
Manjit Pathak 1 2 , Krishna Chetry 1 2 , Dipanjan Mazumdar 1 , Patrick LeClair 1 2 , Gary Mankey 1 2 , Arunava Gupta 1
1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States, 2 Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractCrO2 is the only known half metal oxide with spin polarization of 98% and Curie temperature close to 395K. However, progress on CrO2-based structures such as magnetic tunnel junctions has been limited to date, perhaps in part due to the significantly strained CrO2 (100) films used in previous studies.In this study, CrO2 films of (100) and (110) orientations were epitaxially grown on TiO2 substrates with chemical vapor deposition (CVD) technique. Films on TiO2 (100) follow a layer-by-layer growth mode, with atomically smooth surfaces but significant out-of-plane compressive stress. In contrast, films on TiO2 (110) follow an island-like growth mode, and are found to be essentially strain-free for even the thinnest films studied (~35nm). Both orientations of CrO2 were studied with X-ray magnetic circular dichroism (XMCD). Analysis of our data with the conventional sum rules for 3d elements show almost 50% higher XMCD on (110) CrO2 films compared to (100) CrO2 films at the Cr-L2,3 edge at room temperature, consistent with SQUID magnetometry on the same samples. The orbital moment is found to be similar for both films and no significant XMCD was observed on the O-2p edge. This robust magnetism is attributed to the colinearity of Cr spins in (110) films which grows strain free on TiO2 substrates of same orientation, compared to canted Cr spins in strained (100) CrO2 films. This suggests (110)-oriented films may be more amenable to successful device fabrication.
11:45 AM - E5.6
Defect Controlled Ferromagnetic Properties of Vanadium Dioxide Thin Films: Structure-property Correlations.
Tsung-Han Yang 1 , Sudhakar Nori 1 , Honghui Zhou 1 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractInducing room temperature ferromagnetism (RTFM) in oxide dilute magnetic semiconductors (DMS) and insulators, such as ZnO, SnO2, In2O3 and MgO, has been a subject of intense investigation recently. The spins of the unpaired 3d electrons associated with the host atoms or the magnetic dopants or impurities in DMS materials interact with both defects and the oxygen atoms in a complex way altering the physical properties significantly. Defects arising out of oxygen vacancies play a critical role in determining the physical properties. Here, we present the control of defects in vanadium dioxide thin films by the stoichiometry adjustment route, owing to the narrow stability range in different phases, where the ratio of oxygen and vanadium varies from 1.5 to 2.5 such as V2O3, V3O5, V4O7, V5O9, VO2, and V2O5. Vanadium dioxide (VO2) exhibits an extremely sharp and ultra-fast phase transformation from low temperature monoclinic phase to high temperature tetragonal phase at 68 °C. The characteristics of phase transformation can be also controlled by defects and both coercivity and magnetization have been found to decrease with increasing temperature. In this study, we report ferromagnetism for the first time in VO2 thin film with Curie temperature around 500 K. By controlling the processing parameters, we are able to tune the defect contents and microstructure and consequently the magnetic properties. We also discuss correlations between the characteristics of phase transformation and ferromagnetic properties with the microstructure and the valence charge of undoped VO2 thin films, which are consistent with the analysis of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We believe that VO2 has high potential in multifunctional device applications related to spintronics, switching and magnetic recording.
12:00 PM - E5.7
Correlation Effects in the P-electron Magnet RbO2.
Claude Ederer 1 , Roman Kovacik 1
1 School of Physics, Trinity College Dublin, Dublin, Co. Dublin, Ireland
Show AbstractRubidium superoxide, RbO2, is an antiferromagnetic insulator with a Neel temperature around 15 K. It is thus a rare example of a bulk material with magnetic order resulting from partially filled p states. Here we present results of GGA+U calculations which show that the electron-electron interaction between p electrons on the oxygen sites leads to a strong tendency towards the formation of an orbitally polarized insulating state. This is in contrast to standard first principles calculations using the pure local density or generalized gradient approximations, which lead to a half-metallic ground state for this class of compounds. The energy differences calculated within the GGA+U approach between different orbitally ordered configurations are sizeable, indicating an orbital ordering temperature higher than the antiferromagnetic Neel temperature of ~15 K. Our results demonstrate the importance of correlation effects in p electron magnets such as RbO2, and suggest that this class of materials can provide important benchmark results for the future study of more complicated "d-0 magnets" where holes in the p electron states are induced by defects.
12:15 PM - E5.8
The Electronic Structure of GdN from X-ray Spectroscopy.
Andrew Preston 1 2 3 , Joe Trodahl 2 3 , Ben Ruck 2 3 , Louis Piper 1 , Alex DeMasi 1 , Kevin Smith 1 , James Downes 4 , Walter Lambrecht 5
1 Department of Physics, Boston University, Boston, New York, United States, 2 , The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington New Zealand, 3 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand, 4 Department of Physics, Macquarie University, Sydney, New South Wales, Australia, 5 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractGdN is a bulk ferromagnetic semicondutor with a Curie temperature of ~ 70 K. In recent reports it has been shown that spin-splitting of the valence and conduction bands, caused by the onset of ferromagnetism, significantly narrows the direct band gap [1]. A strong negative magnetoresistance is also seen close to the transition temperature [2]. These results suggest that GdN may be suitable for spintronic applications where a high degree of spin polarization is desirable.Nevertheless, there are numerous open questions surrounding the nature of its ferromagnetism and the nature of its electronic structure both above and below the Curie temperature. These questions are as much theoretical as experimental since typical density functional theory (DFT) based approaches struggle to integrate the half-filled 4f electrons with the other band electrons [3].We have investigated the electronic structure of epitaxial GdN films using soft x-ray spectroscopy. X-ray absorption, emission, and resonant emission data from the N Kedge provide information about the occupied and unoccupied states close to the Fermi level and can be directly compared with theoretical results [4]. Agreement is noted between the spectra and the N 2p partial density of states extracted from DFT (within the local spin density approximation and with Hubbard-U corrections to the Gd 5d and 4f orbitals). The evolution of the N K-edge resonant emission spectra is explained in terms of momentum-selectivity resulting from coherent emission from specific regions in k-space, most notably the X-point. X-ray absorption and emission at the Gd M-edge are explained well by atomic multiplet calculations. Spectra obtained below the Curie temperature provide further information about the ground state electronic structure.This work supported in part by the NSF under DMR-0710485.1. Trodahl et al., Phys. Rev. B 76, 085211 (2007).2. Leuenberger et al., Phys. Rev. B 72, 014427 (2005).3. Larson et al., Phys. Rev. B 75, 045114 (2007).4. Preston et al., Phys. Rev. B 76, 245120 (2007).
12:30 PM - E5.9
Ferromagnetic MnAs Nanowires.
Thomas Nummy 1 , Steve Bennett 2 , Don Heiman 1
1 Physics, Northeastern University, Boston, Massachusetts, United States, 2 Mechanical Engineering, Northeastern University, Boston, Massachusetts, United States
Show AbstractFerromagnetic MnAs nanowires were grown by solid-source molecular beam epitaxy, imaged with electron microscopy, and studied via magnetrometry. Several approaches were used to initiate and direct wire growth, including the vapor-liquid-solid (VLS) method, and substrate crystallographic-directed growth. A variety of substrate materials, substrate orientations, and catalyst thickness were examined. For growth on GaAs[111] substrates the majority of the wires were found to be attached epitaxially to the substrate surface, having rod-like shapes with aspect ratios (length/diameter) as large as 20. Use of a 1 nm thick layer of Au catalyst resulted in micron-long nanowires having diameters of 50-100 nm and lengths of 1-2 microns. Using a thicker 5 nm layer of Au catalyst resulted in larger 100-200 nm diameter wires of similar lengths. By changing the growth conditions, a web-like triangular network of wires can also be synthesized. All of these cases produced a three-fold orientation of the wires along the principle axes of the substrate, similar to GaN nanowires grown on sapphire.[1] Large diameter Au dots were also found and produced short, large diameter pillars directed perpendicular to the substrate surface. Magnetic measurements on these materials revealed a ferromagnetic transition temperature Tc=328 K, significantly higher than that in bulk MnAs where Tc=313 K, but similar to that for MnAs epilayers grown on GaAs. The magnetization of the wires was investigated in order to understand the role of reduced dimensionality and anisotropy on their magnetic properties. Hysteresis loops were far from being square-like, having a coercive field of Hc=220 Oe much smaller than the saturation field of Hs=1000 Oe, due to the magnetic anisotropy induced by the three principle orientations of the wires. These results show that room temperature ferromagnetic nanowires can be grown epitaxially on substrates and that wires can be aligned along crystallographic directions.-- Work supported by the NSF.[1] Z. Wu, M.G. Hahm, Y.J. Jung and L. Menon, "Epitaxially Grown GaN Nanowire Networks," J. Mater. Chem. 19, 463 (2009).
12:45 PM - E5.10
Room Temperature Ferromagnetism of Neodymium-doped GaN via Diffusion.
Melvyn Luen 1 , Pavel Frajtag 2 , Neeraj Nepal 1 , John Zavada 1 , Salah Bedair 1 , Nadia El-Masry 2
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractRare-earth doped GaN is attracting wide-spread attention both as a diluted magnetic semiconductor (DMS) material and for optical devices useful in communications and multi-color semiconductor display technology. Simultaneously wide band gap semiconductors, like GaN, are suitable hosts for the near-infrared (1364nm) emission from Nd3+ ions corresponding to the low-dispersion wavelength of silica-based optical fiber communications. Recently, we demonstrated what appears to be the first successful thermal diffusion of Nd into GaN, and characterization of near-infrared luminescence and above room temperature ferromagnetism of the Nd-doped GaN thin films. This study reports the diffusion of neodymium into undoped, Mg-doped and Si-doped GaN templates that were prepared by MOCVD on (0001) sapphire substrates. We present the room temperature ferromagnetic properties of the diffused thin films as verified by magnetoresistance and alternating gradient magnetometer (AGM) measurements. The diffusion coefficient has been estimated via secondary ion mass spectroscopy (SIMS). The mechanisms for above room temperature ferromagnetism, related to the Nd concentration, will be discussed.
E6: Organic and Molecular Spinvalve, Spin-Dependent Transport
Session Chairs
Wednesday PM, December 02, 2009
Room 103 (Hynes)
2:30 PM - **E6.1
Organic Spintronics for Spin-Based Information Processing.
Supriyo Bandyopadhyay 1 , Bhargava Kanchibotla 1 , Sandipan Pramanik 1 , Sridhar Patibandla 1 , Marc Cahay 2
1 Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Electrical and Computer Engineering, University of Cincinnati, Cincinnati, Ohio, United States
Show AbstractOrganic semiconductors have weak spin-orbit interaction which tends to make the spin relaxation time in these materials exceptionally long. Measurement of the longitudinal spin relaxation time (T1) in the π-conjugated molecule tris(8-hydroxyquinolinolato aluminum), or Alq3, has shown that this time can exceed 1 second above liquid nitrogen temperatures, which is very promising for spin-based classical computing applications. In such applications, an electron’s spin is used to encode a classical bit of information and a long relaxation time ensures that the information is faithfully retained long enough to allow fault-tolerant computing. Measurement of the transverse spin relaxation time (T2) in the same molecule showed that it is about 30 nanoseconds at room temperature, which is sufficiently long to allow fault-tolerant quantum computing, if a qubit is encoded in the spin state of a single delocalized electron in Alq3. We discuss the potential use of Alq3 in quantum information processing systems and elucidate a qubit read-out scheme that has no analog in inorganic semiconductors. Finally, we discuss likely phonon bottleneck effects in this organic which may be harnessed to increase the spin relaxation time even further. We conclude with a discussion of possible spin relaxation mechanisms in this organic in various temperature regimes.
3:00 PM - **E6.2
Charge Transport in Alq3 Spin Valves: Implications for Spin Transport.
J. Jiang 1 , J. Pearson 1 , S. Bader 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractHarnessing the spin degree of freedom in electronic transport has given rise to the field of spintronics, where the vision is to merge magnetics with electronics and photonics for multifunctional devices. Recent reports of GMR in spin valves with nominally thick organic semiconductor spacers (in particular, Alq
3) [1] have raised hopes for “organic spintronics”, which is premised upon spins being injected into and coherently transported within the organic media. The attraction of organic spintronics is in long spin diffusion lengths expected from weak spin-orbit interactions due to the low-Z elements. However, it has also been suggested that these recent observations may be artifacts due to the roughness of the organic layer and
direct tunneling through locally thin regions, instead of spin injection and coherent transport. [2]
Since spin injection and transport in spin valves are necessarily represented by charge injection and transport, we performed a systematic study of charge transport in Alq3-based spin-valve structures aimed at resolving this issue.[3] The current density-voltage characteristics of our non-tunneling Co/Alq3/Fe spin-valve structures are strongly polarity- and temperature-dependent, and are significantly different from those in earlier studies where GMR was reported. By comparing with Co/Alq3/Au unipolar devices, we show that charge injection from 3d-ferromagnetic metals into Alq3 cannot be by electrons, but rather is by thermally activated holes. This is consistent with the known Alq3 HOMO and LUMO levels relative to the metal work functions. The lower spin polarization of thermally-activated charge carriers, together with the “impedance mismatch problem” due to the extremely low mobility of holes in Alq3, preclude spin injection in Alq3-based spin-valve structures. In our Co/Alq3/Fe spin-valve structures where carriers are indeed injected into and diffusively transported through Alq3, we observe no measurable MR. Moreover, we did not observe MR in Co/AlOx/Alq3/Fe structures where spins may be injected from the AlOx tunnel barrier. Thus, electrical spin injection from ferromagnetic metals and transport in Alq3 remain to be demonstrated.
Work supported by U.S. DOE Office of Science, BES, under Contract No. DE-AC02-06CH11357.
[1] Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Nature, 427, 821 (2004); F. J. Wang, Z. H. Xiong, D. Wu, J, Shi, Z. V. Vardeny, Synth. Met. 155, 172 (2005); S. Pramanik, S. Bandyopadhyay, K. Garre, M. Cahay, Phys. Rev. B74, 235329 (2006).
[2] W. Xu, G. J. Szulczewski, P. LeClair, I. Navarrete, R. Schad, G. Miao, H. Guo, A. Gupta, Appl. Phys. Lett. 90, 072506 (2007); H. Vinzelberg, J. Schumann, D. Elefant, R. B. Gangineni, J. Thomas, and B. Büchner, J. Appl. Phys. 103, 093720 (2008).
[3] J. S. Jiang, J. E. Pearson, S. D. Bader, Phys. Rev. B77, 035303 (2008).
3:30 PM - E6.3
Spin Polarized Injection in Magnetic Tunnel Junction System with Organic Semiconductor Barrier.
Kai-Shin Li 1 , Santhanam Agilan 1 , Yin-Ming Chang 1 , Cheng-Yung Hung 1 , Jung-Chi Tai 1 , Minn-Tsong Lin 1 2
1 Physics, National Taiwan University , Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractTo study spin electron transport behavior in organic semiconductor (OS) materials, which have long spin-diffusion length, the perylene-3,4,9,10- tetracarboxylic-3,4,9,10-dianhydride (PTCDA) is used as an OS barrier in magnetic tunnel junction. Two ultrathin Al2O3 films have been deposited in between organic and ferromagnetic layers to prevent the formation of the dipole layer at interface and also to protect the top metal layer from diffusing into soft organic layer. Significant magnetoresistance (MR) have been measured as 10% ~ 20% in the temperature range from 300 K to 20 K. From inelastic tunnel spectroscopy, it is clearly observed that there is a resonance state, which is from C-C=O bend mode in PTCDA, at 47 mV. The bias dependence behavior of MR shows that the rate of decrease of MR at 300 K with increasing applied voltage is larger than that of determined at 20 K. Shorter hopping length at 300 K causes the electron transport to occur via inelastic hopping through the localized states in OS barrier.
3:45 PM - E6.4
Formation and Characterization of Metal-SAM-Metal Junctions Formed via Nanotransfer Printing (nTP) for Application Toward Molecular Spintronics.
Jeremy Niskala 1 , Will Rice 1 , Frank Tsui 1 , Wei You 1
1 , University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractA great deal of organic spintronics has been focused on spun-coat polymeric or thermally deposited organic materials. In contrast, few studies have been focused on spin behavior through molecular layers covalently bound to electrodes such as with self-assembled monolayers (SAM) formed on ferromagnetic metals. One reason is that forming permanent, stable junctions atop molecular layers is not trivial where bottom electrode surface roughness, SAM density, and device stability play critical roles in large area manufacture. We report a simple method to form metal-organic-metal junctions (MOMs) via nanotransfer printing (nTP) with poly(dimethylsiloxane) (PDMS) and low surface energy perfluoropolyether (PFPE) based stamps. Transfer printing is demonstrated onto thermally deposited metal thin film electrodes where the root-mean-squared roughness of these films is controlled by the deposition process and varies by 40% or more. Transfer of Au, Ni, and Co thin films onto Au/SAM, Ni/SAM, and Co/SAM electrodes in well-ordered arrays will be presented. The nanotransfer printing we report is reproducible in high fidelity which is required in the application of practical molecular spintronic devices. The magnetic properties of transferred Ni and Co thins films were probed by MOKE. Current-voltage characteristics, magnetic properties, and preliminary spin-valve behavior will be discussed.
4:30 PM - **E6.5
Spin-dependent Scattering by Organic Radicals.
Patrick LeClair 1 3 , Weihao Xu 2 3 , Greg Szulczewski 2 3
1 Physics & Astronomy, University of Alabama, Tuscaloosa, Alabama, United States, 3 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States, 2 Chemistry, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractElectron tunneling across organic/inorganic interfaces is important to the performance of organic based electronic devices, such as organic light emitting diodes and organic field effect transistors. However, spin dependent tunneling across organic/inorganic interfaces for spintronic applications is not yet well understood. In the first portion of this talk, we will review recent results on injection, transport, and detection of a spin-polarized currents through molecules. Specifically we highlight the results for tetraphenyl porphyrin and Tris(8-hydroxyquinolinato)aluminium. We will review the challenges inherent in fabricating these organic/inorganic structures, and discuss characterization techniques. In particular, Meservey-Tedrow spin-polarized tunneling and inelastic electron tunneling spectroscopy (IETS) are powerful tools for elucidating spin-dependent transport properties, with added advantages when applied to organic-based systems. A model that explains the temperature, bias voltage and sign of the magnetoresistance will be presented. Collectively these results clearly demonstrate that spin polarized electrons tunnel through conjugated organic semiconductors with negligible spin-flipping, and suggest that such molecules may find use in spintronic applications.In the second portion of this talk, we will present recent utilizing halogen-substituted benzoic acid self-assembled monolayers as a “model” system. In these systems, we have observed evidence for spin-dependent scattering by organic radicals in a self-assembled monolayer. We show that in tunneling devices sandwiching a para-substituded benzoic acid derivative between Al/Al2O3 and transition metal electrodes, the conductance can be dramatically modulated by varying the para- substituent and the adjacent metal electrodes. Interface reactions at specific benzoic acid - transition metal interfaces lead to the formation of benzoic acid radicals, whose unpaired spins lead to spin-dependent scattering, as revealed through the Kondo effect. Our results indicate that the conductance through very simple metal-organic interfaces can display a wide range of transport properties. For example, 4-iodobenzoate in contact with Co exhibits the Kondo effect and strong spin-dependent scattering, while 4-fluorobenzoate in contact with Co exhibits only elastic and molecular-vibration-assisted tunneling. Relating specific molecular groups to spin-polarized transport features is of crucial importance, and our results show that the common presumption of negligible spin scattering is not generally well-founded. On the other hand, localization of unpaired spins on organic molecules is a key requirement for the development of molecular magnets. Finally, we believe our results show that transport through molecular devices can be exquisitely controlled through even single-atom changes, providing new constraints for the design of molecular electronic devices.
5:00 PM - E6.6
Investigation of Spin Injection and Transport in a Ferromagnet Fe / Organic Semiconductor CuPC Heterojunction.
Nyun Jong Lee 1 , Jeahyoung Lee 2 , Hyunduck Cho 3 , Changhee Lee 3 , Tae Hee Kim 1
1 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 Memory Division, Samsung Electronics Co., LTD, Yongin Korea (the Republic of), 3 School of Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of)
Show AbstractA fundamental requirement for implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Carrier injection, carrier separation at metal/organic (M/O) interfaces and carrier transport in the organic films are key processes in the OSC devices. The understanding of the electrical properties at M/O interfaces is of great importance to improve device performance as well as to design novel organic devices. Here, we consider a heterostructure system consisting of a highly-qualified interface between the ferromagnet Fe and the OSC Cu-phthalocyanine(CuPC). We focus on interfacial effects on spin injection and on the spin transport properties of CuPC by using the electroluminescent (EL) and tunneling devices with different electrodes, such as polycrystal Fe, epitaxial Fe(001) and permalloy (Py). EL devices with CuPC films were prepared beyond the polycrystal Fe and epitaxial Fe(001) films by the vacuum–deposition at a base pressure of 10
-8 Torr. For the EL devices using both polycrystal Fe and epitaxial Fe(001) electrodes, we observed a magneto-optical effect of about 4% at room temperature. However, for the tunneling devices with an epitaxial Fe(100) electrode (Fe(001)/CuPC/Py), the larger room temperature tunneling magnetoresistance was observed than that of polycrystal Fe/CuPc/Py junction. This work is supported by the Korea Science & Engineering Foundation through the Quantum Meta-Materials Research Center and the Korea Research Foundation Grant (KRF-2006-531-C00026). Corresponding author:
[email protected]5:15 PM - E6.7
Investigation of Spin-dependent Transport Properties and Spin-spin Interactions in a CuPc-Co Nano-composite System.
Zhenyao Tang 1 , Shinyichi Tanabe 1 , Daiki Hatanaka 1 , Takayuki Nozaki 1 , Teruya Shinjo 1 , Yoshishige Suzuki 1 , Masashi Shiraishi 1 2
1 , Graduate School of Engineering Science, Osaka Univ., Osaka, Japan Japan, 2 , JST-PRESTO, Kawaguchi Japan
Show AbstractRecently, in the field of molecule spintronics, single spin manipulation by spin-flip interactions in molecule materials attracts much attention[1]. In this research, considering Copper-phthalocyanine (CuPc) is a popular material in which one isolated spin exists in the copper site and the spin state is near the Fermi level, and is suitable for studies on spin manipulation by external fields and conducting spins, so we have fabricated a series of CuPc–Co nano-composites (the compositional ratio was 4:1=CuPc:Co), in which Co nano-particles were embedded in CuPc molecules, in order to study the spin transport properties and to detect spin-spin interactions between conducting spins and isolated spins in the CuPc. Nonlinear characteristics of an electric current featured by I~V(2N-1), (V : a bias voltage, N : an order of co-tunneling) have been found, where the 5th order co-tunneling was clarified. The order of the co-tunneling effect in CuPc-Co was almost equal to that in a rubrene-Co nano-composite. Because the mean diameter of the Co nano-particles (1.6-2.6 nm) was almost same as that in rubrene-Co [2], the order of the co-tunneling should be decreased if the spin-flip scattering took place. Thus, it is thought that spin-flip scattering which breaks co-tunneling did not occur [3]. This may be attributed to the spin-coupling between the isolated spins in the CuPc and spins in the Co, which was supported by electron spin resonance (ESR) measurements from 4.2 to 297 K, because spin signals from pure CuPc and pure Co disappeared in the nano-composites and only new broadened peaks were found. [1]M. M. Matsushita, H. Kawakami, and T. Sugawara, PRB 77, 195208 (2008)[2] H. Kusai et al., CPL 448 (2007) 106–110[3]Z. Tang et al., in preparation for publication.
E7: Poster Session
Session Chairs
Thursday AM, December 03, 2009
Exhibit Hall D (Hynes)
9:00 PM - E7.1
A Theoretical Study and Realization of New Spin Quantum Cross Structure Devices using Organic Materials.
Kenji Kondo 1 , Hideo Kaiju 1 , Akira Ishibashi 1
1 , Research Institute for Electronic Science, Hokkaido University, Sapporo Japan
Show AbstractRecently, we have proposed a quantum cross structure (QCS) device as a candidate beyond CMOS. The QCS device consists of two metal thin films with their edges crossed. The metal thin films are made by the evaporation method on polyethylene naphthalate (PEN) substrates. We can make QCS devices by using the edges of evaporated metal thin films as both electrodes. Therefore, the area of the cross point of the QCS device can ultimately be reduced to dimensions in the order of a few square nanometers since the area size is determined by the metal-deposition rate, which can be changed from 0.01 nm/s to the order of 0.1 nm/s. In this work, we propose a “Spin Quantum Cross Structure (SQCS)” device, which is a QCS device consisting of two magnetic thin films, or two electrodes with spin injection mechanisms. The SQCS device has potential applications in both switching devices and high-density memory devices by sandwiching a few molecules and atoms. First, we have calculated the current-voltage (I-V) characteristics of SQCS devices within the framework of the Anderson model at room temperature. We assume that the calculated SQCS devices sandwich a single molecule having the two energy levels, and that Ni magnetic thin films are used for both electrodes. In the weak coupling region, where the energy of coupling constant is smaller than that of ambient temperature, the calculated results show sharp steps at the positions of the energy levels of the sandwiched molecule. These results also indicate that we can obtain a high on/off ratio beyond 10000:1 and low energy consumption of less than 10 nW in SQCS devices under the weak coupling condition. Under the strong coupling limit, the calculation results show that the I-V characteristics are Ohmic. This is because the energy levels of the sandwiched molecule are almost continuous because of the wide broadening induced by the strong coupling with the itinerant d electrons of the Ni electrodes. Finally, we have realized an SQCS device with Ni magnetic thin-film electrodes, including poly (3-hexylthiophene) (P3HT)/ 6, 6-phenyl C61-butyric acid methyl ester (PCBM) organic molecules between both electrodes, and measured the I-V characteristics by a four-terminal method. The thickness of Ni thin films is 16 nm, and the area of the cross point is 16 nm x 16 nm. The electronic transport of the fabricated SQCS device shows Ohmic-like I-V characteristics, and agrees well with the theoretical results, quantitatively. The junction resistance is estimated to be 32 Ω, experimentally. The corresponding theoretical result is 26 Ω. Also, we have calculated the magnetoresistance (MR) ratio as a function of renormalized coupling constants, VR and VL, dependent on the spin polarization of both electrodes including magnetostriction effects phenomenologically. The results indicate that the MR ratio can be controlled by changing the renormalized coupling constants, and that the ratio is represented by a new formula.
9:00 PM - E7.10
Electron Exchange in Low-energy Electron – Co-phthalocyanine interactions.
Alexander Kukhta 1 , Iryna Kukhta 1 , Sergey Kazakov 2 , Oleg Khristoforov 2 , Oscar Neyra 3 , Jose Paz 4 , Romano Dorta 4 , Neudo Urdaneta 4
1 , B.I.Stepanov Institute of Physics, Minsk Belarus, 2 , Chuvash State University, Cheboksary Russian Federation, 3 , Popular University of Cesar, Valledupar Colombia, 4 , Simon Bolivar University, Caracas Venezuela, Bolivarian Republic of
Show AbstractCo-phthalocyanine belongs to the class of organic semiconductors which is chemically and thermally stable. Having magnetic atom this molecule is perspective for applications in organic spintronics devices. One of the most direct ways to study electron exchange reactions determining the required properties is an electron energy loss spectroscopy.In this report the relative efficiency of excitation into singlet and triplet states in the region between 1 and 9 eV, their dependence on projectile electron energies for the scattering of monoenergetic electrons with tunable energies from 0 to 50 eV at an angle of 90°, for Co-phthalocyanine molecules in the gas phase are presented.A special electron spectrometer containing an electron gun and 127° electrostatic analyzer with channel electron multiplier were used for the energy loss spectra measurements. A monokinetic electron beam 1.5 mm in diameter is formed in 10-6 Torr vacuum; the electron beam current did not exceed 1 mA ; the full-energy resolution determined using elastic scattering peak was 0.3 eV. The organic vapor density in the collision cell was less than 10-3 Torr to avoid multiple collisions and secondary processes.In order to obtain a new information the calculation has been made in the framework of density functional theory (DFT) using the B3LYP functional and the 6–31+G(d) basis set as implemented in Gaussian 03 quantum chemical package. The gas phase molecular geometries were fully optimized separately in the neutral, anion and cation states. The geometrical and electronic properties such as IP, EA, band gap, electronic excitation spectrum, Mulliken atomic charges, frontier orbitals, and bond lengths have been calculated.In electron energy loss spectra of Co-phthalocyanine the intense low-energy peaks are clearly observed. They decrease strongly with incident electron energy, confirming singlet-triplet nature of these transitions. In the optical absorption and the calculated spectra there are no any peaks in this region. The rest of calculated spectrum is in rather good accordance with experimental data. The obtained results are compared with corresponding data for Cu-phthalocyanine. The reasons of strong singlet-triplet transition Co-phthalocyanine are analyzed.
9:00 PM - E7.11
Oxide Tunnel Junctions Supporting a Two-Dimensional Electron Gas: Resonant Tunneling, Spin-Polarization and Lattice Polarization Effects.
J. Burton 1 , J. Velev 2 , E. Tsymbal 1
1 Physics and Astronomy, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 2 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractThe discovery of a two-dimensional electron gas (2DEG) at the interface between insulating oxides has led to a well-deserved level of excitement due to possible applications as "in-plane" all-oxide nanoelectronics.[1] The the range of possibilities may however be expanded to the realm of "out-of-plane" nanoelectronics by examining such all-oxide heterostructures as barriers in magnetic tunnel junctions. Here we perform first-principles electronic structure and transport calculations of a tunnel junction with a [SrTiO3]4/[LaO]1/[SrTiO3]4 heterostructure tunneling barrier embedded between ferromagnetic SrRuO3 electrodes. The presence of the LaO atomic layer induces the formation of a 2DEG within the tunneling barrier which acts as an extended defect perpendicular to the transport direction, providing a route for resonant tunneling.[2] Our calculations demonstrate that the tunneling conductance in this system can be strongly enhanced compared to a pure SrTiO3 barrier due to resonant tunneling. The contribution from resonant tunneling is controlled by lattice polarization effects that determine the population of the 2DEG. In addition we find that this resonant tunneling is highly selective of the orbital symmetry of the tunneling states due to the "orbital polarization" of the 2DEG.The proposed resonant tunneling junctions (RTJs) may have new interesting properties and potential applications. For example, they can be used for vertical measurements to characterize properties of the 2DEG, as well as introducing the possibility of vertical connectivity to complement the horizontal in-plane architectures already proposed, such as all-oxide high-mobility field-effect transistors. In addition, the RTJs involving magnetic electrodes could have much lower resistance compared to the conventional magnetic tunnel junctions (MTJs) and therefore may be useful for magnetic recording applications. Moreover, recent experimental and theoretical data suggests that the 2DEG could be magnetic. In this case the 2DEG is spin polarized, RTJs could be used for spin filtering and could have applications in generating spin polarized currents.[1] A. Ohtomo, and H. Y. Hwang, Nature 427, 423 (2004).[2] J. D. Burton, J. P. Velev and E. Y. Tsymbal. arXiv:cond-mat/0906.3543
9:00 PM - E7.12
Spin-dependent Transport in C:Co and C:Ni Nanocomposite Thin Films.
Shengqiang Zhou 1 , Markus Berndt 1 , Danilo Buerger 1 , Gintautas Abrasonis 1 , Gyorgy Radnoczi 2 , Heidemarie Schmidt 1 , Andreas Kolitsch 1 , Manfred Helm 1
1 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden Germany, 2 Research Institute for Technical Physics and Materials Sciences, Hungarian Academy of Sciences, Budapest Hungary
Show AbstractNanocomposites exhibiting spin-dependent transport properties show a potential for applications in spintronics. Carbon:transition metal nanocomposites are of particular interest due to their multifunctionality and easiness to control their morphology, and thus properties. In this contribution, we present the study of magneto-transport properties of C:Co and C:Ni nanocomposites. The films were grown by ion beam co-sputtering on thermally oxidized silicon substrates in the temperature range of 200-500°C. Two major effects have been found for C:Co films: (i) a large anomalous Hall effect amounting to 2 μ-Ohm-cm; (ii) a negative magnetoresistance. Both the field-dependent resistivity and Hall resistivity curves well correspond to the magnetization curve, which suggests a spin-dependent transport in the nanocomposite thin films. A similar effect is also reported for C:Ni nanocomposite films. A correlation is reported between the magneto-transport properties and the film microstructure.
9:00 PM - E7.13
Room Temperature Ferromagnetism in GaMnN Dilute Magnetic Semiconductor Devices.
Neeraj Nepal 1 , M. Oliver Luen 1 , John Zavada 1 , Salah Bedair 1 , Pavel Frajtag 2 , Nadia El-Masry 2
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractSpin injection into the semiconductor from ferromagnetic metal suffers with interfacial scattering and has low injection efficiency due to difference in conductivity between ferromagnetic metal and semiconductor. One of the possible solutions is to use dilute magnetic semiconductor (DMS), which exploits both charge and spin of electrons. GaMnN was found to exhibit a range of Tc above room temperature and can be used for room temperature spintronic devices. We report on the room temperature (RT) ferromagnetism (FM) in GaMnN DMS films and devices. A GaMnN layer was grown on top of a n-GaN substrate and found to be almost always paramagnetic. However, when grown on a p-type GaN layer, a strong saturation magnetization (Ms) was observed demonstrating that the FM observed in GaMnN film is carrier-related. We have demonstrated the dependence of the FM on the thickness of the p-GaN in the GaMnN/p-GaN/n-GaN multilayer structure and on the applied bias to the GaN p-n junction. Photolithography and inductively coupled plasma dry etching were used to realize RT spin electronic devices from these multilayer structures. The Ms was measured by an alternating gradient magnetometer and a strong correlation between the hole concentration near the GaMnN/p-GaN interface and the magnetic properties of the DMS devices was observed. The variation of RT FM with the thickness of the top GaMnN layer in the GaMnN multilayer structures will be discussed.
9:00 PM - E7.15
A Density Functional Theory Study on Aluminum Oxide Formation at Al/LSMO Interface for Resistive Switching Application.
Nodo Lee 1 , Yun Hee Jang 1
1 Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractResistance random access memory (ReRAM) is emerging as a next-generation non-volatile memory device. One of the most promising materials for such a device is the composite of a reactive metal such as aluminum (Al) and a mixed-valance manganite such as La1-xSrxMnO3 (x ~ 0.3; LSMO). The current hypothesis on the origin of resistance switching is a voltage-dependent reversible formation of high-resistance aluminum oxide (AlOx) layer at the Al/LSMO interface through oxygen migration from LSMO. We perform density functional theory quantum mechanics calculations to validate this hypothesis on Al/LSMO and AlOx/LSMO interfaces as well as their component materials such as Al, AlOx, (x = 0.5, 1, 1,5), LMO, and LSMO. We investigate the change in structure, electronic structure, energy, and electrical properties during the processes of oxygen vacancy formation in LSMO, oxygen migration through the Al/LSMO interface, and aluminum oxide formation. Based on these results, we suggest a model for resistance-switching phenomena in Al/LSMO interface due to oxygen migration.
9:00 PM - E7.16
Transport Properties of Pr0.7Ca0.3MnO3/Nb:SrTiO3 Heterojunctions.
Zhi Luo 1 , Chi Wah Dennis Leung 1 , Paddy K. L. Chan 2
1 Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong China, 2 Mechanical Engineering , The Hong Kong Polytechnic University, Hong Kong China
Show AbstractResistance switching behavior in manganite heterojunctions has attracted much attention for potential memory applications, but the transport and switching mechanism in such devices are still under debate. Here, results on the study of transport mechanism of Pr0.7Ca0.3MnO3 (PCMO)/ 0.1 wt% Nb-doped SrTiO3 (NSTO) heterojunctions are presented. The junctions were fabricated by depositing epitaxial PCMO thin films on (100) NSTO single crystal substrates by pulsed laser deposition. I-V measurements of the junctions down to 20 K showed clear rectifying behaviour. A forward-to-reverse ratio of 1.8×103 under 0.5 V bias was observed at 280 K. For identical current bias, the IV curves of the junctions shifted to higher voltage values with decreasing temperature, which deviated from the diffusion model commonly employed for p-n junctions. When plotted in log-linear scale, the curves at various temperatures were nearly parallel, suggesting direct or thermally assisted tunneling were dominant transport mechanisms in such junctions. Besides, Hall measurement showed that carrier density of PCMO decreased as the temperature was lowered, while no noticeable changes were observed in that of NSTO. These results therefore suggested that the carrier density of PCMO has a great influence on the transport of the heterojunctions.
9:00 PM - E7.18
Magnetic and Transport Properties of MnAs/GaAs Nanocomposite Films.
Radhika Barua 1 , Ben Chaprut 1 , Anthony Bresnahan 2 , Laura Lewis 1 , Don Heiman 2
1 Chemical Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractThe magnetic and transport properties of MnAs/GaAs nanocomposite films were investigated to understand the role of the large magnetic interphase surface area on the electronic properties. Nanoparticles of MnAs were synthesized by annealing thin (20 nm) films of GaMnAs made by molecular beam epitaxy, with Mn/Ga=0.1. Annealing in an arsenic atmosphere at 600-700 C resulted in the creation of thin disc-shaped MnAs particles with diameters ~20-200 nm embedded in GaAs. Magnetization measurements confirmed that samples with small-diameter MnAs precipitates are superparamagnetic, while those with larger particles remain ferromagnetic (FM) at room temperature. The magnetic domain switching behavior and anisotropy of FM nanoparticles were investigated with variable-temperature magnetic force microscopy (MFM). MFM images captured at room temperature demonstrate that the majority of the larger MnAs nanoparticles possess single-domain character with in-plane anisotropy that can be oriented along a specified direction with ex-situ application of a magnetic field H~0.1 T. This magnetic orientation was randomized by raising the temperature above the MnAs Curie point, Tc=337 K. Conductivity measurements demonstrate an increase in resistivity with decreasing temperature due to carrier freezeout with a local maximum at T=80 K. Interestingly, the resistivity also increases for increasing temperature near room temperature with a local minimum at T=240 K. A positive magnetoresistance was found at all temperatures, in contrast to the negative MR found in GaMnAs. The Hall voltage exhibited a linear field dependence that is dominated by the ordinary Hall effect and provides a room-temperature hole carrier concentration of p=(1-10)E18 cm-3. The resistivity was found to be anisotropic due to the alignment of the MnAs nanoparticles along principle crystallographic directions of the GaAs matrix/substrate, as confirmed by scanning electron microscope images. These features indicate that the electrical properties of nanocomposite films are strongly influenced by the embedded MnAs nanoparticles.
9:00 PM - E7.19
Prediction of a Spin-Polarized Two-Dimensional Electron Gas at the LaAlO3/EuO(001) Interface.
Yong Wang 1 , Manish Niranjan 1 , John Burton 1 , Joonhee An 1 , Kirill Belashchenko 1 , Evgeny Tsymbal 1
1 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractThe discovery of a two-dimensional electron gas (2DEG) at the interface between insulating oxides, i.e. LaAlO3/SrTiO3 [1], is promising in designing novel electronic devices, such as all oxide field effect transistors [2,3]. Recently it was found that at ultra-low temperatures the 2DEG occurring at the interface between the non-magnetic LaAlO3 and SrTiO3 materials may become magnetic [4]. Making 2DEG spin-polarized is a very exciting prospect for spintronics applications, where the involvement of the spin degree of freedom broadens the spectrum of potential applications [5]. Here we propose a method to achieve a spin-polarized 2DEG by employing a ferromagnetic insulator as one of the constituents in the oxide heterostructure [6]. Spin-polarized properties of the 2DEG are, in this case, expected to be inherited from the ferromagnetism of the oxide, and consequently this approach may lead to a more robust magnetism in the 2DEG, which is beneficial for applications. To illustrate the idea we consider EuO as a representative ferromagnetic insulator in conjunction with LaAlO3 to form a spin-polarized 2DEG at the LaAlO3/EuO(001) interface. This polar interface favors electron doping into the Eu-5d conduction bands. Due to the exchange splitting of the Eu-5d states in the ferromagnetic insulator the 2DEG is spin-polarized below the Curie temperature of EuO. This behavior is consistent with a recently discovered high spin polarization of conduction electrons in EuO doped with La [7]. The formation of a spin-polarized 2DEG at the interface between polar and ferromagnetic insulators may provide a robust magnetism of the 2DEG which is interesting for spintronics applications. 1. A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).2. S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, and J. Mannhart, Science 313, 1942 (2006).3. C. Cen, S. Thiel, J. Mannhart, and J. Levy, Science 323, 1026 (2009).4. A. Brinkman, M. Huijben, M. van Zalk, J. Huijben, U. Zeitler, J. C. Maan, W. G. van der Wiel, G. Rijnders, D. H. A. Blank, and H. Hilgenkamp, Nature Mater. 6, 493 (2007).5. I. Zutic, J. Fabian, and S. D. Sarma, Rev. Mod. Phys. 76, 323 (2004).6. Y. Wang, M. K. Niranjan, J. D. Burton, J. M. An, K. D. Belashchenko, and E. Y. Tsymbal, Phys. Rev B, 79, 212408 (2009).7. A. Schmehl, V. Vaithyanathan, A. Herrnberger, S. Thiel, C. Richter, M. Liberati, T. Heeg, M. Rockerath, L. F. Kourkoutis, S. Muhlbauer, P. Boni, D. A. Muller, Y. Barash, J. Schubert, Y. Idzerda, J. Mannhart, and D. G. Schlom, Nature Mater 6, 882 (2007).
9:00 PM - E7.2
Theoretical Studies for the Duality of Quantum Nano-devices.
Morishige Yoneda 1 , Masaaki Niwa 2 , Mitsuya Motohashi 2
1 , Denshi Gakuen Japan Electronics College, Tokyo Japan, 2 Engineering, Tokyo Denki University, Tokyo Japan
Show AbstractRecent advances in nano-fabrication technology enables us to confine electric charge or spin in nano-structures such quantum wires or quantum dots. In such systems, it can not be ignore the quantum effects.The duality of spontaneous magnetization and domain wall or the electric charge and magnetic flux be become pronounced.Our objective is to build the theory of quantum nano devices where the duality of degrees of freedom of a quantum particle play an important role,and it to propose a new principle device.Our main results are listed below. 1.We studied the theoretical aspects of the duality of Josephson effects in both 3D and 2D small systems. 2.An analogy with Josephson junctions,we devised a junction device consisting of a sandwich structure of the ferromagnetic-metal/ superconductor/ferromagnetic-metal. 3.Tunneling device using a quantum domain wall,we devised a junction device consisting of a sandwich structure of the ferromagnetic insulator/superconductor/ferromagnetic-insulator.4. The single-electtron device is a quantum effect device which operates by using the single electron tunnelig created by a coulomb blockade. In this analogy, we proposed a single flux quantum devices and a single quantum domain wall device,which operates by using a single quantum flux tunnelig and a single quantum domain wall tunnelig,respectively.In this talk, We are going to discuss the physical mechanism behind these quantum nano devices, and will show the theoretical conditions to control these devices.
9:00 PM - E7.20
Single Crystalline CrO2 Thin Films by a Low-Pressure CVD Method.
Xing Zhong 1 , Jinwen Wang 1 3 , Manjit Pathak 1 2 , Tonya Klein 1 3 , Patrick LeClair 1 2 , Arunava Gupta 1 3 4
1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States, 3 Department of Chemical and Biological Engineering, University of Alabama, Tuscaloosa, Alabama, United States, 2 Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama, United States, 4 Department of Chemistry, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractThe CVD precursor that has been most extensively utilized thus far for CrO2 growth is CrO3, which is a solid at room temperature and must be heated to relatively high temperatures (>250○C) for sublimation and decomposition. The use of a solid precursor, while highly successful for CrO2 film growth, is unfortunately problematic for obtaining high quality interfaces in heterostructures, due to the need to change precursors. This hinders the possibility of creating heterostructures with high-quality interfaces, necessary for TMR and GMR structures, and prevents in situ characterization of film growth (e.g., RHEED). Another approach to high-quality CrO2 synthesis is required to realize high-quality heterostructures.
In this work, we have explored the fabrication of epitaxial CrO2 films on (100) TiO2 substrates by a low-pressure CVD process (LPCVD). Briefly, CrO2 films were deposited onto heated HF-etched TiO2 (100) substrates by thermal decomposition of chromium carbonyl (Cr(CO)6) using home-built LPCVD system. The precursor temperature is regulated to control its vapor pressure, and held at 3○C during deposition to achieve optimum vapor pressure. Oxygen is used as carrier gas, and the flow rates are maintained at 100 sccm (directly into the chamber) and 15 sccm (through precursor container) respectively. X-ray diffraction shows the existence of (200)-oriented CrO2, with rocking curve full width at half maximum (FWHM) of 0.32○. Compared to the conventional solid precursor CVD method crystal quality of the film still need to be improved, as some Cr2O3 is still present in the film. The films exhibit typical ferromagnetic hysteresis loops, and have a surface mean roughness of 2-3nm. We have found the quality of the resulting films to be highly sensitive to precise control of the substrate temperature, precursor temperature, oxygen and precursor flow rates, and chamber operating pressure. However, using this route, we are able to deposit reasonably phase-pure CrO2 films.
9:00 PM - E7.21
Synthesis of Transition Metals Silicide Nanowires and Electrical Properties for Silicon-based Spintronics Applications.
Luis Valentin 1 , Luis Fonseca 1 , Oscar Resto 1
1 Institute for Functional Nanomaterials and Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractCrSi2, CoSi, and CrFeMnSi alloy nanowires were synthesized by the Chemical Vapor Transport Method. These silicide nanostructures are promising materials due to their electrical and thermoelectrical properties. Their stability at high temperatures and the natural electrical contact they form with silicon wafers make them promising nanostructures for thermal sensing and for silicon based spintronics. In this work we explain the synthesis of the nanostructures and the characterization made by SEM-EDS, TEM-EDS, XRD and Electron Diffraction. We studied their thermal stability up to 1100C and the magnetic properties of CrFeMnSi alloy wires. We also report on the electrical response of silicon grounded nanowires. In particular, I-V curves of individual wires were determined with an In Situ STM-TEM holder. These measurements were also correlated with the field emission response of the nanowires.
9:00 PM - E7.22
Room Temperature Ferromagnetism in Fe0.1Sc0.9N(001)/ScN(001)/MgO(001) Films Grown by Molecular Beam Epitaxy.
Costel Constantin 1 , Kangkang Wang 2 , Abhijit Chinchore 2 , Arthur Smith 2 , Han-Jong Chia 3 , John Markert 3 , Kai Sun 4
1 Physics, Seton Hall University, South Orange, New Jersey, United States, 2 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 3 Physics and Astronomy, University of Texas at Austin, Austin, Texas, United States, 4 Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractConsiderable effort is being allocated in finding a room temperature dilute magnetic semiconductor. Recent theoretical calculations by Herwadkar1 et al. predicted ferromagnetism above room temperature in MnxSc(1-x)N if the electron concentration is kept below 1020 cm-3. In this study, Fe0.1Sc0.9N films were grown on ScN(001)/MgO(001) substrates by radio frequency plasma assisted molecular beam epitaxy. The buffer layer of ScN(001) was grown on top of MgO(001) at Ts ~ 800 oC and with a thickness of ~50 nm. The Fe0.1Sc0.9N films were grown under N-poor conditions with a total metal flux of (JMn + JFe) ~ 1.67, and with a thickness of tFeScN ~ 290 nm. The work-in-progress saturation magnetization vs. temperature data measured with the superconducting quantum interference device show magnetic behavior with a Curie temperature TC ~ 350 - 375 K. The measured electron carrier concentration was nS ~ 3.63E+19 /cm3. This collaborative work is supported by: Seton Hall: University Research Council. Ohio University: DOE-BES Grant No. DE-FG02-06ER46317 and NSF Grant No. 0730257. UT Austin: NSF Grant Nos. DMR-0605828 and DGE-0549417, Welch Foundation Grant No. F-1191. 1 A. Herwadkar, W. R. L. Lambrecht, M. van Schilfgaarde, Phys. Rev. B 77, 134433 (2008).
9:00 PM - E7.23
Non-Kekulé Nitrogen-Substituted m-Phenylenes: Electronic Ground State Diradicals.
Shadi Amiri 1 , Peter Schreiner 1
1 Organische Chemie Institut, Justus-Liebig-Universitaet Giessen, Giessen, Hessen, Germany
Show AbstractParamagnetic single molecules represent attractive building blocks for the development of ferro- and ferrimagnetic materials. Moving towards the goal of preparing stable, neutral open-shell systems at first we investigated the electronic structures of a novel series of substituted 3,5,7,9-hexaazaanthracene zwitterions, which have non-Kekulé hexaazaacene as their basic structures.[1] Based on the results for tricyclic azaacenes, in the next step we identified octahydropyrido-quinoline (8π-electron) as a non-Kekulé-type N-substituted m-phenylene derivative with an exceptionally large singlet–triplet energy separation of +27.9 kcal mol–1, in favor of the triplet.[2] The high-spin structures can readily be obtained by interruption of the full cyclic π-delocalization that avoids cross-conjugation of the nitrogen radical centers. Introduction of additional heteroatoms on the other hand preferentially stabilizes the singlet zwitterionic resonance contributors in these systems. The identified diradicals show strong ferromagnetic exchange interactions between the two radical centers. The relationship between the structures and electronic ground state properties of tricyclic N-substituted m-phenylenes was studied in detail utilizing density functional theory (DFT) at B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p).[1] Langer, P.; Amiri, S.; Bodtke, A.; Saleh, N. N. R.; Weisz, K.; Gorls, H.; Schreiner, P. R. J. Org. Chem. 2008, 73, 5048–5063.[2] Amiri, S.; Schreiner, P. R. J. Phys. Chem. A 2009, 113, in press.
9:00 PM - E7.24
Influence of the Buffer Layers on Magnetic Properties of FePtB Films MBE-deposited.
Jae Young Ahn 1 , Nyun Jong Lee 1 , Jea Hyoung Lee 2 , Anny Michel 3 , Tae Hee Kim 1
1 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 Memory Division, Samsung Electronics Co., LTD, Yongin Korea (the Republic of), 3 Laboratoire de Physique des Materiaux, Universite de Poitiers, Futuroscope-Chasseneuil France
Show AbstractFePt is a leading candidate for future extremely high density magnetic recording media due to its very high anisotropy (Ku) value. However, this Ku gives films with very high coercivity, too high for reliable recording using conventional write heads. Hard/soft magnetic composites could be an excellent solution to tailor the magnetic properties according to specific requirements. Here, the thickness-dependent magnetic response of Fe-Pt-B films prepared by mixing amorphous B and fct FePt phases is presented. The [B(0.03nm)/Fe(0.17nm)/Pt(0.2nm)]N were deposited on an epitaxial Pt/MgO(001) buffer grown on Si(100) substrate by UHV-molecular beam epitaxy (MBE) deposition. The perpendicular magnetic anisotropy (PMA) was found in the films having at least 8 (N=8) repetitions of B/Fe/Pt, after subsequent annealing. The magnetic and structural characterization of the films was performed by VSM and TEM, respectively. One possible explanation for the origin of the PMA could be related to the spin reorientation transition (SRT) from in-plane single-domain-like state to out-of plane multidomain state, with increasing film thickness and temperature. Our results showed that the atomic randomness and strains are responsible for the SRT in magnetic materials. In addition, the tunneling magnetoresistance in the MgO-based magnetic tunnel junctions with perpendicular magnetic L10-FePt electrodes was investigated in this work. We explored the magnetic coupling between perpendicularly magnetized FePt and FePtB electrodes in a fully epitaxial FePt/MgO/FePtB junction. This research was supported by a grant from the Fundamental RD Program for Core Technology of Materials funded by the Ministry of Commerce, Republic of Korea.
9:00 PM - E7.25
Enhancement of Spin Polarization in (Fe,Zn) 3O4 Ferromagnetic Oxide Nano Dot Diodes.
Satoru Yamanaka 1 , Tomoji Kawai 1 , Hidekazu Tanaka 1
1 ISIR-Sanken, Osaka University, Osaka Japan
Show AbstractThe ferromagnetic Schottky epitaxial nano-dot diodes composed of High TC semiconductive ferromagnetic oxide of (Fe,Zn)3O4 were fabricated using Nanoimprint lithography (NIL) based on Mo lift-off technique in combination with pulsed laser deposition (PLD), which allows us to construct highly integrated epitaxial transition metal oxide nano-hetero structures [1]. We constructed Fe2.1Zn0.9O4 (FZO)/ Nb (0.2 wt %)-SrTiO3 (STO) ferromagnetic nano dot diode and measured current-voltage (I-V) characteristics and magnetoresistance (MR). The FZO/ Nb-STO nano dot diodes embedded by SiO2 thin films were constructed whose dot sizes were 1×1μm2, 500×500nm2, and 300×300nm2, respectively. The I-V characteristics were obtained in the temperature range from 50 to 300K, and were well dominated by thermionic emission under 200K. The MR is enhanced by decreasing dot size down to 300×300nm2. Moreover, the spin polarization (P) of FZO which was estimated by the thermionic emission theory in ferromagnetic metal-semiconductor contacts proposed by Ziese et al[2], enhanced up to P = 0.89±0.05 in the nano-dot structure with the size of 300×300nm2 at 50KReferences [1] N. Suzuki, H. Tanaka, S. Yamanaka, M. Kanai, B. K. Lee, H. Y. Lee, and T. Kawai, Small 4 (2008) 1661, [2] Ziese et al, Phys. Rev. B 71 (2005) 180406.
9:00 PM - E7.26
Fe3+, Co2+-Doped Anatase TiO2: Diluted Magnetic Semiconductor Nanoparticles by Sol-Gel Method.
Tushar Jagadale 1 2 , D. Pravarthana 2 , M. Ikram 2 , Amit Chawla 3 , Ramesh Chandra 3 , B. Lefez 4 , B. Hannoyer 4 , Satishchandra Ogale 2
1 Dept. of Physics,, University of Pune, Pune , Maharashtra, India, 2 Physical and Materials Chemistry Division, National Chemical Laboratory, Pune, Maharashtra, India, 3 Institute Instrumentation Centre, Indian Institute of Technology, Roorkee India, 4 , University of Rouen, Rouen France
Show AbstractDiluted Magnetic Semiconductors (DMS) are the most intensely investigated materials in recent times because of their potential for applications in the field of spintronics. Amongst them oxide-based DMS are of particular interest because of their wide band gap leading to the exciting possibility of transparent spintronics. Unfortunately the work done thus far is controversial due to the issues of dopant incorporation and homogeneity. Also, there are contradicting reports regarding the magnetic behavior of such materials. To use DMS materials for the practical spintronics devices, the stringent requirement is that the ferromagnetic ordering should persist much above room temperature. Most work on DMS has been done on thin films. Relatively less amount of work is done on the chemical synthesis of DMS nanoparticles. In the present work we have employed sol-gel route to synthesize (<5%) Fe, Co-doped TiO2 nanoparticles (NPs) using titanium tetra-isopropoxide, ferric nitrate and cobalt nitrate as precursors. For first case, titanium tetra iso-propoxide in ethanol and Fe(NO3)3 were added together in stoichiometric amount. After controlled hydrolysis, the gel was formed. The dried xerogel on subsequent calcination at 300oC yielded Fe-doped TiO2 NPs. Same protocol was followed for the Co-doped TiO2 NPs. Some cases of higher concentrations of magnetic element were also examined to explore the possibility of formation of composite nanoparticles above the solubility limit. Characterizations were done using XRD, FE-SEM, SQUID Magnetometry, XPS, Mössbauer spectroscopy, FTIR, ATR, Raman, PL and UV-VIS-NIR spectroscopy etc. The nanoparticles were also examined for photocatalysis. These data will be analyzed, presented and discussed.Funding Support: CSIR, Govt. of India, CEFIPRA grant no. 3808-2
9:00 PM - E7.27
Electron Transport and Magnetic Ordering in High Quality Epitaxial Films of Co2MnSi and Co2MnSi-NbN Bilayers Prepared by Pulsed Laser Ablation.
Ramesh Budhani 1 , Gyanendra Singh 1 , P. Joshi 1
1 Physics, Indian Institute of Technology, Kanpur India
Show AbstractSuccessful implementation of magnetic tunnel junctions in spintronics related applications at ambient temperatures requires a ferromagnetic metal with high degree of spin polarization combined with a high Curie temperature (Tc) in addition to a large on – off ratio in small magnetic fields. The full Heusler alloys of the type Co2MnSi and Co2FeSi are half metallic systems with a Tc in excess of 1000 K, and hence very promising for magnetic tunnel junction applications. However, for realization of high spin polarization it is essential to have epitaxially grown films stabilized in the ordered phase (L21 type structure). This is quite non-trivial because of the multi-elemental composition of these alloys combined with their propensity for oxidation. Here we describe successful synthesis of epitaxial films of Co2MnSi on (100) MgO by pulsed laser deposition (PLD) from a single alloy target. While as-deposited films at 200 0C are disordered, a highly ordered epitaxial growth is realized on annealing the films at 600 0C in hydrogen environment. Although a similar crystallographic structure is also obtained if deposition is carried out at 600 0C, the surface smoothness of the films grown at 200 0C followed by insitu annealing is much superior. We will present results of a detailed study of surface topography, surface roughness and magnetic domain structure of these films studied with scanning probe microscopy. The PLD grown Co2MnSi films have been successfully integrated with superconducting films of NbN. Some preliminary results of transport measurements in such superconductor - highly spin polarized ferromagnet heterostructures will be presented as well.This research has been supported by grants from the Department of Information Technology and the Board for Research in Nuclear Sciences, Government of India.
9:00 PM - E7.3
Fabrication of Molecular Spin Valve with Spacer of Layer-by-Layer Langmuir-Blodgett Film.
Jung-Chi Tai 1 , Kai-Shin Li 1 , Jang-Chang Huang 1 , Sheng-Syun Wong 1 , Cheng-Yung Hung 1 , Santhanam Agilan 1 , Yin-Ming Chang 1 , Minn-Tsong Lin 1 2
1 Department of Physics, National Taiwan University , Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractOrganic spintronic is a recent emerging field to combine the research of organic electronics and spintronics, and molecular spintronics involves the method to form ordered molecular monolayer or single molecule. Using layer-by-layer molecular method to fabricate molecular spin valve is still a challenge in the research of molecular spintronics. Molecular Magnetic Tunnel Junction (MMTJ) with the spacer of Langmuir-Blodgett (LB) film has been fabricated. LB film is an ordered molecular thin film checked by contact angle between water drop and LB film, and could be constructed as layer-by-layer molecular ultrathin film by repeating transfer processes. Hard and soft Ferromagnetic (FM) layer of spin valve are fabricated in vacuum using sputtering. In the sample of MMTJ with 1 Monolayer (ML) LB film, the Tunnel Magnetoresistance (TMR) could be observed at 12 K, and that consists with the spin configuration according to two step hysteresis loop of MMTJ. Room temperature TMR effect is observed in MMTJ with 2 ML LB film, and also consists with magnetic configuration. Measurement of TMR effect of 2 ML MMTJ at different temperature shows that the shape of state could be influenced by thermal energy, but the TMR ratio is independent of temperature, which needs further investigation to exclude current distribution effect. The current-voltage (I-V) curve of MMTJ with LB spacer indicates the tunnel mechanism in the junction, and the resistance is decreasing with the increase of temperature. This experiment shows that spin could keeps memory in layer-by-layer ordered molecules according to TMR effect of MMTJ, which is potential to be non-volatile flexible electronic device.
9:00 PM - E7.4
Asymmetric Interface between Cobalt and Pentacene on Top and Bottom Electrode Contact for Organic Spintronics.
Yao-Jane Hsu 1 , Ya-Jyuan Hung 1 2 , Ying-Chang Lin 1 , Yu-Ling Lai 1 , Hsu-Ting Chang 1 2 , Yuet-Loy Chan 1 , Jia-Hao Wang 1 2 , Chih-Hao Lee 2 , Der-Hsin Wei 1
1 , National Synchrotron Radiation Research Center, Hsinchu Taiwan, 2 Engineering and System Science, National Tsing Hua University, Hsinchu Taiwan
Show AbstractOrganic spintronics has received extensive attention due to its ability to integrate the advantages of two emerging fields of organic semiconductor electronics and spin electronics. Organic semiconductors (OSE) have been successfully adopted as an active layer in organic electronics owing to their economic cost, simplicity and bendability. Spins as carrier transport for low thermal consumption and high efficiency represent a future trend in electronics. Combining these two emerging fields thus stimulates the advancement of new generation electronics for organic spin valves. The interface between metal electrodes and organic semiconductors determinates the performance of organic electronics-based devices. It is therefore of prime importance to explore the interface coupling and surface properties between organic/magnetic heterostructures. We utilized synchrotron photoemission (PES), near-edge X-ray absorption (NEXAFS) spectroscopy and magneto-optical Kerr (MOKE) measurement to study the interfacial chemical, electronic structures and magnetic coupling of cobalt(Co)/pentacene(Pc) and Pc/Co in a trilayer vertical device structures. The strong polarization dependence of NEXAFS spectra as a function of pentacene thickness illustrates pentacene has a well order packing on cobalt surface. The preferential packing orientation is inclined toward surface to stand-up on bottom cobalt electrode as pentacene coverage is increasing. Through core and valence-level PES, the bottom interface presents a dipole layer surface. Unlike the bottom interface, we found an asymmetric top interface as cobalt deposited on pentacene. The interfacial diffusion from top cobalt into pentacene was observed. The PES data demonstrates the diffused cobalt reacts with π-bonding of pentacene. With more cobalt deposit, pentacene is severely damaged and accordingly generates interfacial carbides. The well order packing of pentacene is thus destroyed due to the loss of π-π interaction between adjacent pentacene. Employing magneto-optical Kerr measurement, the different film roughness and chemical responses was attributed to the magnetic dispairity. 1. M.V. Tiba, W.J. M. de Jonge, B. Koopmans, and H. T. Jonkman, J.Appl.Phys. 100, 93707(2006)2. Z. H. Xiong, Di Wu, Z. Valy Vardeny & Jing Shi, NATURE, 427, 821(2004)3. M. Popinciuca,et al. J.Appl.Phys. 100, 093714(2006)4. Fan Zheng, et al. J.Chem.Phys. 126, 154702 (2007)
9:00 PM - E7.5
Towards Spin Injection from Magnetite into InGaAs/InAlAs Quantum Well.
Takeshi Ejiri 1 , Hidekazu Kumano 2 , Kenji Suzuki 1 , J. Bubesh Babu 1 , and Kanji Yoh 1
1 Research Center for Integrated Quamtum Electronics, Hokkaido University, Sapporo Japan, 2 Research Institute for Electronic Science, Hokkaido University, Sapporo Japan
Show AbstractThe improvement of spin injection efficiency is of primal importance to realize the feasible devices based on the concept, spintronics, where spin degree of freedom is utilized for information technology. Magnetite (Fe3O4), one of ferrimagnet, is a good candidate as spin injector into semiconductors due to its physical properties. First, it has half metallic characteristic in theory. Second, compared to other metals, the conductivity mismatch problem can be suppressed because its resistivity is in the same order of one of semiconductors. And lastly, Curie temperature is higher than room temperature (about 850K). However, there have been few reports about the properties of magnetite thin film and the injection experiment from magnetite into semiconductors. In this report, we present the optical measurement of spin injection efficiency of magnetite thin film.InGaAs/InAlAs spin LED structure was grown by MBE on p-GaAs substrate. The structure is, from the top, n+-Al0.1Ga0.9As (150A) / n-Al0.1Ga0.9As (550A) / Al0.1Ga0.9As (50A) / In0.2Ga0.8As quantum well (100A) / Al0.1Ga0.9As (1500A) / GaAs (500A). The samples were exposed in air and processed with standard photolithography technique to form the light-emitting part and the contact part on which magnetite will be deposited. The magnetite thin film (100A) was grown on the samples at 7.5x10-7 Torr. The spin polarized electrons are injected from the magnetite thin film into the In0.2Ga0.8As quantum well active layer and recombined with unpolarized holes emitting the polarized light according to the electron polarity. All measurement were performed above 120K because magnetite turns into insulator from metal at the temperature (Verwey transition) and the electron injection become very difficult. The external magnetic field was applied perpendicularly to the surface. We observed the intensity between the right and left circular polarized light above 1.0 Tesla. This is consisted to that the saturation magnetic field of the magnetite thin film is about 1.0 Tesla. However, the observed differential intensity was very week. The reason would be the formation of the oxide compound which in intermix spin components at the interface between the magnetite thin film and semiconductor surface. To improve the spin injection efficiency, we have developed in-situ growth in which magnetite thin film successively deposited without breaking vacuum after spin LED structure was grown and confirmed enhanced emission intensity.
9:00 PM - E7.6
Localized Electronic Manipulation of Graphene by Proton-irradiation.
Cheng-Hao Chuang 1 2 , Xingyu Gao 3 , Chia-Hao Chen 2 , Bo-Yao Wang 1 , Der-Hsin Wei 2 , Hung-Wei Shiu 2 , Yuet-Loy Chan 2 , Mark B H Breese 3 , Watt Frank 3 , Chen Shi 3 , T S Wee Andrew 3 , Minn-Tsong Lin 1 4
1 Department of Physics, National Taiwan University, Taipei Taiwan, 2 , National Synchrotron Radiation Research Center, Hsinchu Taiwan, 3 Department of Physics, National University of Singapore, Singapore Singapore, 4 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractEpitaxial graphene is fabricated on the SiC substrate by the vacuum graphitization which could lead to novel graphene-based electronics. It was reported that the graphitic material irradiated by proton beam shows ferromagnetic property around the impact region. Although the origin of ferromagnetic moment should be related to the vacancy generation and implanted proton in the lattice, the change of electronic structure is still unclear, in particular concerning the site-dependent modification with the non-uniform impact. In this work, a 2 MeV proton beam was irradiated onto graphene/SiC. On a space-resolved base, we can distinguish the structural and chemical property on the patterned graphene. Scanning PhotoElectron Microscopy (SPEM) equipped with X-ray photoemission spectroscopy (XPS) and PhotoEmission Electron Microscopy (PEEM) used by Near-Edge X-ray Absorption Fine Structure (NEXAFS) are combined to offer the correlation between chemical and molecular orientation of carbon atoms located at different sites. The irradiated area gives rise to the plateau evolving with the natural boundary between the irradiated and pristine graphene. Since the graphene-like sp2–bonded carbon over the whole irradiated area was partially removed, the lattice vacancy and chemical environment changes in different carbon species could be also observed. In particular, a newly sp3-hybridized component was found exclusively on the boundary, which might be resulted from the non-uniformity of proton ion after the injection. The remaining graphene-like carbon layer represents disordered orientation on the impact center, but on the boundary area its orientation is largely intact to the proton irradiation. The local electronic modification of the graphene caused by the proton irradiation is a promising route for engineering fully graphene-based electronics.
9:00 PM - E7.8
Conduction Mechanism in a Single HCl-Doped Polyaniline Nanofiber.
Yen-Fu Lin 1 , Chien-Hsiang Chen 1 , Sheng-Hsiung Yang 2 , Chain-Shu Hsu 2 , Wen-Bin Jian 1
1 Electrophysics, National Chiao Tung University, Hsinchu Taiwan, 2 Applied Chemistry, National Chiao Tung University, Hsinchu Taiwan
Show Abstract A great amount of theoretical and experimental works in conducting polymers were reported when the first highly doped conjugated polymer, polyacetylene, was discovered to manifest a dramatic decrease in its resistivity via halogen treatment [1] in 1977. Among all the conducting polymers, Polyaniline (PANI) is one of the potential synthetic metals due to its low-cost synthesis, fast processability, and environmental stability in either the doped conducting or the de-doped insulting form. The applications of PANI, such as gas sensors, chemical sensors, solar cells, light emitting devices, and molecular electronics, have been demonstrated as well. Very recently, PANI nanostructures have been synthesized by using the template-free or template synthesis so as to raise the surface/volume ratio and to enhance the sensitivity in comparison with that of their bulk [2-4]. Despite the abundant achievements in applied aspects, there are still a lot of controversial issues in the conduction mechanism of the conducting PANI for a long time. In this study, we employed an interfacial polymerization method to prepare high quality HCl-doped PANI nanofibers. Two types of PANI nanofiber devices were fabricated. The milliscale devices were prepared by dropping of diluted nanofiber solution on the pair electrodes which were separated away from each other for several millimeters. On the other hand, the nanoscale devices were formed by the dielectrophoresis technique to fill the nanometer gap between two electrodes. Subsequently, the nanofiber devices were placed in a liquid-nitrogen cryostat for the temperature dependent evaluations. The current-voltage curves and the resistance, as a function of temperature, of a single HCl-doped PANI nanofiber could be measured for analysis to explore the conduction mechanism. The experimental results were in line with the charge energy limited tunneling base on the granular metallic model (GMM) [5]. According to the model, we can extract the average lengths of the diameter and the separation of metallic grains in PANI nanofibers to be about 1.07 and 0.40 nm, respectively. Additionally, the electrical resistivity of HCl-doped PANI nanofibers was estimated to be about 10 Ω cm.Reference:[1] C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, Phys. Rev. Lett. 39, 1098 (1977)[2] R. V. Parthasarathy and C. R. Martin, Chem. Mater. 6, 1627 (1994)[3] Z. Wei, Z. Zhang, and M. Wan, Langmuir 18, 917 (2002)[4] J. Huang, S. Virji, B. H. Weiller, and R. B. Kaner, J. Am. Chem. Soc. 125, 314 (2003)[5] P. Sheng, B. Abeles, and Y. Arie, Phys. Rev. Lett. 31, 44 (1973)
9:00 PM - E7.9
Structural and Magnetic Properties of the Two Phase Quarternary Heusler Alloy Co2Mn(1-x)TixSn.
Tanja Graf 1 , Peter Klaer 2 , Christian Blum 1 , Hans-Joachim Elmers 2 , Claudia Felser 1
1 Department of Inorganic and Analytical Chemistry, Johannes-Gutenberg University Mainz, Mainz Germany, 2 Institute of Physics, Johannes-Gutenberg-University , Mainz Germany
Show AbstractThe search for new materials with promising properties for spintronic applications is a field of active research. Heusler compounds are interesting candidates due to their tunable magnetic and structural properties. For a successful use of Heusler compounds in spintronic devices the interface structure is crucial. Smooth interfaces are necessary and interdiffusion between different layers should be avoided in order to retain the half-metallic properties.Here the investigation of the quarternary Heusler alloy Co2Mn(1-x)TixSn, {x = 0, 0.2, 0.4, 0.6, 0.8, 1} is presented. The samples were prepared by arc-melting stoichiometric amounts of the concerning elements. The structural properties were determined by powder X-ray diffraction. The materials exhibit the ordered L21 structure over the whole composition range. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis of the bulk compounds reveals microstructures that result from phase separation into a Mn-enriched and a Ti-enriched phase. These regions grow upon further annealing for one week at 1073 K. Additionally size, shape and ratio of the microstructures can be altered by varying the composition. The saturation magnetisation and the Curie temperature of the compounds were determined by SQUID magnetometry.