Symposium Organizers
Stefano Sanvito Trinity College
Olle G. Heinonen Seagate Technology
Valentin Alek Dediu ISMN-CNR
Nick Rizzo Freescale Semiconductor
FF1: Dilute Magnetic Semiconductor and Oxides
Session Chairs
Tuesday PM, April 14, 2009
Room 3007 (Moscone West)
9:15 AM - **FF1.1
Towards Functional Diluted Magnetic Semiconductors and Composite Ferromagnetic/semiconductor Systems.
Tomasz Dietl 1 2
1 Laboratory for Cryogenic and Spintronic Research, Institute of Physics, Polish Academy of Sciences, Warszawa Poland, 2 Institute of Theoretical Physics, University of Warsaw, Warszawa Poland
Show AbstractThe demonstrated and foreseen functionalities of ferromagnetic semiconductors such as (Ga,Mn)As and p-(Cd,Mn)Te [1], have stimulated a considerable effort to develop semiconductor systems in which spontaneous magnetization would persist to above room temperature. Indeed, high-temperature ferromagnetic features have been found in a number of semiconductors and oxides doped with various transition metals and rare earth elements or even in materials nominally undoped with magnetic elements. At the same time, it is becoming clear that the adequate characterization of these systems requires the application of advanced space-resolved and element-specific tools. On the theoretical side, it has been appreciated that conceptual difficulties of charge transfer insulators and strongly correlated disordered metals are combined in these materials with intricate aspects of heavily doped semiconductors and semiconductor alloys, such as Anderson-Mott localization, defect formation by self-compensation mechanisms, spinodal decomposition, and the breakdown of the virtual crystal approximation [2]. In this talk, after presenting the p-d Zener model that describes successfully (Ga,Mn)As and related systems, we will present a phase diagram showing the expected evolution of the Curie temperature when the p-d coupling, and thus hole localization, increases [3]. Interestingly, the theory predicts [3] an anomalous behavior of the exchange valence band spin-splitting in the strong coupling limit observed experimentally [4] as well as provides a novel view on mismatch alloys, such as Ga(As,N). We will then describe the progress in the understanding of the origin of high apparent Curie temperatures observed already in a variety of diluted magnetic semiconductors and oxides [5-9]. It will be shown that doping by magnetic ions above the solubility limit allows one to fabricate a variety of magnetic nanocrystals embedded coherently into the semiconductor host. Importantly, recent experiments [5,6], confirming a theoretical suggestion [7], show that the shape and dimension, and thus the blocking temperature of the magnetic particles, can be controlled by co-doping with shallow dopants and through growth parameters. Appealing functionalities of these multicomponent systems will be presented together with prospects for their applications in spintronics but also in electronics, photonics, and thermoelectricity [8,9].[1] see, e.g., T. Dietl, H. Ohno, F. Matsukura, IEEE—Trans. Electr. Dev. 54 (2007) 945.[2] see, e.g., T. Dietl, J. Phys. Soc. Jpn. 77 (2008) 031005, and references therein.[3] T. Dietl, Phys. Rev. B 77, 085208 (2008).[4] W. Pacuski et al., Phys. Rev. Lett. 100 (2008) 037204.[5] S. Kuroda et al., Nature Mat. 6 (2007) 440.[6] A. Bonanni et al., Phys. Rev. Lett. 101 (2008) 135502.[7] T. Dietl, Nature Mat. 5 (2006) 673.[8] H. Katayama-Yoshida et al., phys. stat. solidi (a) 204 (2007) 15. [9] T. Dietl, J. Appl. Phys. 103 (2008) 07D111, and references therein.
9:45 AM - FF1.2
Effect of Co and Al co-doping on Ferromagnetism in ZnO-based Diluted Magnetic Semiconductors.
Gap Soo Chang 1 , Aziz Dinia 2 , Ernst Kurmaev 3 , Keun Hwa Chae 4 , Jonghan Song 4
1 Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, 2 , IPCMS-CNRS-UMR 7504, Strasbourg France, 3 , Russian Academy of Science-Ural Division, Yekaterinburg Russian Federation, 4 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractThe effect of delocalized free carriers on magnetic properties of ZnO-based diluted magnetic semiconductors (DMSs) has been investigated by co-doping the group III element (Al in our case) into ZnO:Co thin films. The use of ZnO as a host semiconductor for the fabrication of DMS systems opens the possibility for development of spin-photonic devices. In the present study, the 150 nm thick ZnO thin films co-doped with Co and Al were fabricated on A12O3 (0001) substrates using a pulsed laser deposition (PLD) technique. Prior to the PLD process, a Zn0.895Co0.100Al0.005O target was synthesized by a co-precipitation method and sintered at 800 oC for 15 min. We prepared the ZnO:(Co,Al) thin films at different oxygen flow in order to evaluate the effect of partial oxygen pressure on the physical properties of samples. The superconducting quantum interference device and X-ray diffraction measurements reveal that the ZnO:(Co,Al) thin films grown with an oxygen flow (oxygen-rich condition) show ferromagnetic behavior without any noticeable Co aggregation while the Co clustering is observed in a sample prepared under oxygen-deficient regime. To understand ferromagnetism in the ZnO:(Co,Al) system, local electronic structure around magnetic dopants were investigated by employing element- and site-specific X-ray absorption and emission spectroscopy (XAS and XES, respectively). Co 2p XAS and Co L2,3 (3d4s → 2p transition) XES spectra were obtained as well as O 1s XAS and O K (2p → 1s) XES spectra. Resonant Co L2,3 XES spectra at an excitation energy near L2 absorption threshold show that a significant amount of free d carriers are introduced into the oxygen-rich ZnO:(Co,Al) thin films. According to O 1s XAS and O K XES spectra, the presence of free carriers is not related to oxygen vacancies but the doping of Al into ZnO:Co. This is in accordance with the calculated formation energy of various defects in ZnO:(Co,Al) structure which shows much higher formation energy for oxygen vacancies than Al-related defects. Finally, the first-principle calculations within the local spin-density approach were performed for all possible defect configurations involving oxygen vacancies and Zn and/or Al interstitials. causing ferromagnetic properties in ZnO:(Co,Al) system. The overall results suggest that free charge carriers and a strong ferromagnetic exchange interaction can be achieved in ZnO:(Co,Al) thin films when the Al dopants occupy the interstitial sites and hybridize with substitutional Co atoms. The detailed discussion will be presented together with our recent experimental results and analysis.
10:00 AM - FF1.3
Oxygen Vacancy Studies for Ferromagnetic Co-doped Zinc Oxide Films
Daxue Xia 1 , Weiguang Xie 1 , Jianbin Xu 1 , Xiangqian Xiu 1 , Rong Zhang 1
1 , The Chinese University of Hong Kong, Hong Kong China
Show AbstractPure ZnO and Co doped ZnO films on silicon substrate were prepared by sol-gel method and various post-annealing conditions were applied to interrogate the defects in the prepared thin films, which presumably may cause the ferromagnetic behavior of the doped ones. Different characterization techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy in conjunction with UV-visible spectroscopy, photoluminescence, and cathodoluminescence spectroscopy were used for a series of comparative studies. Based on our results, strong evidences for simultaneous existence of substitutional Co2+ and metallic Co cluster were found, which may provide some clues on clarifying the continuing disputes of the mechanism accounting for the ferromagnetic phenomena in transition-metal doped oxides.
10:30 AM - FF1.5
Growth and Structure-magnetic Property Correlations of Nickel-doped Zinc Oxide Thin Films.
Sudhakar Nori 1 , Chunming Jin 2 , Wei Wei 1 , Ravi Aggarwal 1 , D. Kumar 3 , Roger Narayan 2 , Jagdish Narayan 1
1 Materials Science & Engineering Department, 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, 3 Mechanical Engineering, North Carolina A&T State University, Greensboro, North Carolina, United States
Show AbstractDiluted magnetic semiconductor (DMS) thin films have been attracting tremendous attention for the applications in spintronics and other electromagnetic devices. Transition metal doped or alloyed ZnO oxide thin films have been considered as one of the promising materials for such applications. In this work, zinc oxide thin films doped with 5 at.% nickel were deposited using pulsed laser deposition on fused silica and silicon substrate at relatively low substrate temperature about 450°C. X-ray diffraction patterns demonstrated that the films were polycrystalline with c-axis perpendicular to the substrate surface. High resolution TEM investigation confirmed that the nickel atoms were incorporated into zinc oxide host without precipitation. Optical absorption spectra were recorded at room temperature and it was found that the nickel doping did not destruct the excitonic absorption of the films. Luminescence spectra were measured at room temperature with different excitation wavelength. Two exciton-related emission peaks were observed and their intensity varied with excitation wavelength differently. The resistivity and mobility of the films were also measured using four point probe and Hall measurements. The magnetic properties were investigated using a vibrating sample magnetometer (VSM) for both parallel and perpendicular orientations of the films with respect to the magnetic field and at different temperatures in the range from 10 to 300 K. The isothermal M-H curves showed hysteresis loops with significant coercivity values (~750 Oe at 10 K), and saturation of magnetization akin to ferromagnetic materials up to room temperature was observed.
10:45 AM - FF1.6
Origin of Ferromagnetism in Co-doped (La,Sr)TiO3.
Thomas Fix 1 , Lata Sahonta 1 , Mark Blamire 1
1 Department of Materials Science, University of Cambridge, Cambridge United Kingdom
Show AbstractDilute magnetic semiconductors (DMSs) have been under intensive study over the past few years, as they would allow manipulating both the spin and charge degrees of freedom in the same material, providing an alternative route for spin injection into semiconductors. While the DMS Ga1-xMnxAs has been proven to be an intrinsic ferromagnet [1], its Curie temperature is far below room temperature (<200K) and therefore practical applications are limited. More recently, intensive theoretical and experimental studies have focused on oxide-based DMSs like doped ZnO, TiO2, SnO2, and HfO2, some of which providing room temperature ferromagnetism. The origin of the ferromagnetism in these materials is still in dispute as in most cases a contamination with magnetic elements or the precipitation of magnetic metallic clusters cannot be excluded, causing the origin of the ferromagnetism to be extrinsic.Another diluted magnetic oxide system is Co-doped (La,Sr)TiO3 [2], where room temperature ferromagnetism and large spin polarization is reported [3], while the host oxide is a strongly correlated metal. This material is particularly interesting because it allows control of both the magnetic and electrical properties as the La/Sr concentration ratio induces a variation from an insulator to a metal or semiconductor. We investigate the influence of the La/Sr concentration ratio and the structural, magnetic and transport properties of this material by x-ray diffraction, transmission electron microscopy and electron energy loss spectroscopy, x-ray photoemission spectroscopy, x ray magnetic circular dichroism, magnetometry and Hall effect. All these techniques are used to prove or disprove the intrinsic character of the ferromagnetism in the material. The spin polarization of the material is also investigated.Our experiments show that while the as-grown samples are not ferromagnetic at room temperature or at 10 K, ferromagnetism at room temperature appears after annealing the films in reducing conditions and disappears after annealing in oxidizing conditions. However there is no correlation between the carrier density and the ferromagnetism, which indicates that the ferromagnetism might not be carrier-induced but related to oxygen defects.[1] H. Ohno et al., Appl. Phys. Lett. 69, 363 (1996).[2] Y. G. Zhao et al., Appl. Phys. Lett. 83, 2199 (2003).[3] G. Herranz et al., Phys. Rev. Lett. 96, 027207 (2006).
11:30 AM - FF1.7
On the Defect Induced Ferromagnetic Ordering Above Room Temperature In Pure ZnO Films.
Mukes Kapilashrami 1 , Jun Xu 1 2 , Valter Strom 1 , V. Rao 1 , Lyubov Belova 1
1 Dept. of Materials Science, Royal Institute of Technology, Stockholm, Stockholm lan, Sweden, 2 State Key Laboratory of Materials Modification, Department of Physics, Dalian University of Technology, Dalian China
Show AbstractExistence of long range ferromagnetic order above room temperature, RTFM, in compounds containing no nominal magnetic ions reported in both undoped and doped oxides has been a topic of great interest of late. Among these, of particular interest are the reported magnetic properties of pure ZnO, In2O3, and TiO2 thin films [1] which in their doped form are considered to be potential RTFM semiconductors suitable for applications in spintronics. ZnO, for example, has been identified as a suitable host semiconductor (due to the possible high solubility of dopants), both from a theoretical and experimental point of view. Thus arises the question regarding the origin of RTFM in doped dilute alloys if the ZnO matrix itself develops a defect induced ferromagnetic order with a significant magnetic moment per unit cell. In this presentation we report a systematic study of the RTFM in highly transparent ZnO thin films deposited by DC Magnetron Sputtering technique. We have studied the thickness dependency of the magnetization (thickness ranging from 120-720 nm) and find an optimum thickness of ~480 nm under an oxygen ambience of appropriate pressure during the deposition to result in 0,62 emu/g saturation magnetization at room-temperature. As far as we know this is the highest known MS value for any otherwise diamagnetic bulk oxide material. We discuss the origin of such large moments in terms of the possible defects that could arise from Zn vacancies, unpaired electrons at the O sites surrounding a Zn vacancy etc. We also show that in a ZnO matrix suitably tuned for the maximum intrinsic defect induced moment, on doping with Mn, a much larger net moment per Mn atom is obtained. With this approach of doping a tailored matrix, we obtain routinely room temperature ferromagnetism in Mn doped DMS. This work is supported by the Swedish funding Agency VINNOVA, Swedish Research Council and the Hero-M Excellence Center KTH. XJ acknowledges a post doctoral fellowship from Carl Tryggers Foundation in Sweden.[1] Nguyen Hoa Hong et al, Phys. Rev 73, 132404 (2006)
11:45 AM - FF1.8
Controlling the Aggregation of Magnetic Cations in GaN.
Alberta Bonanni 1 , Andrea Navarro-Quezada 1 , Tian Li 1 , Vaclav Holy 2 , Guenther Bauer 1 , Bogdan Faina 1
1 Institute for Semiconductor and Solid State Physics, Linz University, Linz Austria, 2 Condensed Matter Physics, Charles University, Prague Czech Republic
Show Abstract We have carried out detail studies of MOVPE-grown (Ga,Fe)N [1-4], either undoped or co-doped with Si or Mg, combining magnetic (SQUID and EPR), magnetooptical, and XANES investigation with a comprehensive structural and chemical characterization (SIMS, TEM, EDS, synchrotron XRD). Our finding demonstrate that the Fe ions are distributed in the nitride matrix in a way giving rise either to a diluted random alloy [1-4] or to ferromagnetic FeNx nanocrystals that aggregate by precipitation [1,2,4] or by spinodal decomposition into (Ga,Fe)N regions more or less rich in the magnetic component [2,4]. Since this aggregation correlates with the ferromagnetic response and no spontaneous magnetization is observed in the films without Fe, we take our results as a strong support of the notion [5] that the high-TC ferromagnetism discovered in a number of magnetically doped oxides and semiconductors results from a non-uniform distribution of the magnetic component. Interestingly, we find that the aggregation of Fe cations exhibits a strong dependence on the Ga flow rate, allowing – when appropriately mastered – a control of the solubility limit of the transition metal ions in GaN. Moreover, the formation of the Fe-rich nanocrystals and, hence, the ferromagnetic response of (Ga,Fe)N can be affected by co-doping with donors (Si) and acceptors (Mg). Our findings guide us to the far-reaching conclusion that the attractive force between magnetic cations can be adjusted by varying their charge state [4]. Finally, by elaborating novel protocols for incorporating magnetic and shallow impurities in a delta-like fashion we start to take control over the magnetic nanocrystal shapes (dots vs. nanocolumns), a finding with significant consequences for possible functionalities of these systems.[1] A. Bonanni, M. Kiecana, C. Simbrunner, Tian Li, M. Sawicki, M. Wegscheider, M. Quast, H. Przybylinska, A. Navarro-Quezada, R. Jakiela, A. Wolos, W. Jantsch, and T. Dietl, Phys. Rev. B 75, 125210 (2007).[2] M. Wegscheider, Tian Li, A. Navarro-Quezada, B. Faina, A. Bonanni, W. Pacuski, R. Jakiela, and T. Dietl, J. Phys.: Cond. Mat. 20, 454222 (2008).[3] W. Pacuski, P. Kossacki, D. Ferrand, A. Golnik, J. Cibert, M. Wegscheider, A. Navarro-Quezada, A. Bonanni, M. Kiecana, M. Sawicki, T. Dietl, Phys. Rev. Lett. 100, 037204 (2008).[4] A. Bonanni, A. Navarro-Quezada, Tian Li, M. Wegscheider, R.T. Lechner, G. Bauer, Z. Matej, V. Holy, M. Rovezzi, F. D’Acapito, M. Kiecana, M. Sawicki, and T. Dietl, Phys. Rev. Lett. 101, 135502 (2008).[5] see, e.g., A. Bonanni, Topical Review, Semicond. Sci. and Technol. 22, 41 (2007).
12:00 PM - FF1.9
Novel Electrical and Ferromagnetic Properties of Vanadium Dioxide Thin Films: Structure-property Correlations.
Tsung-Han Yang 1 , Sudhakar Nori 1 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, NSF Center for Advance Materials and Smart Structures, Raleigh, North Carolina, United States
Show Abstract Vanadium dioxide (VO2) is an interesting oxide material system which shows a very sharp transition in its electrical resistivity above room temperature close to 340 K, which is accompanied by a structural transformation from a monoclinic semiconducting phase to a tetragonal metallic phase. In addition, it also shows thermal hysteresis (width of hysteresis loop~5-7K) in electrical resistivity making it one of the most sought material and this property indeed can be exploited for fabricating sensor- and memory-type applications in solid state devices. High quality epitaxial vanadium oxide (VO2) thin films on sapphire (0001) substrate have been grown by pulsed laser deposition technique and the as-grown films exhibited excellent properties without further annealing treatments. The epitaxial growth has been achieved via domain matching epitaxy, where matching of integral multiples of planes occur across the film-substrate interface. The magnetic properties of pure vanadium oxide (VO2) film investigated at different temperatures in the range 10 to 360 K showed significant hysteresis as well as saturation of the magnetic moment. The origin of ferromagnetic properties is discussed in the light of X-Ray Photoelectron Spectroscopy (XPS) used to measure magnetic impurities in VO2 thin films.1 J. Narayan and V. M. Bhosle, Journal of Applied Physics, 100 (2006) 1035242 Joyeeta Nag and R F Haglund Jr, J. Phys.:Matter, 20, (2008), 264016
12:15 PM - FF1.10
Ferromagnetism and Surface Morphology in a Candidate Dilute Magnetic Semiconductor: The Case of Cr-doped Ga2Se3 Thin Films on Si(001).
Esmeralda Yitamben 1 , Tracy Lovejoy 1 , Dennis Paul 3 , John Callaghan 3 , Fumio Ohuchi 2 , Marjorie Olmstead 1
1 Physics, University of Washington, Seattle, Washington, United States, 3 , Physical Electronics, Chanhassen, Minnesota, United States, 2 Materials Science Engineering, University of Washington, Seattle, Washington, United States
Show AbstractThe motivation for spin-based electronics, spintronics, resides in the use of both the spin and charge of an electron to transmit information. Spintronics will require new materials that are both silicon compatible, and ferromagnetic at room temperature. Semiconductors that incorporate a few percent of transition metal dopants known as dilute magnetic semiconductors (DMS), are likely candidate for such a material. The intrinsic vacancy semiconductor Ga2Se3, which may be grown epitaxially on Si, presents several interesting issues for the study of DMS. Transition metal doping may lead to the occupation of vacancy sites or Ga sites in the lattice which could lead to either n- or p-type doping. The vacancy-induced anisotropy and wide bandgap (2.5 eV) may also lead to high Curie temperatures. To probe the interaction between magnetism and free carriers in this new class of magnetic materials, experimental investigations of Cr-doped Ga2Se3 epitaxially grown on Si(100):As have been pursued. Inclusion of Cr into the Ga2Se3 lattice at high concentration results in new states at the Fermi edge, signaling a metallic structure; while at low concentration, the films appear to be semiconducting. The films are ferromagnetic at room temperature even at low concentrations. Scanning tunneling microscopy reveals the formation of islands within trenches whose shape and size depend on the Cr concentration and whether or not a Ga2Se3 buffer layer is deposited first. Scanning Auger Microscopy reveals a compositional difference between the islands and the terraces surrounding them, with a larger Cr:Ga ratio in the islands. We suggest this concentration difference and the resultant strain and/or difference in chemical potential, may control the size and shape of the trenches surrounding the islands.
12:30 PM - FF1.11
Formation of Cr-rich Nano-clusters and Columns in (Zn,Cr)Te Grown by MBE.
Shinji Kuroda 1 , Koichiro Ishikawa 1 , Ke Zhang 1 , Yotaro Nishio 1 , Masanori Mitome 2 , Yoshio Bando 2
1 Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki, Japan, 2 Advanced Materials and Nanomaterials Laboratories, National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractSearch for novel ferromagnetic semiconductors with a high transition temperature is one of the most challenging topics in today’s materials science and engineering. Recently, the distribution of magnetic elements in the host crystal has become realized as a key dominating the magnetic properties of diluted magnetic semiconductors (DMSs)[1]. Indeed, it has been reported that nano-scale regions containing high-content magnetic elements are formed and the ferromagnetic properties are accordingly enhanced in various DMSs[2-4]. In our recent study on (Zn,Cr)Te[5], we have found that the Cr distribution can be controlled in a systematic way by the MBE growth condition; Cr-rich regions are formed in the crystal co-doped with iodine (I) as a donor impurity or grown under the surplus supply of Zn flux during the MBE growth. The formation of Cr-rich regions is controlled by manipulating the aggregation energy between Cr ions through the shift of Fermi energy due to the co-doping of donor impurity or the deviation from stoichiometry in the growth under different Zn/Te flux supply ratios[6]. In the present study, we have investigated the structural and magnetic properties of (Zn,Cr)Te films grown by MBE with a systematic variation of growth conditions such as the substrate temperature during the growth and the flux supplies. Combined analyses of the Cr distribution and the magnetic properties using the spatially resolved energy-dispersive X-ray spectroscopy (EDS) in the TEM observation and SQUID magnetometry reveal that the crystallinity and the shape of Cr-rich regions formed in I-doped Zn1-xCrxTe depend on the substrate temperature. With the increase of the substrate temperature TS, the crystal quality is ameliorated with the reduction of stacking faults. In the case of a relatively high average Cr composition around x ~ 0.2, it is found that the shape of Cr-rich regions is transformed from isolated clusters into one-dimensional nanocolumns at a higher substrate temperature TS=360oC. These Cr-rich nanocolumns are slanted against the growth orientation for the growth on the (001) surface. From the angle of this slant, it is considered that Cr-rich nanocolumns have a tendency to be formed along the {111} plane of the zinc-blende crystal structure. The magnetization measurements reveal the increase of the blocking temperature and anisotropic magnetizations between the configurations under magnetic fields parallel and perpendicular to the film plane, both of which are likely due to the formation of one-dimensional ferromagnetic columns.[1] K. Sato et al., Jpn. J. Appl. Phys. 44, L948 (2005).[2] L. Gu et al., J. Magn. Magn. Mater. 290-291, 1395 (2005).[3] M. Jamet et al., Nature Mater. 5, 653 (2006).[4] D. Bougeard et al., Phys. Rev. Lett. 97, 237202 (2006).[5] S. Kuroda et al., Nature Mater. 6, 440 (2007).[6] T. Dietl, Nature Mater. 5, 673 (2006).
12:45 PM - FF1.12
Mn Doped CuO as a New Material for Spintronics: A Theoretical Study.
Yousong Gu 1 , Liqing Pan 2 , Zhanjun Gao 1 , Yue Zhang 1
1 Department of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing, Beijing, China, 2 Department of Physics , University of Science and Technology Beijing, Beijing, Beijing, China
Show AbstractDiluted magnetic semiconductors (DMSs) have attracted much attention because of their potential applications in spintronics due to the promised efficient spin injection and transport, and seamless integration with current semiconductor technology. In this work, first principle calculations are performed to study the possibility of CuO to be a DMS, and the fitness for spin injection.The calculations are performed by the VASP package using density functional theory, pseudo-potential and plane wave basis-set. PAW (Projected Augmented Wave) pseudo potential and PBE (Perdew-Burke-Ernzerhof) type of exchange correlation, and LSDA+U type on site Coulomb interaction are employed in the calculation.Mn doping is constructed by building supercells and replace 2 Cu ions with Mn, and Cu vacancy is done by just delete a Cu ion. The density of states, the projected band population and magnetic moment of the ions are calculated to help us to understand the electronic structure, and further understand the ferromagnetic and transport properties.Calculation on undoped CuO is done in a (101)×(010)×(-101) supercell ( 8 Cu and O ions ) and the experimentally observed antiferromagnetic arrangement along [-101] direction is reproduced. The magnetic moment of the Cu ion is about 0.70μB, and the induce moment on O ion is 0.10μB. It is an insulator with a band gap of about 2eV.Two Mn ions substitute Cu ions in the above superlattice in order to study the magnetic coupling of the Mn ions. The favorable sites of the Mn ions in the supercell were studied by doing a configuration calculation, and the ground state energy calculation show the Mn ions prefer to stay at the Cu-O chain along the [101] direction (the zigzag shape chain). The ground state energies of the ferromagnetic and antiferromagnetic states show the ferromagnetic are favored by 12meV. The magnetic moment of the cell is 8.00μB, and the magnetic moment of Mn, Cu and O are 4.64, 0.72 and 0.10μB, respectively. DOS(density of state) calculation show that Mn doping provide an impurity band just above the valence band, but it is filled and the Fermi level move upward, since Mn and Cu both are 2+ ions. The conductivity is still a insulator. The β angle increase from 82 to 88 degree, and the lattice changes from a monoclinic to almost orthorhombic.In order to have conductivity, Cu vacancy is introduced by deleting a Cu ion in the supercell as it is often the case in experiments. Configuration calculation showed that deleting a Cu ion near the Mn-O chain with antiferromagnetic magneitc coupling is preferred in energy. The Band structure calculation show that the impurity band of Mn ion moved above the Fermi surface and became unfilled. This will facilitate the electron movement and become a conductor. The magnetic moment of the cell is 7.00μB, and the magnetic moment of Mn, Cu and O are 4.11, 0.76 and 0.10μB, respectively, due to the charge transfer between the ions.
FF3/H3: Joint Session: Magnetic Resistive RAM
Session Chairs
Tuesday PM, April 14, 2009
Room 2007 (Moscone West)
4:30 PM - **FF3.1/H3.1
Spin Torque Effects in Nanoscale Magnetic Tunnel Junction Structures.
Robert Buhrman 1
1 Applied and Engineering Physics, Cornell University, Ithaca, New York, United States
Show AbstractThere has been remarkable progress in advancing the fundamental understanding of spin torque effects in magnetic nanostructures, and in successfully moving this phenomenon towards technological implementations, particularly for spin-torque MRAM and spin-torque microwave oscillator applications that have the potential for broad impact. In this presentation I will discuss some recent work that has sought to contribute to this rapidly advancing spin-torque research effort, to better understand quantitatively the details of the phenomenon, and to understand and enhance the efficiency of the effect, particularly in systems that utilize magnetic tunnel junctions. I will summarize results from studies of spin-transfer switching and microwave excitation in magnetic tunnel junctions (MTJs), which include the use of the spin transfer phenomenon to quantitatively determine, through spin-torque-excited ferromagnetic resonance (ST-FMR), both the bias dependent efficiency of the spin torque in high quality MTJs and the magnetic damping of individual free layer nanomagnets. I will also describe some recent three-terminal spin torque nanodevice experiments where a low impedance spin valve contact is employed to reverse a nanomagnet by a non-uniform injection of spin currents while a high impedance magnetic tunnel junction contact is employed to read-out the nanomagnet’s orientation. Finally I will present some recent results that have been obtained by employing several different magnetic materials engineering approaches to substantially reduce the current density required for ns-scale reversal of a thermally stable free layer, including the development of high TMR tunnel junction technology that utilizes magnetic free layers with low saturation magnetization.
5:00 PM - **FF3.2/H3.2
Material and Device Properties of MgO-Based Magnetic Tunnel Junctions for Spin Torque MRAM.
R. Dave 1 , N. Rizzo 1 , F. Mancoff 1 , P. Mather 1 , K. Smith 1 , B. Butcher 1 , T. Andre 1 , J. Slaughter 1 , S. Tehrani 1
1 , Everspin Technologies, Inc., Chandler, Arizona, United States
Show AbstractSpin torque (ST) switching is a candidate programming method that may enable higher density and lower power operation for future magnetic random access memory (MRAM). In this talk, we discuss the optimization of MgO-based magnetic tunnel junction (MTJ) material to meet requirements for ST-MRAM, including: efficient and repeatable ST switching, high magnetoresistance ratio (MR) and low resistance-area-product (RA). Different oxidation processes of Mg were evaluated to obtain key properties of high-quality MgO tunneling barrier. We discuss the evaluation of CoFeB as a suitable free layer for optimum device properties, including high MR for high spin-torque efficiency, moderate to low magnetization, and low magnetic damping. MTJ devices were fabricated on 200 mm Si wafers using optical lithography to form bits with sizes as small as 100 nm×180 nm. Low-bias MR values over 100% were obtained in patterned bits with CoFeB free layers with RA~6 Ω−μm2 using MgO tunnel barriers formed from natural oxidation of Mg. Quasistatic switching current densities of 3 MA/ cm2 were obtained with thermally-stable bits having a thermal energy barrier of 50kT, which corresponds to high-speed switching current density Jc0≈ 5 MA/ cm2 (extrapolated to 1 ns). We also present results on the performance of ST-MRAM arrays integrated with CMOS, including within-die read distributions, MgO barrier reliability under spin torque writing conditions, and distributions of switching voltage. Arrays with a switching to breakdown voltage ratio Vsw /Vbd ≈0.5, and switching distributions ≈5% have been achieved. The ST switching distributions are tighter than the field-switching distributions for the same bits, which are >10%, and exhibit less shape dependence.
5:30 PM - FF3.3/H3.3
Presence of B Oxide in the MgO Barrier in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions and Its Effect on Tunneling Magnetoresistance.
Judy Cha 1 , J. Read 2 , R. Buhrman 1 , David Muller 1
1 School of applied and engineering physics, Cornell University, Ithaca, New York, United States, 2 Department of Physics, Cornell University, Ithaca, New York, United States
Show AbstractMagnetic tunnel junctions (MTJs) are an essential component in magnetic random access memories (MRAMs). Achieving a high tunneling magnetoresistance (TMR) ratio while maintaining a low resistance-area (RA) product is crucial for MRAMs. Since the theoretical prediction of over 1000% TMR [1], MgO-based MTJs became an excellent candidate for non-volatile memory devices and have been extensively studied. The steady increase in TMR [2] has led to a commercial MRAM 4 Megabit memory [3].
Despite the remarkable experimental progress, there is a discrepancy between the theoretical predictions and the experimental results. Theoretically, the high TMR value is attributed to a slowly decaying, coherent tunneling pathway available only to a majority spin state, which is made possible by lattice-matching the interface between the MgO barrier and the ferromagnetic electrodes [1]. Experimentally however, RF-sputtered MTJs, whose MgO/electrode interface is not controlled at atomic precision, produce high TMR values [4]. Moreover, using an amorphous CoFeB electrode and annealing the device afterward is found to produce higher TMR values. With the CoFeB electrodes, we observe that B diffuses into the RF-sputtered MgO barrier and becomes oxidized [5, 6]. Furthermore, we observe a decreasing TMR with increasing barrier thickness in as-grown junctions, opposite to the theoretical prediction. These observations suggest that the tunneling mechanism through the MgO layer cannot be fully accounted for by the current theoretical model [1].
To further investigate the effect of B oxide within the Mg-B-O layer on TMR, we examined Mg-B-O-based MTJs with varying tunnel barrier thicknesses and different metal alloy electrodes. The MTJs consist of a Si/SiOx / seed layer / 25 nm IrMn / 4nm CoFeB / Mg-B-O / 3nm CoFeB / capping layer. The capping layer is 8 nm Ta / 7nm Ru and the seed layer is 5 nm Ta / [20 nm CuN / 3 nm Ta]x4. We measured TMR and RA products of our samples and characterized them using electron energy-loss spectroscopy. Spectroscopic imaging on a MgO / CoFeB single interface was performed to get a 2D map of B oxide concentration in the Mg-B-O layer. We report the relative concentration of B oxide in the Mg-B-O layer with respect to the O content. We will discuss our TMR results and correlate them with the amount of B oxide found in the Mg-B-O layer.
[1] W. H. Butler, X. G. Zhang, T. C. Schulthess, et al., Phys. Rev. B 63, 054416 (2001).
[2] S. Ikeda, J. Hayakawa, Y. Ashizawa, et al., Appl. Phys. Lett. 93, 082508 (2008).
[3] Freescale, News Release, http://media.freescale.com/phoenix.zhtml?c=196520&p=irol-newsArticle&ID=880030 (2006).
[4] S. S. P. Parkin, C. Kaiser, A. Panchula, et al., Nat. Mater. 3, 862 (2004).
[5] J. J. Cha, J. C. Read, R. A. Buhrman, et al., Appl. Phys. Lett. 91, 062516 (2007).
[6] J. C. Read, P. G. Mather, and R. A. Buhrman, Appl. Phys. Lett. 90, 132503 (2007).
5:45 PM - FF3.4/H3.4
Novel Magnetoresistive Structures Using Self-Assembly and Nanowires on Si.
Mazin Maqableh 1 , Xiaobo Huang 1 , Liwen Tan 1 , Beth Stadler 1
1 , U Minnesota, Minneapolis, Minnesota, United States
Show AbstractAnodic Aluminum Oxide (AAO) was grown both as free-standing membranes and as integrated layers on Si as templates for arrays of magnetoresistive nanowires. These structures will be useful for applications such as current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) sensors, magnetic random access memory (MRAM) and microwave oscillator arrays. As the AAO was formed, using a two-step anodization process, columnar nanopores self-assembled inside the oxide to form a close-packed array. The pore diameters could be varied from 10-60nm by changing the anodization conditions. As the diameter of the AAO nanopores decreased, the distance between the nanopores also decreased. The free-standing membranes had pores with lengths of 17um. The pores that were grown directly onto Si with an adhesion layer of Ti were 600nm in length. In addition to growing these latter pores directly onto Si, they were also grown onto Co/Cu/Co thin films that were evaporated onto the Si. Au nanocontacts were electroplated into these nanopores to study point-contact magnetoresistance and microwave response. For the magnetoresistive nanostructures, multilayered Co/Cu nanowires were fabricated via electrochemical deposition. The samples were measured with vibrating sample magnetometry (VSM) and also using ac and dc magnetotransport systems. The highest magnetoresistance was found in nanowires that had hysteresis loops that were identical as measured in plane and perpendicular to the plane. The highest measured MR (Delta R/R = 11%) of the multilayers was calculated as 33% by subtracting the resistance of the Cu leads on either side of the multilayers from the denominator. Shorter wires are currently under construction to avoid this effect. Spin transfer torque (STT) was also measured in the samples. For 10-60nm diameter nanowires, the change in resistance due to STT was around 6% which represents the full magnetoresistance of the larger wires, but only half that of the smaller nanowires. It is therefore concluded that the 10-nm Co layers do not align antiparallel to parallel as fully at the switching current density of JAP-P = 2.7 x 108 A/cm2 compared to the larger wires which switch at JAP-P = 3.2 x 107 A/cm2. With diameters in the 10-60 nm range and integration with Si, these nanostructures have great potential for future nanosensors, MRAM and microwave oscillator arrays.
Symposium Organizers
Stefano Sanvito Trinity College
Olle G. Heinonen Seagate Technology
Valentin Alek Dediu ISMN-CNR
Nick Rizzo Freescale Semiconductor
FF5: Single Spin Dynamics/DMS I (III-V and Group IV)
Session Chairs
Wednesday PM, April 15, 2009
Room 3007 (Moscone West)
2:30 PM - FF5.1
Experimental Evidence of the Hyperfine Interaction between Hole and Nuclear Spins in InAs/GaAs Quantum Dots.
Benoit Eble 1 , Christophe Testelin 2 , Pascal Desfond 3 , Frederick Bernardot 4 , Andrea Balocchi 5 , Thierry Amand 6 , Audray Miard 7 , Aristide Lemaitre 8 , Xavier Marie 9 , Maria Chamarro 10
1 , Université Paris 6; INSP, Paris France, 2 , Université Paris 6; INSP, Paris France, 3 , Université Paris 6; INSP, Paris France, 4 , Université Paris 6; INSP, Paris France, 5 , Universite de TOulouse; INSA, Toulouse France, 6 , Universite de TOulouse; INSA, Toulouse France, 7 , Laboratoire de Photonique et Nanostructures , Marcoussis France, 8 , Laboratoire de Photonique et Nanostructures , Marcoussis France, 9 , Universite de Toulouse;INSA, Toulouse France, 10 , Université Paris 6; INSP, Paris France
Show AbstractHyperfine interaction of an electron with nuclear spins has recently been demonstrated to be the most effective mechanism of electron spin relaxation in quantum dots (QDs) at low temperature. Here, we evidence for the first time the coupling between holes and nuclear spins. Moreover, we demonstrate that the hole spin coupling is only one order of magnitude weaker than the electron spin coupling. We have measured the carrier spin dynamics in p-doped InAs/GaAs QDs by pump-probe and time-resolved photoluminescence experiments. The analysis of the decay of the circular photoluminescence polarization leads to an electron spin dephasing time of 500 ps, which is comparable to previous results in InAs QDs. To probe the resident hole spin polarization, we measured the photo-induced circular dichroism (PCD) in the QD sample. A circularly polarized pump beam propagating along the growth axis z is tuned to the energy of the lowest-allowed optical transition of InAs QDs, containing a single resident hole (E = 1.35 eV). This excitation creates a positive trion, in its ground state. This transient complex consists of two holes with opposite spins forming a singlet (the photo-generated one and the resident one due to doping), and of a photo-generated electron with its spin pointing down or up depending on the σ+ or σ- circularly polarized excitation, respectively. During the lifetime of these photo-generated electrons, the electron-nuclear hyperfine interaction leads to an efficient coherent coupling of the two electron spin states. The spontaneous decay of the trion state by emission of a polarized photon leads to efficient hole spin polarization. To obtain a direct evidence of the hole-nuclear spin interaction, we have applied an external magnetic field parallel to the growth axis z of the sample. We observed that PCD signal at a fixed pump-probe delay of 13ns increases dramatically with very small magnetic fields (few mT). As for an electron, the hyperfine interaction of a hole in a QD with the surrounding nuclei can be described by a frozen effective nuclear field. The only difference is the dipole-dipole origin of this hyperfine interaction instead of a contact Fermi origin. The hole spin precesses about this effective field, which presents a random distribution of magnitude and amplitude from a QD to another, and results in decay of the hole spin polarization in the QD ensemble. If the amplitude of the external field is larger than the dispersion of the in-plane fluctuations of the nuclear hyperfine field the average hole spin dephasing can be reduced and then the average hole spin polarization increased. We have calculated dipole-dipole hole-nuclear coupling and we have also taken into account the mixing character of heavy-holes experimentally observed in self-assembled QDs. Our calculations lead to a hole spin dephasing time for an ensemble of dots of 14 ns in close agreement with experiments.
2:45 PM - FF5.2
Electrical Single Spin Manipulation in Gated Quantum Dots via Closed Loop Trajectories.
Michael Fearn 1 , John Jefferson 1
1 Malvern Technology Centre, QinetiQ, Malvern United Kingdom
Show AbstractWe show, by numerical simulation, how arbitrary rotations of a single spin may be performed for a quantum dot defined within a quantum well with surface gates subjected to sequential electrical pulses. Spin manipulation is via the spin-orbit interaction with the gate pulses giving rise to closed-loop trajectories of the confining potential in the plane of the quantum well. Within the adiabatic approximation the Bloch sphere spin rotation depends only on the geometry of the closed-loop trajectory and may be accumulated with multiple gate-pulse cycles. Typical length and time scales are presented for a quantum dot based on an InSb quantum well structure and we show that spin rotations are feasible within a spin coherence time for loop trajectories, consistent with recent experiments on coherent control [1]. Approximate analytic results are presented and compared with the simulations, including deviations from the adiabatic approximation. [1] K.C. Nowack, F.H.L. Koppens, Yu V. Nazarov and L.M.K. Vandersypen, Science vol.318, p.1430 (2007).
3:00 PM - **FF5.3
Thermodynamics of Quantum Dot Nuclear Spins.
Atac Imamoglu 1 , Patrick Maletinsky 1
1 , ETH Zurich, Zurich Switzerland
Show AbstractOptical orientation of nuclear spin ensembles of both bulk [1] and mesoscopic [2] semiconductor structures is a generic research concept of solid-state physics. It is typically assumed that an established nuclear spin polarization (NSP) can be associated with the reduction of a nuclear spin temperature. Achieving efficient nuclear spin cooling is interesting from a fundamental and an applied point of view: A sufficiently "cold"nuclear spin ensemble is expected to undergo interesting magnetic phase transitions [3] and/or to increase the spin coherence of electrons trapped in semiconductor quantum dots (QDs) [4]. However, experimental observation of such phenomena requires both a deeper understanding of the thermodynamic properties of the QD nuclear spin ensemble and means of controlled manipulation of the nuclear spins.To that end, we use optical preparation and detection of spin and energy of QD electrons to both manipulate and measure the average nuclear spin polarization (NSP) in a single, self-assembled QD [5]. In particular, optical pump-probe sequences allow us to measure the dynamics of NSP and to identify single excess QD electrons as the main source of nuclear spin relaxation [6]. Remarkably, NSP shows a lifetime exceeding hours in the absence of extra QD charges, leaving room for further manipulation of the polarized nuclear spin ensemble. In particular, we are able to implement an adiabatic demagnetization experiment [7] and demonstrate that the mesoscopic nuclear spin ensemble of a QD can in general not be described by a spin temperature. We associate the striking deviations from a thermal nuclear spin state with the presence of strong quadrupolar interactions within the QD. Our results indicate that for a variety of QD systems, the well-established thermodynamic treatment of nuclear spins might not be applicable. Ultimately, further understanding of the subtle mechanisms that lead to the breakdown of the nuclear spin temperature approach in QDs could lead to an improved spin coherence time for QD electrons.[1] F. Meier, Optical Orientation (North Holland, Amsterdam, 1984).[2] D. Gammon et al., Science 277, 85 (1997).[3] P. Simon et al., Phys. Rev. Lett 98, 156401 (2007).[4] A. Imamoglu et al., Phys. Rev. Lett. 91, 017402 (2003).[5] C.W. Lai et al., Phys. Rev. Lett. 96, 167403 (2006).[6] P. Maletinsky et al., Phys. Rev. Lett. 99, 056804 (2007).[7] P. Maletinsky et al., http://arxiv.org/abs/0901.2289 (2009).
3:30 PM - **FF5.4
Manipulating Single Spins and Coherence in Semiconductors.
David Awschalom 1
1 Center for Spintronics and Quantum Computation, University of California, Santa Barbara, California, United States
Show AbstractDemonstrations of coherent single spin control include electron spins in semiconductor quantum dots and nitrogen-vacancy centers in diamond [1]. Recently, diamond-based materials have recently emerged as a unique platform for quantum science and engineering. Spins of single nitrogen-vacancy (N-V) color centers in diamond can be imaged, initialized and read-out optically [2] at room temperature, and measurements show remarkably long room-temperature electron spin coherence times well into the microsecond regime [3]. We demonstrate the coherent control and coupling of single spins in diamond. Using magneto-optical imaging and electron spin resonance (ESR) measurements, we investigate single N-V center spins that are coupled to electron spins of nearby nitrogen (N) defects. Some of the N-V centers are strongly coupled to only one single N spin, allowing the controlled polarization and readout of this single `dark' N spin [4] with remarkably long lifetimes. We show room temperature coherent control of the N-V center spin using optical detection of pulsed ESR and spin echo techniques. Finally, we exploit diamond in order to study the coherent dynamics of a single central spin coupled to an adjustable bath of spins [5]. These experiments reveal that both the internal interactions of the bath and the coupling between the central spin and the bath can be tuned in situ with an applied magnetic field, allowing access to regimes with surprisingly different behavior. Combining these elements with precise implanting techniques paves the way towards future devices based upon the quantum coherent control of multiple coupled spins in diamond.[1] R. Hanson and D. D. Awschalom, Nature 453, 1043 (2008).[2] D.D. Awschalom, R.J. Epstein, and R. Hanson, Scientific American 297, 84 (2007).[3]S. Takahashi, R. Hanson, J. Van Tol, M. Sherwin, and D.D. Awschalom, Phys. Rev. Lett. 101, 047601 (2008).[4] R. Hanson, F. M. Mendoza, R. J. Epstein and D. D. Awschalom, Phys. Rev. Lett. 97, 087601 (2006).[5] R. Hanson, V. V. Dobrovitski, A. E. Feiguin, O. Gywat, and D. D. Awschalom, Science 320, 352 (2008).
4:30 PM - FF5.5
Suppression of Nuclear Polarization in Optically Pumped GaAs.
Michael Fitzsimmons 1 , B. Kirby 1 4 , N. Hengartner 1 , F. Trouw 1 , M. Erickson 2 , S. Flexner 2 , T. Kondo 2 , C. Adelmann 1 , C. Palmstrom 3 , P. Crowell 2 , W. Chen 4 , T. Gentile 4 , J. Borchers 4 , C. Majkrzak 4 , R. Pynn 5
1 , LANL, Los Alamos, New Mexico, United States, 4 , NIST, Gaithersburg, Maryland, United States, 2 , University of Minnesota, Mineapolis, Minnesota, United States, 3 , University of California at Santa Barbara, Santa Barbara, California, United States, 5 , Indiana University, Bloomington, Indiana, United States
Show AbstractWe measured the spin dependence of polarized neutron beams reflected by a GaAs sample at 20 K in a magnetic field of 250 Oe applied along the sample’s surface normal. Neutron data were acquired with left and right circularly polarized light (0.25 W/cm2 and 808 nm) illuminating the sample. These conditions yielded nuclear polarization in the same sample (before and after the neutron experiment) of several percent as evidenced by a shift of a peak in the optical Hanle curve. The neutron data exhibit a correlation with light polarization and thus nuclear polarization. Quantitative analysis of the spin dependence of the polarized neutron reflectivities indicates nuclei within 50 nm of the sample’s surface are not polarized, and then nuclear polarization increases to a small value in the bulk. We attribute suppression of nuclear polarization near the sample’s surface to the electric field in the depletion layer that inhibits binding of spin polarized carriers to donor sites and to the electric field gradient at the nuclei (induced by the electric field in the depletion layer) that depolarizes nuclei with quadrupole moments such as Ga and As.
4:45 PM - FF5.6
Spin-precession Induced Voltage in a Co/InAs/Al Hybrid Structure Incorporating a Low-dimensional Electron System.
Andreas Wittmann 1 2 , Bernhard Botters 1 , Sebastian Neusser 1 , Christian Heyn 2 , Wolfgang Hansen 2 , Dirk Grundler 1
1 Physik-Department, Technische Universitaet Muenchen, Garching b. Muenchen Germany, 2 Institut fuer Angewandte Physik, Universitaet Hamburg, Hamburg, Hamburg, Germany
Show AbstractDynamic spin pumping due to precessing spins in a ferromagnetic thin film is of great current interest in spintronics. In multilayer structures consisting of magnetic and nonmagnetic metals the spin pumping has recently been observed via additional damping of the ferromagnetic resonance (FMR) [1] and an FMR-induced dc voltage drop across the interfaces.[2] The so-called spin battery, originally proposed for ferromagnet/semiconductor hybrid structures, [3] has so far been realized only in all-metal devices, e.g., Pt/permalloy/Al devices.[2]We report on InAs/Co/Al hybrid structures specifically designed to perform spin pumping experiments on a semiconductor. For this, we cleaved a modulation-doped InAs/InGaAs heterostructure in vacuum and deposited a thin Co layer on the cleaved facet. By this overgrowth technique, we generated an ohmic contact between the Co layer and the two-dimensional electron system (2DES) formed in a 4 nm wide InAs quantum well (for details on the heterostructure and preparation see Refs. [4,5]). A further contact to the Co was introduced via an additional Al layer. We mounted this InAs(2DES)/Co/Al hybrid device on a coplanar waveguide [6] and excited the spin precession in the Co layer in the GHz frequency regime at room temperature. We observed a dc voltage drop of a few 10 nV when the microwave source was in resonance with the FMR of the Co. Interestingly, the sign of the induced voltage depended on the strength of the applied field. The sign changed when we increased the field beyond the anisotropy field of the Co layer. To our knowledge, such a characteristic sign change at an anisotropy field has not been reported so far for the all-metal devices and provides further insight into the mechanism of spin precession induced effects in spintronic hybrid devices.References:[1] B. Heinrich et al., Phys. Rev. Lett. 90, 187601 (2003).[2] M.V. Costache et al., Phys. Rev. Lett. 97, 216603 (2006).[3] A. Brataas et al., Phys. Rev. B 66, 060404(R) (2002).[4] D. Grundler, Phys. Rev. Lett. 84, 6074 (2000).[5] M. Hoener et al., J. Appl. Phys. 99, 036102(R) (2006).[6] J. Podbielski et al., Phys. Rev. Lett. 96, 167207 (2006).
5:00 PM - FF5.7
Optical Pumping in Bulk Silicon Studied by Spin-resolved Low-energy Photoemission Spectroscopy.
Frederic Roux 1 , Simon Holzberger 1 , Georges Lampel 1 , Yves Lassailly 1 , Jacques Peretti 1
1 Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique - CNRS, Palaiseau France
Show AbstractOptical orientation of electron spin in semiconductors has been evidenced for the first time in silicon 40 years ago by observing the induced enhancement of 29Si nuclei polarization under circularly polarized light illumination [1]. Since that time, optical pumping and related techniques (such as polarized photoluminescence) have furnished a great number of results on spin dynamics in direct bandgap semi-conductors and on spin injection in semiconductor-based devices [2]. In silicon, although the electron spin relaxation time is known to be large, the situation is not so favourable because of the indirect band gap structure, the weak spin-orbit interaction and above all, because of the large value of the carrier lifetime relative to the spin relaxation time. In particular, this makes luminescence techniques hardly usable. Thus, very few experimental results have been obtained [3].Here, we present very recent results on optical orientation of electron spin in bulk silicon obtained by spin-resolved low-energy photo-emission spectroscopy. In these experiments, p-type silicon is activated to negative electronic affinity (NEA) by co-adsorption of caesium and oxygen. In this situation [4], electrons excited under optical pumping conditions, i.e. with near-band-gap circularly polarized light, can be emitted into vacuum where their energy and spin-polarization can be analyzed. We have measured both the energy distribution curves and the polarization distribution curves of the photo-emitted electrons measured for different excitation energies ranging from 1.17 eV, which gives access to optical transitions at the indirect gap Eg = 1.12 eV, up to 3.53 eV, which is slightly larger than the direct gap E1 ≈ 3.30 eV. In the photoemission process, the relevant time scale is given by the escape time which may be significantly shorter than the spin relaxation time. In particular, we will show that for excitation energies very close to the direct gap E1, the spin-polarization of hot electrons is positive and reaches values close to the maximum initial polarization theoretically predicted. For electrons emitted below the L minimum of the conduction band, the spin-polarization can be either positive or negative depending on the excitation energy and on the temperature.These results demonstrate that spin-resolved low-energy photoemission spectroscopy can be a useful tool for the study of the physics of conduction electron spin in silicon.[1] G. Lampel, Phys. Rev. Lett. 20, 491 (1968).[2] I. Zutic, J. Fabian, S. Das Sarma, Review of Modern Physics 76, 323 (2004).[3] B.T. Jonker, et al., Nature Physics 3, 542 (2007).[4] Y. Lassailly, P. Chiaradia and G. Lampel, Phys. Rev. B 41, 1266 (1990).
5:15 PM - FF5.8
Room Temperature Ferromagnetism in GaMnN Spin Electronic 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 AbstractFerromagnetic semiconductor devices utilizing both the charge and spin of electrons can be used to process and store information. The most studied ferromagnetic semiconductor material is GaMnAs that has a cryogenic Curie temperature (Tc) of ~ 170 K. However, 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 magnetic and electrical properties of metal-organic chemical vapor deposition (MOCVD) grown GaMnN heterostructures and devices. Magnetic and electrical properties were studied by Alternating Gradient Magnetometer (AGM) and Hall measurement, respectively. The GaMnN layer on top of n-GaN is almost paramagnetic, however, on p-type (AlGaN/GaN) strained-layer superlattices templates show strong saturation magnetization (Ms). Ms increases with the number of periods (N) of SLSs and saturates for N≥15 (0.25μm). Photolithography and inductively coupled plasma dry etching were used to realize room temperature spin electronic devices from these heterostructures. AGM measurement on these devices shows the decrease in saturation magnetization by manipulating the hole-concentration. These current controlled room temperature ferromagnetic devices would have application in non-volatile memory storage.
5:30 PM - FF5.9
Four Stable Magnetization States Formed in the Single Layer of GaMnAs Ferromagnetic Film.
Sanghoon Lee 1 , SunJae Chung 1 , X. Liu 2 , Jacek Furdyna 2
1 Physics, Korea University, Seoul Korea (the Republic of), 2 Physics, University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractThe process of magnetization reversal in ferromagnetic GaMnAs epilayers has been systematically investigated using the planar Hall effect (PHE). Interestingly, we have observed a pronounced asymmetry in the PHE hysteresis when the range of the field scan is restricted to fields below the final magnetization transition. The observed behavior indicates that multidomain structures are formed as magnetization undergoes a reorientation and the domain landscape formed in this way remains stable even after the magnetic field is switched off. Such stable multidomain states result in four stable planar Hall resistance (PHR) states at zero magnetic field. The PHR can be systematically controlled by changing the direction and the scanning range of the applied magnetic field. The possibility of a quaternary memory device based on the observed four PHR states is demonstrated by obtaining consistent results in the writing process using appropriate sequences of magnetic field pulses.
5:45 PM - FF5.10
Structure of High Curie Temperature Ge1-xMnx Nanocolumns: A Combined Theoretical and Experimental Investigation.
Emmanuel Arras 1 , Ivetta Slipukhina 1 , Pascal Pochet 1 , Matthieu Jamet 1 , Thibaut Devillers 1 , Andre Barski 1 , Pascale Bayle-Guillemaud 1 , Vincent Favre-Nicolin 1 , Thierry Deutsch 1 , Mauro Rovezzi 2 , Francesco d'Acapito 2 , Samuel Tardif 3 , Salia Cherifi 3 , Joel Cibert 3
1 INAC, CEA, Grenoble France, 2 CNR-INFM-OGG GILDA CRG, ESRF, Grenoble France, 3 Institut Néel, CNRS-Université Joseph Fourier, Grenoble France
Show AbstractHigh Curie temperature (> 400K) has been recently reported for GeMn alloys grown by low temperature molecular beam epitaxy [1]. These GeMn samples are composed of Mn-rich nanostructures (nano-columns) embedded in a nearly pure germanium matrix. The composition of these Mn-rich nano-columns is close to Ge2Mn, and their crystalline structure is related to the diamond structure of germanium, but the detail of the structure is still puzzling. Our recent Extended X-Ray Absorption Fine Structure (EXAFS) study reveals that these nano-columns show a complex local structure that does not correspond to any known stable GeMn compound [2]. Electronic structure calculations give information about the magnetic and structural properties of a possible Ge1-xMnx compound which may form inside the nano-columns. The calculated total magnetization, magnetic ordering, and Curie temperature are compared to experimental results [3]. Moreover the results of additional specific characterization tools (EXAFS, X-ray absorption near edge structure, transmission electron microscopy, X-ray magnetic circular dichroism and X-ray diffraction) are compared to their computed counterpart based on this possible Ge1-xMnx compound.[1] M. Jamet, A. Barski, T. Devillers, V. Poydenot, R. Dujardin, P. Bayle-Guillemaud, J. Rothman, E. Bellet-Amalric, A. Marty, J. Cibert, R. Mattana, and S. Tatarenko, Nat. Mater. 5, 653 (2006). [2] M. Rovezzi, T. Devillers, E. Arras, F. d’Acapito, A. Barski, M. Jamet, and P. Pochet, Appl. Phys. Lett. 92, 242510 (2008). [3] T. Devillers, M. Jamet, A. Barski, V. Poydenot, P. Bayle-Guillemaud, E. Bellet-Amalric, S. Cherifi, and J. Cibert, Phys. Rev. B 76, 205306 (2007).
FF6: Poster Session: Spintronics Materials
Session Chairs
Thursday AM, April 16, 2009
Salon Level (Marriott)
9:00 PM - FF6.1
Ferromagnetism and Near-infrared Luminescence in Nd-doped GaN via Diffusion.
Melvyn Luen 1 , Neeraj Nepal 1 , John Zavada 1 , Salah Bedair 1 , Ei Ei Brown 3 , Uwe Hommerich 3 , Pavel Frajtag 2 , Nadia El-Masry 2
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Department of Physics, Hampton University, Hampton, Virginia, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractRare-earth (Nd, Er) 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. Until recently, transition-metal-doped GaN has been considered for magnetic properties with observable ferromagnetism in Mn-doped GaN with the Curie temperature above room temperature. Recently, rare-earth doped GaN also has demonstrated above room temperature ferromagnetism. Simultaneously wide-bandgap semiconductors, like GaN, are suitable hosts for the near-infrared (1364nm) emission from Nd3+ ions corresponding to the low-dispersion wavelength and near-infrared (1540nm) emission from Er3+ corresponding to the low-loss wavelength for silica-based optical fiber communications, respectively. In this study, we report the diffusion of neodymium and erbium into undoped, Mg-doped and Si-doped GaN templates that were prepared by MOCVD on (0001) sapphire substrates. Ferromagnetic properties show a preference for undoped and n-type GaN. The diffusion coefficient has been measured via secondary ion mass spectroscopy (SIMS), and room temperature ferromagnetic properties were measured using an alternating gradient magnetometer (AGM). The photoluminescence (PL) emission of the Nd-diffused GaN is observable in the near-infrared (NIR) region of the spectrum via a Nd:YAG laser and InGaAs detector. The mechanisms for above room temperature ferromagnetism and NIR emission intensity, related to the Nd concentration, will be discussed.
9:00 PM - FF6.10
Cr Moment Coupling in Cr-containing Diamond-like Carbon Thin Films.
Juan Colon 1 , Varshni Singh 2 , Vadin Palshin 2 , Andre Petukhov 3 , Yaroslav Losovyj 2 , Andrei Sokolov 1 , Peter Dowben 1 , Ihor Ketsman 1
1 Physics & Astromomy, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 2 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rougue, Louisiana, United States, 3 Physics, South Dakota School of Mines, Rapid City, South Dakota, United States
Show AbstractInvestigation into the structural and magnetic properties of the Cr-doped hydrogenated diamond-like carbon (Cr-DLC ) and chromium carbide (Cr2C3) hydrogenated diamond-like carbon alloy thin films has showed promising results for magnetic and spintronics application, even at room temperature. The films were deposited on silicon substrates by a hybrid plasma-assisted PVD/CVD process. Extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge spectroscopy (XANES) and superconducting quantum interference device (SQUID) magnetometry were employed for studies of the local structure and magnetic properties of the films in dependence of the chromium concentration and temperature. At low concentrations, Cr forms a substitutional solid-solution compound substituting for carbon in a diamond-like amorphous matrix. In this concentration range, magnetization curves indicate ferromagnetic behavior of the films at low temperatures (10 K) but are more indicative of some sort of compensated magnetism at higher temperatures. Towards the upper end of the concentration range (~20 at%) Cr precipitates in the form of chromium carbide nanoclasters. A successful heterojunction device was constructed using n-type silicon substrate with deposited chromium containing DLC film in a wide range of Cr concentrations. While at low chromium contents the properties of the heterojunction device are dominated by a high capacitance, for the films of higher Cr concentration (11-20 at%), I-V measurements reveal a large coefficient (50-100%) of negative magneto-resistance at room temperature dependent on the direct bias voltage and the forward bias current increasing with magnetic field. This negative magnetoresistance of I-V curves , which can be ascribed to the uncompensated spins at the surface of the antiferromagnetic chromium carbide clusters, suggests the possibility of using the material for spin-electronics applications.
9:00 PM - FF6.12
Electrical Properties of (Fe, Co):Si nanowires
Tae-Eon Park 1 , Han-Kyu Seong 1 , Hwangyou Oh 1 , Joonyeon Chang 2 , Suk-Hee Han 2 , Heon-Jin Choi 1
1 Department of Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of), 2 Center for Spintronics Research, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractThe perspective of simultaneously manipulating the charge and spin of a single semiconductor medium leads to the exciting area of spintronics. Among the most promising candidates for such applications is the diluted magnetic semiconductors (DMSs), which transforms spin-frustrated semiconductors to ferromagnets by magnetic doping. Most of the studies thus far have focused on the DMSs in III-V semiconductors, however, those semiconductors are not compatible with current Si-based complementary metal-oxide-semiconductor (CMOS) processing. In this regard, Si-based spintronics is attractive though Si-based magnetic materials have not been developed yet. In this work, we investigated the electrical properties of single crystalline transition metal, M, doped M1-xSix (M : Fe or Co, x = 0 ~ 4) and MSi-silicide nanowires (NWs). M1-xSix (M : Fe or Co, x = 0 ~ 4) and MSi-silicide, and MSi-silicide/Si longitudinal heterostructure NWs were synthesized by transporting metal sources onto the as-grown Si nanowires at 600 ~ 800 °C. To determine the electrical properties of the NWs, NW-based field effect transistors (FETs) were fabricated. In case of transition metal doped (Fe, Co): Si NWs, the current versus the gate bias voltage curves (I – VG) and temperature dependent resistivity showed semiconductor behaviors and the mobility decreased with doping concentration. In case of MSi-silicide NWs, the resistivity increased with temperature and then decreased. Aiming injection and detection of a spin-polarized current in Si devices, the electrical properties these NWs will be discussed.
9:00 PM - FF6.14
Studies of Structural, Electrical and Vibrational Properties of Delafossite CuFeO2 at Ambient Temperature.
Shojan Pavunny 1 , Ashok Kumar 1 , Ram Katiyar 1
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractSingle phase delafossite p-type CuFeO2 (CFO) semiconductor was synthesized by modified solid state reaction technique. X-ray diffraction (XRD) and X-ray photo spectroscopy (XPS) studies suggest pure phase of CFO and Energy dispersive X-ray spectroscopy (EDX) also revealed that the atomic ratio Cu and Fe is 1:1. The well defined layered granular microstructure was observed from the Secondary Electron Microscopy (SEM). It also revealed the presence of pores in microstructure due to low sintering temperature. The XPS spectra showed two intense Cu 2p3/2 and 2p1/2 peaks at 932.5 eV and 952 eV and two Fe 2p3/2 and 2p1/2 peaks at 710 eV and 725 eV suggesting Cu and Fe ions are in +1 and +3 state with high spin S=5/2. The turn on field which is the macroscopic field needed to get an emission current of 9 nA was calculated as 5.72 Volts/µm. The frequency dependent conductivity follows Arrhenious relation near room temperature with activation energy ~ 0.048 eV. Moreover the conductivity increases with increase in temperature indicating p type semiconduting behavior (room temperature conductivity ~ 0.01 Scm-1). Although single phase compound was prepared and analyzed by XRD and XPS, we obtained weak antiferromagnetic hysteresis loop of CFO at room temperature with very weak coercive field ~ 132 Oe and a remnant magnetization ~ 0.0017 emu). This weak magnetization may be due to impurities/carrier mediated vacancies/ weak canted magnetic dipole. The room temperature Raman spectra of CFO displayed two main strong active modes at 351 cm-1 and 692 cm-1 that matched with other delaffosite structure. Temperature dependent Raman spectra indicate that the lowest mode vanished or overdammped at ~ 400 K where as higher modes shifted to lower frequency side with significantly decreased in intensity. We have also observed a low frequency (E2 low) mode at 79 cm-1 using 532nm (<5MHz line width) laser line. The line width and intensity of the lowest mode indicates temperature independent behavior.
9:00 PM - FF6.15
Effect of Doping Density, Growth Temperature and Partial Oxygen Pressure on Structural, Electrical, and Magnetic Properties of Cobalt Doped Indium Oxide Thin Films.
A. Ghosh 1 , N. Ukah 1 , Ram Gupta 1 , P. Kahol 1 , K. Ghosh 1
1 Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri, United States
Show AbstractDilute magnetic semiconductors are ferromagnetic semiconductors recently discovered in nitride and oxide semiconductors by incorporating a small percentage of magnetic atoms into the semiconductors host. Recently it is reported that the structural and electrical properties of pure indium oxide can be modify by growth parameters. In this paper we investigate magneto-transport properties of Co-doped In2O3 dilute magnetic semiconductors thin films grown on sapphire substrates using pulsed laser deposition technique. The effect of doping density, growth temperature, and partial oxygen pressure on structural, electrical, optical, and magnetic properties was discussed in details. The crystallinity of the films largely depends on growth temperature. Magneto-transport properties such as temperature dependent resistivity and magneto-resistance were found to be very sensitive to the micro-structural properties such as crystalinity as well as oxygen defect. Temperature dependent resistivity measurements show semiconducting and metallic behavior depending on growth parameters. Some films show transition from semiconducting to metallic behavior also. The electrical carrier density of the films depends on oxygen pressure and a change of 103 orders of magnitude is observed. We observe 0.8% magneto-resistance in these films. Depending on growth parameters, both positive and negative magneto-resistance is observed. Detailed temperature dependent resistivity and magneto-resistance data will be presented.
9:00 PM - FF6.16
Synthesis and Characterization of Manganese Silicide Nanowires.
Hana Yoon 1 , Kwanyong Seo 1 , Juneho In 1 , Bongsoo Kim 1
1 Chemistry, KAIST, Daejeon Korea (the Republic of)
Show AbstractTransition metal silicides can be used as ohmic or schottky barrier contacts and low resistance interconnects in electronic devices. Compared with other metal silicides, chemical compositions of manganese silicides can vary more flexibly and thus manganese silicides are expected to possess richer physical properties. For example, MnSi~1.7 is semiconducting and a promising material for silicon-based optoelectronic devices as well as a thermoelectric material operating at high temperatures. Mn5Si3 and MnSi compounds are metallic in bulk and MnSi is ferromagnetic with a Curie temperature of 30 K, which may find application in the developing field of “spintronics”. Among the various manganese silicides, MnSi has attracted great interest because it continues to provide surprise in the area of metal physics. We report here synthesis of single-crystalline MnSi nanowires via a simple vapor transport method. Free-standing MnSi nanowires are grown on a Si substrate with no catalyst. We have studied the crystal structure of the MnSi nanowires. Transmission electron microscopy studies confirm the single crystalline nature of the simple cubic MnSi nanowires. The typical size of the MnSi nanowire is several micrometers in length and 70-150 nm in diameter.
9:00 PM - FF6.2
Synthesis and Characterization of Cr-doped In2O3 Diluted Magnetic Semiconductor Films.
N. Ukah 1 , A. Ghosh 1 , Ram Gupta 1 , P. Kahol 1 , K. Ghosh 1
1 Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri, United States
Show AbstractThin films of Cr-doped indium oxide were grown on sapphire substrate under different partial oxygen pressures using pulsed laser deposition (PLD) technique. The effect of partial oxygen pressure on the structural, electrical, and magneto-transport properties was investigated. X-ray diffraction study reveals significant shifts to lower angles as well as a reduction in the full width at half maximum of the (222) diffraction peaks as the oxygen growth pressure increased from 1 × 10-5 mbar to 1 × 10-2 mbar. The optical band gap of the films was observed to decrease with increase in oxygen pressure - traceable to the significant improvement in crystal quality. An analysis of the ordinary component of the Hall effect revealed a decrease in the films’ conductivity with increase in oxygen growth pressure, brought about by the reduction in oxygen vacancies. Temperature dependent resistivity measurements showed that dc electrical transport in the films grown at low oxygen pressure was governed by two different transport mechanisms. Anomalous Hall effect – a signature of ferromagnetic ordering was observed in our films up to 300 K. Detailed study based on effect of partial oxygen pressure on structural and optoelectrical properties will be presented.
9:00 PM - FF6.3
Effects of Cu-doping on the Spin-lattice Interaction in Colossal Magnetoresistance Manganites: La1-xPbxMnO3.
Tze Chien Sum 1 , Ali Sucipto Tan 1 , Bang Chuan Zhao 1 , Lixiu Guan 1 , Diyar Talbayev 2 , Lan Wang 1 , Antoinette Taylor 2 , Cheng Hon Alfred Huan 1
1 Division of Physics and Applied Physics, Nanyang Technological University, Singapore Singapore, 2 Center for Integrated Nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractColossal magnetoresistance manganites have been subjected to extensive research due to their unique magnetic and electrical properties. The physical origin of this phenomenon has been attributed to the double-exchange model. However, this double-exchange mechanism of carriers between Mn2+ and Mn3+ ions alone cannot account for all the observed changes such as metal-insulator transition in these materials. Other factors include the highly correlated nature of the spin, lattice, charge and orbital degrees of freedom. Ultrafast optical techniques are powerful probes for investigating the quasi-particle and spin relaxation dynamics in these strongly correlated systems. Recently, Zhao et. al. [1] reported a re-entrant metal insulator (M-I) transition in single crystals of Cu-doped manganites La1-xPbxMnO3 (x = 0.14). The re-entrant M-I transition at low temperatures was observed in samples with Cu-doping (y ≥ 0.02) and was attributed to the charge carrier localization due to lattice distortion caused by Cu doping at Mn sites. However, there have been no reports on the effects of Cu-doping on the spin-lattice interaction in these materials. In this work, we investigate the spin-lattice relaxation and photo-induced magnetization dynamics in single crystals of La1-xPbxMn1-y-zCuyO3 (x = 0.14, y = 0, 0.01, 0.02, 0.04, 0.06 and z = 0.02, 0.08, 0.11, 0.17, 0.20) that were grown by the flux melt technique. The substitution of Mn sites with a transitional element Cu allows one to study the critical role of Mn ions in CMR materials. Transient reflection spectroscopy and time-resolved magneto-optical kerr spectroscopy have been employed to probe the quasi-particle relaxation and the photo-induced dynamics of magnetization [2, 3]. Through comparing these results, the charge and spin dynamics were independently investigated. The origins of the re-entrant M-I transition in this novel system were also discussed in light of these new findings. References:[1] B. C. Zhao, W. H. Song, Y. Q. Ma, R. Ang, S. B. Zhang and Y. P. Sun, Phys. Rev. B 72 132401 (2005)[2] R. D. Averitt and A. J. Taylor, J. Phys.: Condens. Matter 14 R1357 – R1390 (2002)[3] T. Ogasawara, K. Ohgushi, Y. Tomioka, K. S. Takahashi, H. Okamoto, M. Kawasaki and Y. Tokura, Phys. Rev. Lett. 94 087202 (2005)
9:00 PM - FF6.4
Novel Room-temperature Ferromagnetic Semiconductors Pb1-x-yCaxCryTe.
Evgeny Skipetrov 1 , Elena Zvereva 1 , Nikolay Pichugin 1 , Alexey Primenko 1 , Evgeny Slyn'ko 2 , Vasily Slyn'ko 2
1 Faculty of Physics, Moscow State University, Moscow Russian Federation, 2 , Institute of Material Science Problems, Chernovtsy Ukraine
Show AbstractFerromagnetic semiconductors (FMSs), which combine semiconductor properties and ferromagnetism, are in focus of a great interest in recent years because of their important applications in the field of spintronics. One of the major challenges is to obtain a FMS with the Curie temperature TC exceeding the room temperature. The room-temperature ferromagnetism has been revealed recently in Zn1-xCrxTe (TC up to 300 K) [1] and Pb1-x-yGexCryTe (TC up to 280 K) [2]. Here we report on the magnetic properties of novel diluted magnetic semiconductors Pb1-x-yCaxCryTe, which discovers a ferromagnetic ordering at temperatures higher the room temperature. The single crystals Pb1-x-yCaxCryTe(0.06≤x≤0.11, y≤0.06) have been grown by Bridgman method. The temperature and magnetic filed dependencies of magnetization have been measured using the vibrating sample magnetometer EG & G PARC M155 (T=5-330 K, B≤0.5 T). Magnetization curves display a clear hysteresis loop over the whole temperature range investigated. With increasing Cr concentration magnetic saturation moment increases rapidly and achieves 3.0 emu/g, the coercive force passes through the maximum (0.13 T at 5 K) and decreases. The temperature dependence of the magnetization M(T) is rather concave type with a broaden maximum at temperatures about 140 K for all samples under study. The amplitude of this maximum decreases with decreasing the magnetic field applied. On the temperature dependencies of residual magnetization Mr(T) this maximum disappears and we observed a plateau at lower temperatures. The Curie temperature TC was obtained by extrapolating of the linear part of M2(T) dependence to its temperature intercept. It was found that TC weakly depends on the alloy composition and its maximal value achieves 345 K. We have observed also the difference between ZFC and FC magnetization curves for the alloys with low chromium concentration up to relatively high blocking temperature Tb≈120 K that may indicate the existence of spinglass phase at low temperatures. Both with an increase in Cr concentration and in magnetic field Tb shifts towards the lower temperature and finally ZFC curve merges with FC one. Possible mechanisms of ferromagnetic ordering were discussed including: i) direct exchange interaction in the frame of the mean-field model; ii) superexchange interaction between chromium ions [3]; iii) exchange, resulting from the interaction of the band states and the localized spins and from the hybridization of the band states with impurity states [4]; iv) clustering effect.[1] H. Saito, V. Zayets, S. Yamagata, K. Ando, Phys. Rev. Lett. 90, 207202 (2003); [2] E.P. Skipetrov, F.A. Pakpour, L.A. Skipetrova, N.A. Pichugin, V.E. Slyn’ko, Inst. Phys. Conf. Ser. 187, 27 (2006);[3] J. Blinowski, P. Kacman and J.A. Majewski, Phys. Rev. B 53, 9524 (1996); [4] V.K. Dugaev, V.I. Litvinov, J. Barnas, A.H. Slobodskyy, W. Dobrowolski and M. Vieira, J. Supercond.: Incorporated Novel Magnetism 16, 67 (2003).
9:00 PM - FF6.5
Spin Polarization of Electrons Injected from Fe into GaAs Quantum Well Characterized using Oblique Hanle Effect.
Eiji Wada 1 , Mitsuru Itoh 1 , Tomoyasu Taniyama 1 2 , Masahito Yamaguchi 3
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 PRESTO, Japan Science and Technology Agency, Tokyo Japan, 3 Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya Japan
Show AbstractSpin injection into semiconductor is a crucial issue for realizing spintronic devices. A number of studies on spin injection have been reported and spin-LED structures are receiving much attention since the structure enables us to estimate the spin polarization of electrons injected into semiconductor using an optical means. In this study, we report on spin injection from an Fe layer into a GaAs quantum well (QW) structure, where the spin polarization of the electrons is estimated using oblique Hanle effect in low magnetic fields.A typical spin-LED structure used is Fe/n-Al0.1Ga0.9As/i-GaAs QW/p-Al0.3Ga0.7As with an Au capping layer. Spin polarized electrons are injection from the Fe layer into the GaAs quantum well electrically. The spin polarization of the electrons was estimated using a combination of electroluminescence (EL) and oblique Hanle effect, which can circumvent the inevitable application of large magnetic field of ~20 kOe to saturate the magnetization of Fe: spin precession about an oblique magnetic field induces the effective spin polarization of electrons normal to the layer plane, allowing spin injection into a quantum well even in a low magnetic field of ~1 kOe. An EL spectrum from a 20-nm-thick QW of a spin-LED structure was measured at 30 K. The spectrum consists of four peaks at 1.48, 1.50, 1.53, and 1.54 eV. The peaks at 1.48 and 1.50 eV are identified as emission from the GaAs buffer layer by comparing with an EL spectrum from a reference sample without a QW, while the other two peaks arise from emission from the GaAs quantum well. Since the EL peak at 1.54 eV is the most prominent, we hereafter focus on the field dependence of the circular polarization of EL peak at 1.54 eV. The field dependence of the circular polarization is well fitted with a Hanle curve1, indicating that the circular polarization at 1.54 eV is originating from the Hanle effect of spin polarized electrons injected from the Fe layer. The spin polarization estimated, however, is rather low, i.e., 5.3%. This suggests that the spin polarization of Fe at the Fe/n-Al0.1Ga0.9As interface becomes low due to a modification of the electronic structure at the interface. Another cause is the low spin injection efficiency across the Schottky barrier at the Fe/n-Al0.1Ga0.9As interface, presumably due to spin relaxation at the interface, in the AlGaAs just underneath the Fe layer, or in the GaAs QW. It should also be noted that magnetic circular dichroism (MCD) effect in the Fe layer is negligibly small in this EL spectrum since circular polarization of photoluminescence (PL) measured at the same time does not show any magnetic field dependence, in contrast to the clear field dependence of the EL spectrum. We will discuss the detail of the origin of the low spin polarization.[1] V.F.Motsnyi, P. Van Dorpe, W.Van Roy, E Goovaerts, V.I.Safarov, G.Borghs, and J. De Boeck, Phys. Rev. B 68, 245319 (2003).
9:00 PM - FF6.6
Physical Properties of Co-Mn-Si Films on SOI Prepared by Silicidation Process.
Hwanwoong Cho 1 , Jin Bae Kim 1 , Taewoon Eom 1 , Nguyen Dai 1 , Young-Pak Lee 1 , Seongjae Lee 1 , Moongyu Jang 2
1 Physics, Hanyang University, Seoul Korea (the Republic of), 2 , ETRI, Daejeon Korea (the Republic of)
Show AbstractDas-Datta spin transistor has attracted a lot of attention for the possible application as a next-generation switching device in the field of spintronics because it has the same architecture as the conventional field effect transistor with an electric field controlling spin degree of freedom for switching. One of the desirable device scheme based on the silicon technology is the SB-MOSFET(Schottky Barrier Metal-Oxide-Semiconductor Field-Effect Transistor) where the source/drain metallic electrodes are composed of a ferromagnetic material. Co2MnSi is a promising candidate for this scheme because it is predicted to be a half-metal with Tc ~ 900K to have 100 % spin polarization at room temperature. We have measured the structural, magnetic, transport properties of Co-Mn-Si films prepared on SOI wafers by two different silicidation processes: rapid thermal annealing (RTA) at 650 °C for 3 and 5 min. X-ray analysis revealed that the RTA at 650 °C for 5 min produced a single phase Co2MnSi film while the RTA at 650 °C for 3 min produced a multi-phase film composed mainly of CoSi2 and Co2MnSi. For the single phase Co2MnSi films, we obtained the saturated magnetization 4.1 μB/f.u. and the residual resistivity 8.5 μΩ-cm at 4.2 K, which are close to the values of single crystal Co2MnSi. Also, the absence of the low-field magnetoresistance (MR) for the single phase film in contrast to the hysteretic MR for the multi-phase film suggests that our single phase Co2MnSi film has a single crystal-like quality.
9:00 PM - FF6.7
Room-Temperature Ferromagnetism of Undoped and Co-Doped TiO2-δ Nanobelts Synthesized by Metallorganic Chemical Vapor Deposition.
Nguyen Thi Quynh Hoa 1 , Young-Soo Park 1 , Zonghoon Lee 2 , Velimir Radmilovic 2 , Eui-Tae Kim 1
1 Department of Materials Science & Engineering, Chungnam National University, Daejeon Korea (the Republic of), 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractOne dimensional (1-D) TiO2 diluted magnetic semiconductor (DMS) nanostructures are one of intense interest for fundamental research and promising spintronic device applications. Among potential DMS materials, transition metal-doped TiO2 has been extensively studied as one of the most promising candidates since cobalt (Co)-doped TiO2 (Ti1-xCoxO2) was shown to be ferromagnetic with TC up to 400 K. Moreover, the origin of the room-temperature ferromagnetism (RTFM) of Ti1-xCoxO2 DMS is the most important issue, but, whether it is due to an intrinsic or an extrinsic effect (transition-metal clusters) remains controversial. Recently, the room-temperature ferromagnetism was observed in undoped TiO2 thin film by utilizing defect states. However, litter information is available on the synthesis and characteristics of 1-D TiO2 and Ti1-xCoxO2 nanostructures. In this study, we mainly focus on the room-temperature ferromagnetic (RTFM) properties of undoped and lightly Co-doped TiO2-δ nanobelts (x<2 at.%). The nanobelts were synthesized without using any metal catalysts by metallorganic chemical vapor deposition. The nanobelts consisted of ~10-20 nm size nanocrystallites, which were dominantly rutile structure. Both undoped and Co-doped TiO2-δ nanobelts showed ferromagnetic behaviour at room temperature. We will further discuss the growth mechanism and the origin of the RTFM of undoped and Co-doped TiO2-δ nanobelts.
9:00 PM - FF6.8
Tunable Spin-dependent Transport in La0.7Sr0.3MnO3/n-Si Heterojunction
Cheong Chong 1 , Daniel Hsu 2 , Jauyn Grace Lin 2 , Li-Chyong Chen 2 , Kuei-Hsien Chen 3 , Yang-Fang Chen 1
1 Physics, National Taiwan University , Taipei Taiwan, 2 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 3 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractIn very recent years, intensive studies on the colossal magnetoresistance (CMR) manganites based p-n junction have opened up a new way to utilize this correlated electron system for multifunctional device applications. While most of the demonstrations are based on the oxide-based substrates, the transport properties for the CMR/Si heterojunction is less reported. Silicon manufacturing is a well established IC compatible technology, it’s highly rewarding to exploit the potential in integrating these spintronics materials with the Si-based technology. On the other hand, the well defined band diagram of Si will help to understand the transport mechanism in these heterojunctions, which is still not fully understood, thus far. In this work, heterostructure La0.7Sr0.3MnO3/n-Si were fabricated using pulsed laser deposition method. Transport mechanism has been studied by temperature-dependent magnetoresistance (MR) measurement. Our works reveal that the MR behavior in this heterojunction strongly depends on magnetic field, external bias and temperature. MR can be tuned from negative to positive by increasing the bias (Vb = 0.8 -3.0 V) in the temperature range from 125 K to 30 K. Meanwhile, crossover from the positive to negative MR can be achieved by applying higher magnetic field (H = 0.2-0.5 T). We propose that the creation of the minority spin down carriers at the interface and the intrinsic bulk properties of LSMO are the origins for the tunable MR transport. By modulating the Fermi level in Si by Vb, one can “select” either spin up or spin down at the Fermi surface in the depletion region. While increasing the magnetic field (0 - 0.2 T), spin scattering due to the anti-parallel spin configuration will lead to positive MR. Simultaneously, further increase of the magnetic field will enhance the intrinsic negative MR of LSMO. Based on this proposed scenario, room temperature tunable MR could be realized by finely tuning the carrier concentration in LSMO and Si. Competition between the intrinsic MR transport and the interface transport will eventually allow us to tune the MR in La0.7Sr0.3MnO3/n-Si heterojunction.
9:00 PM - FF6.9
Magnetic Properties of Cobalt Nanodeposits Created by Focused Electron Beam Induced Deposition.
Amalio Fernandez-Pacheco 1 2 , Jose De Teresa 2 , Rosa Cordoba 1 , Ricardo Ibarra 1 2 , Dorothee Petit 3 , Dan Read 3 , Liam O'Brien 3 , Huang Zeng 3 , Emma Lewis 3 , Russel Cowburn 3
1 Instituto de Nanociencia de Aragón, University of Zaragoza, Zaragoza, Aragón, Spain, 2 Instituto de Ciencias de Materiales de Aragón, University of Zaragoza, Zaragoza, Aragón, Spain, 3 Nanoscale Magnetics Group, Department of Physics, Blackett Laboratory, Imperial College London, London United Kingdom
Show AbstractThe local deposition of materials by means of a focused electron or ion beam in presence of a precursor material (FEBID and FIBID, respectively) is an interesting way to grow nanostructures1. The beam decomposes the adsorbed molecules as it is scanned on the surface, inducing a localized deposit. This simplicity is the main advantage with respect to other more well-established lithographic techniques, where several steps, usually involving resist, are required. Regarding the deposition of magnetic nanostructures by these techniques, results in the literature can be found for iron2, and cobalt 3,4. As the deposits are usually formed from an organo-metallic precursor, the fraction of carbon present becomes an important parameter which can dramatically alter the quality of the deposited material. Thus, the magnetic characterization of these materials is an essential issue.In this contribution we characterize magnetically Co nanowires (NWs) created by FEBID by means of magnetotransport measurements5, as well as by spatially resolved magneto-optical Kerr effect (MOKE). The high magnetic character found in the measurements is a consequence of the high purity of the grown cobalt (around 90% atomic), as found by EDX and HRTEM measurements. In the case of the electrical transport, the resistivity-versus temperature curves are fully metallic. The Hall effect presents anomalous and ordinary contribution. The magnetoresistance in different geometries are studied as a function of temperature. The saturation magnetization inferred from such measurements is 1329±20 emu/cm3.The switching properties of single FEBID Co nanostructures have been studied by creating L-shaped NWs for several aspect ratios, by means of MOKE. By applying different in-plane magnetic field sequences, the magnetization of the structure is initialized in a different magnetic configuration, measuring in some cases the domain wall propagation, and in others the nucleation field. Clear differences for both fields have been observed in all the NWs studied. This, together with the advantages in simplicity and flexibility of this technique with respect to others to create nanometric structures, makes this material a good candidate to be used in fields such as magnetic logics based on domain wall propagation6, or Spintronics.References[1] I. Utke, P. Hoffmann, and John Melngailis, J. Vac. Sci. Technol. B, Vol. 26, No. 4, Jul/Aug 2008[2] M. Takeguchi, M. Shimojo , R. Che, K. Furuya, J. Mater Sci 41 (2006) 2627–2630[3] G. Boero, I. Utke, T. Bret, N. Quack, M. Todorova, S. Mouaziz, P. Kejik, J. Brugger, R.S. Popovic, and P. Hoffmann, Appl. Phys. Lett. 86, 042503 (2005)[4] Y. M. Lau, P. C. Chee, J. T. L. Thong, and V. Ng,, J. Vac. Sci. Technol. A 20.4., Jul/Aug 2002[5] A. Fernández-Pacheco, J.M. De Teresa, R. Córdoba, and M.R. Ibarra, submitted to J. Phys. D: Appl. Phys.[6] D. A. Alwood, G. Xiong, C.C. Faulkner, D. Atkinson, D. Petit, R.P. Cowburn, Science 309, 1688 (2005)
Symposium Organizers
Stefano Sanvito Trinity College
Olle G. Heinonen Seagate Technology
Valentin Alek Dediu ISMN-CNR
Nick Rizzo Freescale Semiconductor
FF9: Organic and Molecular Spintronics
Session Chairs
Thursday PM, April 16, 2009
Room 3007 (Moscone West)
2:30 PM - **FF9.1
Spin Tunneling and Transport Studies in Organic Semiconductors.
Jagadeesh Moodera 1
1 Francis Bitter Magnet Lab, MIT, Cambridge, Massachusetts, United States
Show Abstract Spin transport in molecular organic semiconductors (OS) is an area that has become active in recent years due to its high potential for new fundamental science and possible technological application. Electrical transport in OS is complicated and is just beginning to be investigated and understood. Adding spin degree of freedom to this is bound to create further challenges. However, in addition, the possibility for a bottom-up approach to chemically tune and build an all OS that can not only act as a spin source/detector but also is one of the transporting medium is certainly one of the main attractions to explore the field. In the past we have shown successful spin transport with ultrathin layers of tris(8-hydroxyquinolinato)aluminum (Alq3) and Rubrene by spin polarized tunneling. In magnetic tunnel junctions with these OSs as tunnel barriers, finite tunnel magnetoresistance (TMR) has been measured at room temperature. Using Meservey-Tedrow technique, with a superconducting spin detector, direct measurement of tunnel current spin polarization through these OS layers was carried out. With the proper interfacial engineering OS shows that elastic tunneling characteristics are enhanced, showing reduced defects and thereby greatly improving the spin transport. The highly encouraging result is the large spin diffusion length (λs) in amorphous Rubrene measured directly by spin polarized tunneling. This is in contrast to traditional semiconductors such as amorphous Si and Ge where no spin information is retained. Further interfacial morphology and barrier properties are being investigated by other sensitive techniques such as neutron reflectivity and inelastic tunneling spectroscopy. Reducing the defect density, improving the carrier mobility in say single crystals of rubrene and epitaxial thin films of Cu pthalocyanine (CuPc) one can expect λs to be much higher and could lead to the development of organic spintronics. We will present and discuss our research in this area. Work done in collaboration with Karthik V. Raman, Jenny Shim, Carina Faber and Geetha Berera, supported by KIST-MIT project fund and ONR grant.1. T. S. Santos, J. S. Lee, P. Migdal, I. C. Lekshmi, B. Satpati, and J. S. Moodera, Phys. Rev. Lett. 98, 016601(2007) 2. Shim, K.V. Raman, Y.J. Park, T.S. Santos, G.X. Miao, B. Satpaty and J. S. Moodera, Phys. Rev. Lett. 100, 226603 (2008)
3:00 PM - **FF9.2
Spintronics with Organic Semiconductors.
Luis Hueso 1 , Ilaria Bergenti 2 , Alberto Riminucci 2 , Valentin Dediu 2
1 , CIC nanoGUNE, San Sebastian Spain, 2 ISMN, CNR, Bologna Italy
Show AbstractSpin electronics, in its aim to compete with conventional electronics, is always looking for new and more performing materials. Conjugated organic semiconductors could represent a breakthrough in the discipline, mainly due to their weak spin-orbit scattering and their chemical flexibility. Continuous effort in organic spintronics has led to the realization, for example, of vertical organic devices with different organic semiconductor layers and of organic tunnel barriers. However, there is still a lack of understanding on the mechanism that governs spin injection and transport in organics, leading to general disagreement even on the expected sign of the devices output magnetoresistance. In this talk, I will briefly review the development of the field of organic spintronics since its birth less than a decade ago. I will outline the main experimental results and their connections with areas such as light emitting diodes and electronic memory devices. In the final part, I will discuss the current challenges in the field and possible strategies to overcome them. In particular, I will concentrate on recent advances on surface characterization and on techniques to improve the performance of vertical ferromagnetic/organic vertical devices.
3:30 PM - FF9.3
Investigation of an Origin of Large Magnetoresistance in Molecular Nano-composite Spin Devices.
Masashi Shiraishi 1 2 , Daiki Hatanaka 1 , Ryo Nouchi 1 , Takayuki Nozaki 1 , Teruya Shinjo 1 , Yoshishige Suzuki 1
1 , Osaka University, Toyonaka Japan, 2 , JST-PRESTO, Kawaguchi Japan
Show AbstractObservation of a resistance hysteresis and a magnetoresistance (MR) effect induced by a magnetization alignment of ferromagnetic nano-particles in molecular nano-composites and inorganic granular spin devices provides a good platform for discussing interesting physics in spin-dependent tunneling, single charge (spin) transport and an induced large MR effect. However, underlying physics of the large MR ratio which exceeds a theoretically predicted value has not been conclusive yet, although it is believed that the CT and the CB effects may play an important role. It was reported that the CB effect induced the enhancement of the MR ratio and its oscillation for an applied bias voltage, which is due to spin accumulation in ferromagnetic particles [1]. The CT-induced enhancement of an MR effect within a Coulomb gap was explained by a theoretical model (Takahashi-Maekawa model [2]) where the enhancement was formulated within a second order effect of the CT and the MR ratio should be flat in the Coulomb gap. We have reported the large MR ratio of 78% in rubrene-Co nano-composite spin devices [3], and have tried to clarify the detailed mechanism of the enhancement by using investigating spin echo signals [4]. During the course of this study, we have found that unprecedented enhancement of the MR ratio that is ascribed to a higher order (at maximum, 5th order) CT effect, which has not been reported in previous studies [5].References:[1]K. Yakushiji et al., Nature Mat. 2005. [2]S. Takahashi and S. Maekawa, Phys. Rev. Lett. 1998. [3]H. Kusai, M. Shiraishi et al., Chem. Phys. Lett. 2007. [4]M. Shiraishi et al., Appl. Phys. Lett. 2008. [5]D. Hatanaka, M. Shiraishi et al., submitted.
3:45 PM - FF9.4
Magnetic Nanoclusters Synthesized by a New Gas-stabilised Aggregation Technique.
Ovidiu Crisan 1 2 , K. von Haeften 2 , C. Binns 2 , A. Ellis 3
1 , National Institute for Materials Physics, Bucharest-Magurele Romania, 2 Consensed Matter Physics, University of Leicester, Leicester United Kingdom, 3 Department of Chemistry, University of Leicester, Leicester United Kingdom
Show AbstractNanoscale organized materials for different purposes, such as nano-electronics, biomedicine, or magnetic applications are extensively studied. Novel nanoscale architectures are now engineered by using molecule-by-molecule or atom-by-atom assemblies, into building blocks for nanoscale devices, with a variety of properties, within the so-called “bottom-up” approach. A novel method for production of free clusters as building blocks for nanoscale devices is presented. These clusters may be subsequently functionalized in-situ by adding atoms/molecules of different nature, on the surface of readily formed clusters. The method uses a cold beam of rare gas (Ar) molecules and clusters, passing through a region of low-pressure atomic vapour, in an ultra-high-vacuum multiple chamber facility. The atoms are collected and condensed by the rare gas beam in the pick-up zone, and a large variety of very small cold clusters, metals, oxides, molecules, etc. are thus formed. We prove that the cluster size is extremely well controlled by the vapour pressure of the picked-up species. The method is versatile, since it allows multiple pick-up processes within the same rare gas cluster for producing, for example metal/oxide core-shell nanoparticles that are furthermore functionalized by attaching to the surface of the cluster, various molecules such as pentacene, for applications in nanoelectronics and aptamers, nucleotides, antibodies, for applications in biomedicine. Initial formation of Fe gas-stabilised clusters, Fe/Fe oxide core-shell nanoparticles, their structure and morphology, are presented and discussed.
4:30 PM - **FF9.5
Single-molecule Spintronics: Three-terminal Transport through Individual Magnetic Molecules.
Herre van der Zant 1
1 , Kavli Institute of Nanoscience, Delft Netherlands
Show AbstractWith electromigration, we create nanogaps in thin gold wires. From solution magnetic molecules are trapped and at low temperature transport measurements are used to identify the single-molecule properties in the Coulomb blockade regime. We find that metal complexes containing a single Mn2+ atom are in the low-spin state when contacted by the gold elctrodes and that spin interactions between the metal core and charges on the ligands lead to an interesting interplay between singlet and triplet states. In the bulk, on the other hand the neutral complexes are in the high-spin state as evidenced by magnetic susceptibility measurements. Possibly, charging of the molecule in the electromigrated junction induces the transition from high to low spin. A different class of molecules is formed by the single-molecule magnets, which have a high-spin ground state. In a Fe-4 compound, we observe features that confirm the presence of a high-spin state including Kondo behaviour for S>1/2 and zero-field splitting. Work done in collaboration with Kasper Moth-Poulsen, Jens Paaske, Karsten Flensberg, Per Hedegård, Jesper Bendix and Thomas Bjørnholm from Copenhagen, Maarten Wegewijs from Jülich/Aachen and Andrea Cornia from Modena.
5:00 PM - **FF9.6
Ab initio Simulations of the Transport Properties of Mn12 Based Spin-devices.
Sri Chaitanya Das Pemmaraju 1 , Ivan Rungger 1 , Stefano Sanvito 1
1 School of Physics and CRANN, Trinity College, Dublin, IRELAND, Dublin, Dublin, Ireland
Show AbstractSingle-molecule magnets (SMMs) represent a unique playground for fundamental quantum physics and exhibit exotic phenomena such as magnetic hysteresis as well as magnetization reversal through quantum tunneling [1]. Recently, transport measurements on Mn12 based molecular magnets in single-molecule transitor devices have been realized [2]. In this work we present ab initio transport calculations of Mn12 molecules functionalized by thioether groups and sandwiched between gold contacts. We find the transport properties of these SMMs to be dominated by tunneling type behaviour across the organic functional groups and asymmetric coupling to the leads. We observe asymmetric I-V curves under positive and negative bias. In addition we demonstrate that the I-V characteristic changes upon changing the internal magnetic state of the molecule, suggesting that electrical single-spin detection is indeed a possibility from a detailed knowledge of the I-V. References: [1] D. Gatteschi and R. Sessoli, Agnew. Chem., Int. Ed. 42, 268 (2003) [2] H. B. Heersche et. al, Phys. Rev. Lett, 96, 206801 (2006)
5:30 PM - FF9.7
Planar-type Spin-valves Based on Organic Single Crystals with Ferromagnetic Electrodes.
Hirokazu Tada 1 , Tomonori Ikegami 1 , Iwao Kawayama 2 , Masayoshi Tonouchi 2
1 Division of Materials Physics, Osaka University, Toyonaka Japan, 2 Institute of Laser Engineering, Osaka University, Suita Japan
Show AbstractConsiderable attention has recently been paid to the study of the active control and manipulation of spin degrees of freedom in organic materials because of their potential applications to spintronics devices. In the present study, we have studied spin injection and transport properties in organic single crystals utilizing planar-type sandwich structures with half metal electrodes, La0.67Sr0.33MnO3 (LSMO). The LSMO films with the thickness of approximately 100 nm were grown epitaxially on SrTiO3 by pulsed laser beam deposition. The LSMO electrodes with a spacing in the range from 100 to 300 nm were prepared by electron-beam lithography and dry etching. Single crystals of organic semiconductors such as pentacene and bis(l,2,5-thiadiazolo)-p-quinobis(l,3-dithiole) (BTQBT) were placed on the electrodes and fixed using a silver paste. The magneto-resistance (MR) of the device was measured at the temperature varied from 5 K to 300 K under the magnetic field up to 4 kOe. The devices showed clear spin-valve behaviors with a MR ratio up to 20 % at 5K. It was found that the materials having high carrier mobility values showed a large MR ratio. The MR ratio varied depending upon the gap spacing of the electrodes, temperature and bias voltage applied.
5:45 PM - FF9.8
Role of Interfacial States in Alq3-based Organic Spin Valves.
Ilaria Bergenti 1 , Valentin Dediu 1 , Alberto Riminucci 1 , Mirko Prezioso 1 , Francesco Borgatti 1 , Patrizio Graziosi 1 , Francesca Casoli 2 , Donald MacLaren 3 , John Chapman 3
1 , ISMN CNR, Bologna Italy, 2 , IMEM-CNR, Parma Italy, 3 Dept. of Physics and Astronomy, University of Glasgow, Glasgow United Kingdom
Show AbstractApplications of organic semiconductors into spintronic domain, motivated by unquestionable advantage of weak spin-orbit coupling, have been recently successfully demonstrated; in particular magnetoresistive spin valves (SV) and magnetic tunnel junction (MTJ) based on archetypal organic materials Alq3 have been experimentally obtained[1-3].In spite of the different mechanism, injection/transport for SV and tunnelling for MTJ, performances of such devices are affected not just by the properties of its constituent but also by their metal/organic interfaces.Considerable interest has been devoted to the investigation of Alq3 based vertical spin valves containing half metallic manganite (LSMO) as bottom electrode and Cobalt as the top one.While the deposition of Alq3 on LSMO give rise to a well defined interface which energetics has been investigated [4], it has been claimed by several authors that the Co/alq3 interface is intrinsically “ill defined” [1] and transport properties are strongly affected by the Co clusters that can be found inside the organic material down to 50 nm from the surface.The introduction of a thin (1-2 nm) Al2O3 barrier between the Alq3 and the Co results in a sharper definition of the metal/organic interface. From X-ray reflectivity measurements, the intermixing region at the interface between Alq3 and Co can be estimated to be around 2.5 nm, while the direct interface suffers from a thicker degradation layer. These results have been confirmed by cross Sectional TEM and High Energy Photoemission Spectroscopy (HAXPES) measurements, which indicate that the tunnel barrier prevents also the reaction of Co with the organic molecules. Scarce control of interfacial layer for the direct deposition of Co on top of Alq3 gives rise to a variety of spurious effect on the device operation such as high closure field and not uniform and defined thickness of organic layer. In this latter condition, the estimation of length over which coherent spin transport is achieved is hardly reproducible. We present a full characterization of LSMO/Alq3 and Alq3/Co interfaces in presence of buffer insulating layer and we correlate the results with the operation of spin valve device. The presented results clearly indicate that extension of hybrid spintronic devices to practical application requires the engineering of interfaces between organic and the spin polarized materials.References[1] Z.H. Xiong, D.Wu, Z.V Vardeny, J.Shi Nature , 427, 821(2004)[2] T. S Santos, et al. Phys. Rev. Lett. 98, 016601 (2007).[3]V. Dediu et al., Phys. Rev. B, 78, 115203, (2008).[4]Y. Q. Zhan et al., Phys. Rev. B,76, 45406 (2007)
FF10: Poster Session: Spintronics Transport and Imaging
Session Chairs
Friday AM, April 17, 2009
Salon Level (Marriott)
9:00 PM - FF10.1
Resonant Electronic States and I-V Curves of Fe/MgO/Fe(100) Tunnel Junctions.
Stefano Sanvito 1 , Ivan Rungger 1
1 School of Physics, Trinity College, Dublin Ireland
Show AbstractUsing the ab initio code SMEAGOL [1] we calculate the electronic transport properties of Fe/MgO/Fe(100) tunnel junctions for applied biases up to 2 Volt, and for barrier thicknesses of 4 and 8 MgO monolayers. We show that the correct bias-dependent occupation of the interface states in the Fe/MgO junction is crucial in order to obtain a physically meaningful potential drop and current. The I-V characteristics are mainly determined by the bias-dependent position in energy of interface states and of the Fe delta_1 band-edges. For a 4 MgO monolayers barrier the low bias region is characterized by a non-monotonic behavior of the TMR, which is caused by resonant scattering across the barrier due to the highly localized interface states. In the high bias region (V > 0.4 Volt) the TMR decreases monotonically and eventually becomes negative. This is mainly due to Fe minority spin band edges of the states with high transmission moving closer to and finally entering the bias window. Random disorder can suppress the transport through sharp interface states, and results in a monotonic decay of the TMR with bias for all voltages. The same behavior is also found when the thickness of the MgO barrier is increased to 8 monolayers. Finally we show that oxygen vacancies inside the MgO barrier quench the TMR if they are within the first few layers from the interface.[1] A.R. Rocha, V.M. Garcia-Suarez, S.W. Bailey, C.J. Lambert, J. Ferrer and S. Sanvito, Phys. Rev. B. 73, 085414 (2006)
9:00 PM - FF10.11
Single Wall Carbon Nanotube Field Effect Transistors in a Ferrofluidic Environment.
K. Narayan 1 , Shruti Badhwar 1
1 , JNCASR, Bangalore India
Show AbstractSingle Wall Carbon Nanotube Field Effect Transistors (SWNTFETs) provide a unique platform to probe optical1-2, dielectric and chemical processes in the vicinity of the nanotube by closely following and studying the subtle changes in the FET characteristics. We present the changes in SWNTFET characteristics upon introduction of well dispersed magnetite based ferrofluid. The magnetic nanoparticles get organized around the nanotube upon drying. A clear change in the state of the nanotube, in form of a gate independence of the drain-source current, is observed beyond a threshold level of the magnetite concentration. This metallic-type response of SWNT in the proximity of magnetic particles is speculated to arise from spin interactions across the interface.References1)Control of single-wall-nanotube field effect transistors via indirect long-range optically induced processes, Narayan K.S, Rao M, Zhang R, Maniar P, Applied Physics Letters,88, 243507 (2006).2)Semiconducting polymer coated single wall nanotube field effect transistors discriminate holes from electrons, J.S. Chawla, D. Gupta, K.S Narayan, Applied Physics Letters, 91,043510(2007).
9:00 PM - FF10.12
Morphologic, Compositional and Magnetic Characterization of Sputtered CoCr Thin Films for Applications in MTJs as Hard Spin Injectors
Paola Martino 1 , Alessandro Chiolerio 1 , Paolo Pandolfi 1 , Paola Tiberto 1 , Paolo Allia 1
1 Physics, Politecnico di Torino, Turin Italy
Show AbstractSpin-polarized currents across an insulating tunnel barrier, needed for the development of efficient magnetic tunneling junctions (MTJs), may be obtained using hard spin injector electrodes [1]. Thin films of CoCr solid solutions have been fabricated involving two main steps: 1) deposition of Co/Cr alternated layers via RF magnetron sputtering and 2) diffusion and field cooling treatment, realized in sequence keeping the sample in the high vacuum chamber, and instrumental to the increase of the sample coercivity [2].Both silicon (100) substrates and thermally oxidized wafers have been used in order to better characterize the residual anisotropy given by conformal growth on the monocrystalline material or by free growth on the amorphous silica. 1 cm2 wide multilayers have been realized, respecting the structure (Co10/Cr2)3Co10 where the subscript represents the thickness in nm and the superscript the number of repetitions. The thermal treatment was done in a partial Ar pressure of 1 mTorr (base vacuum ~10-7 Torr) at 450°C for 1 hour, cooling the sample in a uniform field generated by an electromagnet (600 Oe field strength). A detailed numerical correlation of field effect SEM images and EDX micromaps was used to evaluate the oxygen diffusion on the magnetic film, while an alternating gradient force magnetometer (AGFM) allowed us to evaluate at room temperature both coercivity and magnetic bias obtained after the field cooling treatment. The effect of standard thermal treatment on the homogeneity of the films is discussed, and a possible alternative heating technique is proposed.[1] J.S. Moodera and G. Mathon, J. Magn. Magn. Mater. 200 (1999) 248;[2] Y.C. Feng, D.E. Laughlin and D.N. Lambeth, IEEE Trans. Mat. 30 (1994) 3948.
9:00 PM - FF10.13
Real-time, in-situ Resistance Measurements for Determining the Evolving Properties of Ultrathin Magnesium Layers Being Grown in a RF Sputtering Apparatus.
Alessandro Chiolerio 1 , Paolo Pandolfi 1 , Paola Martino 1 , Paolo Allia 1
1 Physics, Politecnico di Torino, Turin Italy
Show AbstractWe report on real-time resistance measurements performed in-situ during RF-sputtering deposition of a metallic layer. It is shown that an accurate control of the substrate sheet resistance allows one to detect and follow both the early deposition stages (nucleation of metallic islands on the substrate) and the ensuing process of uniform film growth after island coalescence. The measurement setup, based on a 4-point contact probe with a delimited area exposed to the plasma is demonstrated during the growth of a ultrathin layer of pure Mg, which is subsequently oxidized through an ozone treatment to give rise to an efficient MgO barrier for use in magnetic tunnel junctions (MTJs) [1]. The growing interest towards the MgO barrier is related to the high TMR values predicted for MTJ containing this interlayer [2]. However, the overall quality of a MTJ is extremely sensitive to thickness, stoichiometry, homogeneity and degree of oxidation of the tunneling barrier. .The measuring setup is comprised of optically lithographed copper electrodes deposited onto a thermally oxidized silicon substrate. The electrodes allow the sheet resistance to be measured across a wide area of 1x5 mm2. The electrical resistance continuously measured via a 4-point probe as a function of the time elapsed during deposition of the metallic layer is acquired by a computer with a low sample rate. The very first deposition step is shown to be affected by the substrate temperature, which results in a different behavior of the resistance decay from the high values of the clean substrate to the values corresponding to a non-percolative metallic layer. Subsequently, the R(t) curve is characterized by an hyperbolic behavior, which is explained by the Fuchs – Sondheimer (FS) thin-film conduction model. Fitting the experimental data to the FS model allows one to extract physical parameters, such as the mean free path in bulk magnesium and the deposition rate. The measurements are shown not to be affected by any interference of the RF signal. As a consequence, it may be proposed as a powerful tool aimed not only to characterize the process of thin film growth, but also to determine the onset of full percolation in ultrathin films as well as the actual deposition rate.[1] T. Dimopoulos, G. Gieres and J. Wecker, J. Appl. Phys., 98 (2005) 073705 [2] X.G. Zhang and W. H. Butler, J. Phys. : Condens. Matter, 15 (2003) R1603 – R1639
9:00 PM - FF10.14
Electronic Structure and Magnetic Properties of Ti-doped ZnO.
Soumia Lardjane 1 , Ghouti Merad 1 , Houda Imane Faraoun 1
1 LEPM-URMER, Physics Dpartment, A. Belkaid University, Tlemcen Algeria
Show AbstractRecent experiments suggest that Ti doped ZnO can be a promising room temperature dilute magnetic semiconductor (DMS) and a potentially useful material for spintronic devices. Furthermore, the fact that Ti doped ZnO shows ferromagnetic behaviour despite it contains no magnetic element makes this system good candidate for theoretical investigation regarding the controversies about the origin of ferromagnetic ordering in TM-doped ZnO. In this work, we calculate the electronic and magnetic structures of Ti-doped ZnO using the full potential linearized augmented plane wave (FPLAPW) method of the density functional theory (DFT) along with the local spin density approximation (LSDA) for the exchange correlation potential. Results are obtained for different doping concentrations and are used to discuss the Curie temperature of this system, the origin of the ferromagnetism, and the contribution of different atoms for magnetic moments.
9:00 PM - FF10.15
Ferromagnetic and Structural Response of Fe3Si Thin Films to Deposition Rate and Post-Deposition Annealing.
Siao Li Liew 1 , Hwee Leng D. Seng 1 , Hui Ru Tan 1 , Dongzhi Chi 1
1 , IMRE, Singapore Singapore
Show AbstractFerromagnetic Fe3Si on semiconducting substrates is attractive for spintronics applications due to matching lattice constants between Fe3Si and the substrate which leads to a sharp interface, enhancing its potential as a spin injection source. However non-epitaxial Fe3Si and its properties have not been reported widely so far though other ferromagnetic materials in polycrystalline form were studied for application in spintronic functionalities. We performed a comparative study of Fe3Si thin films formed on Si(001) by magnetron d.c. sputtering with low and high deposition rates. With low deposition rate used, the as-deposited Fe3Si film has a coercive field Hc of 5 Oe, squareness ratio Mr/Ms of 0.9 and saturation magnetization Ms of ~940 emu/cm3. On the other hand, the deposited Fe3Si film using high deposition rate has a Hc of 1 Oe and Mr/Ms of 0.1. Structural investigations revealed that a low sputtering rate deposition resulted in the formation of poly-crystalline Fe3Si in as-deposited films after room temperature deposition, while a high rate deposition produced amorphous Fe3Si in as-deposited films. No noticeable change in magnetic and structural properties with post-deposition annealing (up to 350 oC) was observed for the films formed with a low deposition rate, while a phase transformation from amorphous to poly-(or nano-) crystalline occurred at 350 oC, along with a drastic change in magnetic properties, in the films grown with high deposition rate – namely Hc increased to between 14 and 24 Oe while Ms reduced by 35%. These differences in structural and magnetic properties were attributed to the influence of deposition rate on the grain nucleation/growth and film structure evolution.
9:00 PM - FF10.2
Opto-electronic 1D Polaritronic Quantum Processor Based on Selective Long Range Two Spin Qubits Couplings Mediated by Quasi-0D/quasi-1D Virtual Excitonic Polaritons.
Jerome Tribollet 1
1 , Independent Researcher, Tours France
Show AbstractIn this presentation, I describe the theory of a selective long range coherent coupling between two nearest neighbor spin qubits being part of a 1D array of spin qubits contained in a 1D solid state photonic waveguide. This coupling is mediated by quasi-0D/quasi-1D waveguide excitonic polaritons virtually excited by a coherent radiation. The quasi-0D/quasi-1D excitonic polaritons are laterally confined by the 1D photonic waveguide structure, and longitudinally confined by means of local electric fields applied to the waveguide at positions corresponding to the middle of preselected nearest neighbor spin qubits pairs. The local electric fields being independently and reversibly controlled by electronic means, many two-qubits couplings can be switched on/off selectively and simultaneously. It is also described in this presentation, how those selective long range couplings between spin qubits can simplify the practical implementation of new solid state quantum processors with available technologies. ZnO-based 1D photonic nanostructures are identified as particularly appropriate candidate nanostructures for the practical implementation of a quantum processor device based on selective spin-spin couplings mediated by quasi-0D/quasi-1D excitonic polaritons.
9:00 PM - FF10.3
Imaging of the Evolution of Spin Reorientation Transition in a Glassy Co-Fe-Ta-B Film.
Tomoyasu Taniyama 1 2 , Mitsuru Itoh 1 , Parmanand Sharma 3 , Hisamichi Kimura 3 , Akihisa Inoue 3
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 PRESTO, Japan Science and Technology Agency, Tokyo Japan, 3 Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractSpin reorientation transition (SRT) in ultrathin ferromagnetic layers such as Ni/Cu(001) has attracted much attention due to its potential application to spintronic devices in ferromaget/semiconuctor heterostructures. It has been believed that SRT arises from a competition between interfacial anisotropy and magnetoelastic anisotropy, which favors in-plane magnetization and out-of-plane magnetization, respectively. Therefore, ultrathin layer structures are considered to be essential to observe SRT. A recent report, on the other hand, showed a temperature driven SRT even in Co-Fe-Ta-B glassy films with a thicknesses of sub-micrometers. In order to clarify the origin of the SRT, we report on direct observation of the temperature driven SRT in the Co-Fe-Ta-B glassy film using magnetic force microscopy (MFM) as a function of temperature. In fact, we clearly demonstrate a crossover of different magnetic domain structures across the SRT temperature. A 0.6-μm-thick Co-Fe-Ta-B glassy film was deposited on Si substrates by magnetron sputtering. Details are found in elsewhere[1]. Preliminary magnetization measurements showed that the in-plane magnetization at 5 K transformed into out-of-plane magnetization at 200 K, i.e., SRT occurs at 200 K. To demonstrate the temperature evolution of SRT, we performed imaging of magnetic domain structures using MFM with temperature control capability. The sample was initially saturated in-plane and transferred into the MFM chamber at room temperature. The initial magnetic domain structure at room temperature shows stripe domains with up and down out-of-plane magnetization orientation. The width of up and down domain stripes was 0.8 μm. As the sample was cooled down to 158 K, on the other hand, the clear stripes of the magnetic domain structures completely disappear and any magnetic contrasts are not seen at 158 K below the SRT. The results indicate that the magnetization of the Co-Fe-Ta-B glassy film lies in-plane at 158 K and no magnetic poles appear at the surface of the film. With increasing temperature, a slight magnetic contrast with a wider stripe width of 1.0 μm appears at 250 K. Of particular interest is that the magnetic stripes are not straight along the initial magnetic field any more and the domain structures are split into a number of small domains. The magnetic contrast becomes clearer with further increasing temperature and the width of the magnetic stripe is 0.9 μm at 300 K, where the magnetic domain structure is completely different from that of the initial one before cooling the film. The temperature evolution of the magnetic domain structure we observe is direct evidence of the SRT in a Co-Fe-Ta-B glassy film at around 200 K, in good agreement with the magnetic measurements reported previously. [1] P. Sharma et al., J. Appl. Phys. 100, 083902 (2006).
9:00 PM - FF10.4
Soft X-ray Imaging of Vortex Dynamics in Trilayer Patterned Magnetic Elements.
Brooke Mesler 1 2 , Dong-Hyun Kim 3 , Peter Fischer 1
1 CXRO, Lawrence Berkeley National Lab, Berkeley, California, United States, 2 Applied Science and Technology, UC Berkeley, Berkeley, California, United States, 3 Physics, Chungbuk National University, Cheongju Korea (the Republic of)
Show AbstractSoft X-ray microscopy provides element specific magnetic imaging with a spatial resolution down to 15nm. At XM-1, the full-field soft X-ray microscope at the Advanced Light Source in Berkeley, a stroboscopic pump and probe setup has been developed to study fast magnetization dynamics in ferromagnetic elements with a time resolution of 70ps which is set by the width of the X-ray pulses from the synchrotron. Previous studies of patterned permalloy elements have revealed complex magnetization dynamics. Results obtained with a 2µm x 4µm x 45nm rectangular permalloy sample exhibiting a seven domain Landau pattern reveal dynamics up to several nsec after the exciting magnetic field pulse. Domain wall motion, a gyrotropic vortex motion, and a coupling between vortices in the rectangular geometry are observed. On going studies of patterned trilayer elements, composed of magnetic permalloy and cobalt layers separated by a copper spacer layer, will probe the dynamics of the trilayer system. Of particular interest is observing how the coupling between the magnetic layers affects the vortex dynamics.
9:00 PM - FF10.5
Monte Carlo Study of the Spin Current in Magnetic Multilayers
K. Akabli 1 , H. Diep 1
1 Lab. Physique Theorique et Modelsation, UMR8089, University of Cergy-Pontoise, CNRS, Cergy-Pontoise France
Show AbstractThe spin current is known to depend on the orientation of the itinerant spins with respect to the lattice spins: if they are parallel then the current is strong, if not the spin current is very weak. This is the origin of the so-called giant magnetoresistance (GMR) discovered 20 years ago by Fert and Grunberg in magnetic multilayers and awarded with the Nobel prize in 2007. As far as the temperature dependence is concerned, it has been observed that the magnetic phase transition gives rise to an anomalous temperature-dependence of the electron resistivity in ferromagnetic crystals. Phenomenological theories based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. It has been theoretically shown earlier in 1958 by de Gennes and Friedel that the spin resistivity depends on the spin-spin correlation. However, experimentally various behaviours have been observed. These depend on the type of the material, i. e. the details of interaction between spins in the system. Using Monte Carlo simulations, we study the spin current across a sandwich composed of several magnetic thin films as a function of temperature. The itinerant spins interact between themselves and with the lattice localized spins. We use here the Ising spin model. We consider two cases: the in-plane (CIP) and perpendicular (CPP) configurations. In the CIP case, we show that the spin resistivity undergoes a huge peak at the phase transition temperature for a single ferromagnetic film, while it does not show such a peak in the antiferromagnetic film. These results can be interpreted using the structure of clusters formed in the critical region: instead of calculating the spin-spin correlation, we calculate the distribution of clusters in the critical region using the Hoshen-Kopelman algorithm. We show that the resistivity depends on the number and the size of clusters of opposite spins. Such an interpretation is more satisfactory than the one using the spin correlation function. In the CPP case, the effect of interfaces between the films is crucial: we show that the so-called spin accumulation at interfaces depends strongly on the interface parameters such as the interaction range, the sign of interaction and the presence of impurities. The spin current is very sensitive to the variation of these ingredients. Various physiqcal quantities will be shown and discussed. Comparison with experiments will be given.
9:00 PM - FF10.6
Giant Asymmetric Soft X-ray Magnetic Scattering Between Opposite Circular Polarizations Near the Brewster Angle.
Dae-Eun Jeong 1 , Sang-Koog Kim 1
1 Research Center for Spin Dynamics & Spin-Wave Devices and Nanospinics Laboratory, Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractSoft or hard x-ray resonant magnetic scattering (XRMS) has been widely used to probe and study charge, orbital, and spin degrees of freedom in multicomponent, ordered magnetic materials because it offers exceedingly enhanced, element-specific sensitivity to such different scattering sources interacting with incident x rays of energies close to the absorption edges of elements of interest. Due to a rich variety of microscopic interactions between incident photons and each of those scattering sources in a probed sample, as well as their angular and polarization dependences in the XRMS, the initial polarization state of incident photons can be converted to the different polarization states of the scattered photons with their angular variations. These alsp make it possible to determine element specific charge, orbital, and spin orderings by analyzing the changes of the angular dependent polarization states of the scattered soft x rays. Thus, the physical interpretation of the XRMS using the circular-polarization-mode basis is much more informative than the previously implemented linear polarization-mode basis to the fundamental understanding of interactions between photons and various scattering sources as well as their essential polarization and angular dependences. In this presentation, we report on the first observation of giant asymmetry in soft x-ray resonant reflectivities of the circular polarization components of incident p-polarized x rays from magnetic 3d transition-metal thin film, in proximity to the normal Brewster angle . This phenomenon is found to be caused by a totally destructive interference occurring selectively for either circular polarization mode of photons scattered individually from charge, orbital, and spin degrees of freedom. This work is studied not only by the theoretical derivation of the XRMS amplitudes and phases of individual scattering sources using the circular-polarization mode basis newly developed from this work, but also by the numerical calculations of the individual intensities of the RCP and LCP components using the circular-mode-based magneto-optical Kerr matrices [1,2]. Also, continuously variable polarization state, from the RCP, through the linear s polarization, to the LCP (or vice versa) is observed in a wide angle region around .This work was supported by Creative Research Initiatives (ReC-SDSW) of MEST/KOSEF.References*corresponding author,
[email protected][1] D. -E. Jeong, K. -S. Lee, and S. -K. Kim, J. Korean Phys. Soc. 46, 1180 (2005).[2] D. -E. Jeong and S. -K. Kim, Phys. Rev. B 78, 012412 (2008).
9:00 PM - FF10.7
Magnonic-Crystal Waveguides Applicable for Broad-Band Microwave Spin-Wave Filter
Ki-Suk Lee 1 , Sang-Koog Kim 1
1 Research Center for Spin Dynamics & Spin-Wave Devices and Nanospinics Laboratory, Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractOver the past decade, photonic crystals[1] have been a hot research subject in the field of condensed matter physics and optics, owing to their rich applications to optical nanodevices[2]. Meanwhile, in the research areas of nanomagnetism and spin dynamics, magnonic crystals(MCs) as the magnetic counterpart of the photonic crystal are also of growing interest because of its applications to spin-wave(SW) devices such as a SW filter[3]. Recently, many theoretical and experimental studies have been performed on various types of MCs including 1D structures[4] such as periodic multilayers as well as including 2D or 3D structures [5]. In such MC structures, the allowed and forbidden SW modes (magnons) are controllable by a variety of artificial periodic structures of magnetic media with different magnetic properties such as magnetic and geometric parameters. Despite such recent theoretical and experimental studies, there are few studies on simple planar structures of MC waveguides for practical applications.In this presentation, we propose a new type of MC waveguides of a simple-planar-patterned thin-film nanostrip in which magnonic bands and band gaps can be manipulated by means of the periodic modulation of different strip widths. In order to understand the physical origins of the allowed and forbidden bands in proposed nanostripe MCs and elucidate their relations to geometrical width modulation, we performed micromagnetic simulations and analytical calculations of SW propagations in nanostrip MCs. As a model system, we used 10 nm thick Py nanostrips of different width (24 and 30 nm) modulations with periodicity P ranging from 12 to 42 nm. From micromagnetic simulation results and their comparisons with analytic calculations, we have found that a new type of MCs of the periodic modulation of different strip widths yields complex DESW band structures and wide band gaps(~10GHz), originating from the coupling between two identical as well as between different modes. It has been found that the width, position, and the number of the magnonic band gaps are controllable by means of the periodicity and the motif of the periodic strip-width modulation. Moreover, we propose promisingly efficient MC waveguides that consist of serially connected segments of different-width-modulated nanostrips made of a single material, for narrow-band-pass SWs retaining ~10% intensity after having traveled distances as long as ~1 µm. This work paves the way for effective manipulation of magnonic bands and band gaps of SWs in the GHz range, providing a route for realizing a promisingly efficient SW filter using a simple planar-patterned thin-film structure from a single magnetic material. This work was supported by Creative Research Initiatives(ReC-SDSW) of MEST/KOSEF.*corresponding author,
[email protected][1]Yablonovitch,PRL58,2059(1987)[2]Engheta,Science317,1698(2007)[3]Choi et al.,PRL98,087205(2007)[4]Hillebrands,PRB37,9885(1988)[5]Vasseur et al.,PRB54,1043(1996)
9:00 PM - FF10.9
Controlling Vortex-core Oscillations by Spin-polarized Out-of-plane DC Current.
Youn-Seok Choi 1 , Ki-Suk Lee 1 , Sang-Koog Kim 1
1 Research Center for Spin Dynamics & Spin-Wave Devices and Nanospinics Laboratory, Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe spin-transfer torque (STT) of spin-polarized currents is a promisingly reliable means of switching the orientation of magnetizations in magnetic elements of submicron size (or smaller). Nonuniform magnetizations such as the magnetic vortex structure can also be excited by spin-polarized currents, and thus this study becomes emerging in the research fields of spin dynamics and nanomagnetism owing to the potential application of vortex oscillations to future solid-state information storage devices [1-4] as well as tunable frequency oscillators [5-7]. Very recently, resonant vortex gyrotropic motions and vortex-core reversals in confined nanodots excited by currents flowing in the direction normal to the dot plane have been demonstrated with nano-pillar multilayer structures that consist of a magnetic spin polarizer, a non-magnetic layer, and a free magnetic layer[8,9]. In the present work, we propose a generic control of vortex-core oscillations in a soft magnetic nanodot by spin-polarized out-of-plane dc current with extremely low current density flowing through a spin polarizer with perpendicular magnetization, by analytical calculation and micromagnetic simulation studies. The spin-transfer torque along with the accompanying Oersted field of the out-of-plane dc current together yield the nontrivial novel dynamic behaviors of vortex oscillations, providing a simple way for the generic control of the frequency and the orbital-radius amplitude of the vortex core oscillations. Here we will present some key parameters to control and determine the frequency and steady-state orbital motion of vortex-core oscillations in various vortex states having different vortex chirality and polarization.This work is supported by Creative Research Initiatives (ReC-SDSW) of MEST/KOSEF.References*corresponding author,
[email protected][1] K. Yamada et al., Nat. Mater. 6, 269 (2007). [2] S.-K. Kim et al., Appl. Phys. Lett. 91, 082506 (2007); K.-S. Lee et al., Phys. Rev. B 76, 174410 (2007).[3] S.-K. Kim et al., Appl. Phys. Lett. 92, 022509 (2008). [4] K.-S. Lee et al., Appl. Phys. Lett. 92, 192513 (2008); K.-S. Lee et al., Phys. Rev. B. 78, 014405 (2008).[5] V. S. Pribiag et al., Nat. Phys. 3, 498 (2007).[6] Q. Mistral et al., Phys. Rev. Lett. 100, 257201 (2008).[7] M. Bolte et al., Phys. Rev. Lett. 100, 176601 (2008).[8] J.-G. Caputo et al., Phys. Rev. Lett. 98, 056604 (2007). [9] B. A. Ivanov et al., Phys. Rev. Lett. 99, 247208 (2007).
Symposium Organizers
Stefano Sanvito Trinity College
Olle G. Heinonen Seagate Technology
Valentin Alek Dediu ISMN-CNR
Nick Rizzo Freescale Semiconductor
FF12: Multifunctional Materials and Devices
Session Chairs
Friday AM, April 17, 2009
Room 3007 (Moscone West)
11:30 AM - **FF12.1
Intrinsic and Artificial Multiferroics for Spintronics.
Agnes Barthelemy 1 , Manuel Bibes 1 , Helene Bea 1 3 , Vincent Garcia 2 1 , Martin Gajek 1 , Gustau Catalan 2 , Stephane Fusil 1 , Karim Bouzehouane 1 , Cyrile Deranlot 1 , Eric Jacquet 1 , Neil Mathur 2 , Patricja Paruch 3 , Josep Fontcuberta 4 , Jim Scott 2
1 , Unité Mixte de Physique CNRS/Thales, Palaiseau France, 3 , University of Geneva, Geneva Switzerland, 2 , University of Cambridge, Cambridge United Kingdom, 4 , Institut de Ciencia de Materiales, Barcelona Spain
Show AbstractMultiferroics are singular materials that can exhibit simultaneously electric and magnetic orders, with in some cases a coupling between them. As such, these compounds bring new functionalities to spintronics [1] and new device possibilities, such as multiple state memory elements or magnetic random access memories with electrical write operation. For practical purposes, the main problem of multiferroics is their scarcity. Notable examples in the perovskite family include the low temperature ferromagnetic ferroelectric material (La,Bi)MnO3 (LBMO), and the room temperature antiferromagnetic ferroelectric compound BiFeO3 (BFO). We have used 2-4 nm thick ferromagnetic LBMO layers as tunnel barriers in junctions with a La2/3Sr1/3MnO3 (LSMO) bottom electrode and a Au top electrode. In these junctions, the tunnel current is modulated by the ferromagnetic order parameter of the barrier material via the spin-filter effect, giving rise to a TMR of up to ~100% [2,3]. Remarkably, 2 nm-thick LBMO films are also ferroelectric [4]. Accordingly, the concomitance of ferromagnetism and ferroelectricity in a unique tunnel barrier allows obtaining a four level resistance state from the combination of the spin-filter effect and the influence of ferroelectricity on tunneling [4]. The interest of BFO is for the room-temperature control of magnetization by the application of an electric field. We will show that BFO films can be used to establish a robust exchange-bias effect on CoFeB, Co, NiFe layers [5]. The characterization of the magnetic and ferroelectric domain structure [6] of BFO films by neutron diffraction and piezoresponse force microscopy evidences the magnetoelectric coupling between the two orders. Remarkably, the exchange field correlates with the multiferroic domain size, as expected from Malozemoff's model of exchange bias [7] extended to multiferroics [8]. Perspectives for electric control of spintronics devices will be given.A way to overcome the scarcity of room-temperature multiferroic materials is to design artificial multiferroic architectures by combining ferroelectric and ferromagnetic materials. We will present experiments on heterostructures combining ferroelectric tunnel barriers and ferromagnetic electrodes and discuss their transport properties. \\ [1] M. Bibes and A. Barthélémy, IEEE Trans. Electron Dev. 54, 1003 (2007)[2] M. Gajek et al, J. Appl. Phys 97, 103909 (2005)[3] M. Gajek et al, Phys. Rev. B 72, 020406(R) (2005)[4] M. Gajek et al, Nature Materials 6, 296 (2007)[5] H. Béa et al, Appl. Phys. Lett. 89, 242114 (2006)[6] G. Catalan et al, Phys. Rev. Lett. 100, 027602 (2008)[7] A.P. Malozemoff, Phys. Rev. B 35, 3679 (1987)[8] H. Béa et al, Phys. Rev. Lett 100, 017204 (2008)
12:00 PM - FF12.2
Hybrid Spin-Filter / Magnetic Tunnel Junction Heterostructures with Room Temperature Ferrimagnetic Barrier Layers.
Brittany Nelson-Cheeseman 1 , Franklin Wong 1 , Rajesh Chopdekar 1 , Elke Arenholz 2 , Yuri Suzuki 1
1 Dept of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractMagnetic tunnel junctions (MTJ) with ferrimagnetic barrier layers exhibit both spin filtering and MTJ device characteristics, providing a promising means to enhance spin transport over conventional MTJs. In such all-magnetic devices, the magnetic coupling behavior at the electrode-barrier interfaces is crucial to successful device operation. We have investigated the interfacial magnetism of this novel device architecture in the presence of a barrier layer with long-range magnetic order at room temperature. Heterostructures of highly spin polarized La.7Sr.3MnO3 (LSMO) and Fe3O4 electrodes sandwiching an insulating NiFe2O4 (NFO) (TC = 850 K) barrier layer were grown by pulsed laser deposition on (110) SrTiO3 substrates. This heterostructure incorporates an isostructural spinel-spinel (NFO-Fe3O4) interface and a non-isostructural perovskite-spinel (LSMO-NFO) interface. X-ray diffraction in the 2θ-θ geometry shows only (110)-oriented peaks for both the perovskite and spinel crystal structures, indicating that the films grow epitaxially on one another with high crystallinity. Bulk-sensitive hysteresis loops taken at both 300K and 5K show sharp, independent magnetic switching of both the LSMO and Fe3O4 electrodes, creating well-defined parallel and antiparallel states despite the presence of a robust ferrimagnetic barrier layer between them. X-ray magnetic circular dichroism (XMCD) was used to probe the magnetic structure at each electrode-barrier interface with elemental specificity and surface sensitivity by total electron yield. Using various truncated heterostructures to probe the two different electrode-barrier interfaces separately, we have found independent magnetic switching between the Mn in LSMO and the Ni and Fe in NFO at the non-isostructural perovskite-spinel interface. However, we have found strong magnetic coupling and coincident switching of the magnetic cations across the isostructural spinel-spinel interface. The decoupled perovskite-spinel interface is crucial for independent electrode switching as observed in magnetotransport, while the isostructural spinel interface helps preserve interfacial spin polarization of Fe3O4, which has shown to be less robust when using non-isostructural barrier layers. The combination of a magnetically-coupled isostructural interface and a magnetically-decoupled non-isostructural interface is a key feature of this new hybrid device architecture whose room temperature operation is possible with NiFe2O4 barrier layers. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
12:15 PM - FF12.3
Electric-field Modulation of Magnetization in Multiferroic Devices.
Martin Gajek 1 , Lane Martin 2 , Ying Hao Chu 2 , Mikel Holcomb 1 2 , Ramamoorthy Ramesh 1 2
1 Department of Physics, University of California, Berkeley, Berkeley, California, United States, 2 Materials science, University of California, Berkeley, Berkeley, California, United States
Show AbstractThe control of magnetization exclusively by electrical means is one of the most important topics in spin electronics. A promising route towards achieving this goal is the use magnetoelectric multiferroics, where the magnetization and ferroelectric polarization are coupled, as pinning layers for ferromagnetic electrodes in devices such as spin valves or tunnel junctions. In this work, we will show how multiferroic BiFeO3 can be used as an electrically controllable pinning layer at room temperature and discuss some recent device schemes implemented in our group.
12:30 PM - FF12.4
Organic Semiconductors for Multifunctional Electronic Devices.
Alberto Riminucci 1 , Carol Newby 3 , Alek Dediu 1 , Ilaria Bergenti 1 , Luis Hueso 2 , Mirko Prezioso 1
1 , ISMN CNR, Bologna Italy, 3 Material Science and Engineering, Cornell University, Ithaca, New York, United States, 2 School of Physics and Astronomy, University of Leeds, Leeds United Kingdom
Show AbstractAn important step to further the development of Information Technology can be achieved with the novel paradigm of multifunctional electronic devices.We are studying the use of organic semiconductors (OSs) which are good materials for such devices due to both their spin transport and their electrical bistability properties.In particular, here we present results obtained both on Alq3 and Pentacene based spin valve (SV) devices. The former is one of the most used OS in OLED technology while the latter is known for its high mobility. The samples’ structure is La0.7Sr0.3MnO3 (LSMO)/organic semiconductor(s)(OS)/Al2O3/Co.Alq3 based samples showed both SV effect and a clear electrical resistance bistability effect, which makes them good canditates as memory devices. Pentacene based configurations showed a MR that was strikingly dependent on the applied voltage, with the SV effect turned on and off by a change of less than 20 mV in the applied voltage. These are therefore an example of multifunctional magnetic/electronic devices, which pave the way to 2 bits per cell memory units.Moreover, in pentacene based devices, we were able to measure a SV effect of 1% up to room temperature. The maximum SV MR was 9% measured at low temperature.
12:45 PM - FF12.5
Integrating Magnetic Nanostructures into Functional 3-D Devices
Vitali Metlushko 1 , Joshua Sautner 1 , Paolo Vavassori 2 3 , Bojan Ilic 4 , Marcos Grimsditch 5 , John Unguris 6
1 Electrical and Computer Engineering, University of Illinois at Chicago, Chicago, Illinois, United States, 2 , CIC nanoGUNE Consolider, San Sebastian Spain, 3 Dipartimento di Fisica, Università di Ferrara, CNISM, CNR-INFM S3, Ferrara Italy, 4 Cornell Nanofabrication Facility, School of App. and Eng. Physics, Cornell University, Ithaca, New York, United States, 5 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 6 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractMost of magnetic nano-structures today are ultrathin or nanostructured films and multilayers. The main challenge is to find a suitable technology to integrate and to contact nanostructures in a reliable manner. Here, we investigate the problem of contact integration into functional 3-D devices and evaluate the influence of 3-D magnetic layer geometry on performance of magneto-electronic devices. Real devices are truly 3-dimensional structures. Their topography must absolutely be taken into consideration during the design phase since their inherent non-planarity will profoundly affect their magnetization profile. Our initial results strongly indicate that the “non-flatness” of magnetic layer strongly influences the possible magnetic states, alters the switching mechanism and leads to totally new behavior, which was not observed in classic 2-D thin film magnetic structures. Our experimental results will be compared with detailed micromagnetic simulations.V.M. acknowledges support by the U.S. NSF, Grant ECCS-0823813