Symposium Organizers
Claudia Felser Johannes Gutenberg University of Mainz
Arunava Gupta The University of Alabama
Burkard Hillebrands Technische Universitaet Kaiserslautern
Sabine Wurmehl Eindhoven University of Technology
L1/C4: Joint Session: Magnetoelectric Coupling in Multiferroics
Session Chairs
Arunava Gupta
Darrell Schlom
Tuesday PM, December 02, 2008
Room 210 (Hynes)
9:30 AM - **L1.1/C4.1
New Routes to Electric-field Control of Magnetism.
Nicola Spaldin 1
1 Materials Department, University of California, Santa Barbara, California, United States
Show AbstractIn materials which exhibit magnetoelectricity, a magnetic field can be used to modify the electric polarization and an electric field to tune the magnetism. In addition to fundamental interest in controlling order parameters by fields other than their conjugate fields, the electric-field control of magnetism is technologically appealing because of its low power demands and potential for miniaturization. Here we review the fundamentals of the magnetoelectric effect and describe its origin in established magnetoelectric materials. We outline the difficulties associated with calculating magnetoelectric response, then propose some new routes to enhanced magnetoelectricity that have been identified using modern density functional methods.
10:00 AM - **L1.2/C4.2
Dynamical Magnetoelectric Effects in Oxide Multiferroics.
Yoshinori Tokura 1 2
1 Department of Applied Physics, University of Tokyo, Tokyo Japan, 2 ERATO Multiferroics, JST, Tokyo Japan
Show Abstract10:30 AM - L1.3/C4.3
Ferroelectric Control of Carrier Mediated Ferromagnetism in (Fe,Zn)3O4 with High Curie Temperature in Field Effect Transistor Structure.
Hidekazu Tanaka 1 , Junichi Takaobushi 1 , Teruo Kanki 1 , Tomoji Kawai 1
1 ISIR-Sanken, Osaka University, Osaka Japan
Show AbstractFe-based oxides with spinel structure exhibit ferri/ferromagnetism with very high Curie temperature (TC) exceeding 800K. The solid solution system of Fe3-xZnxO4 is one of the best materials for oxide semiconductor spintronics application with high spin polarization above room temperature from the point of view on the large anomalous Hall effect at room temperature, wide tunability on TC, carrier concentration and Density of State at Fermi level [1], [2], [3] with sustainability. There characteristics will lead to advanced spintronics application such as electric field control of ferromagnetism in field effect transistor (FET) structure above room temperature. We report that ferroelectric field control of ferromagnetism at room temperature was achieved in the field effect heterostructure combining the Fe2.5Zn0.5O4 (FZO) with ferroelectric PbZr0.2Ti0.8O3 (PZT). PZT thin film as gate insulator was fabricated using a pulsed-laser deposition (PLD) technique. Nb doped SrTiO3 (Nb-STO) (1wt%) (100) single crystal was used as a substrate and bottom electrode. FZO thin film as channel layer was fabricated on PZT / Nb-STO using a PLD technique. Oxygen atmosphere, PO2, was set to 1.0×10-3 Pa, and substrate temperature TS was 320°C during the deposition. Film thickness of FZO was fixed to 10 and 20 nm. The FZO channel resistance systematically was modulated by ferroelectric polarization through carrier modulation. The Magneto Optic Kerr Effect (MOKE: λ=670nm) confirmed the modulation of ferromagnetism at channel layer in FZO/ PZT FET structure (10nm thickness sample) after applying positive gate bias voltage of +15V (positive ferroelectric remnant polarization (Pr state)) and after applying negative gate bias voltage of -15V (negative Pr state) for ferroelectric gate layer, indicating large ferroelectric remnant polarization (40μC/cm2) modulates ferromagnetism of FZO. The magnetic coercive field (HC) are about 80 Oe in positive Pr state and about 50 Oe in negative Pr state. These results indicate that the predicted ferromagnetic double exchange interaction in spinel ferrite [4, 5] was modulated by ferroelectric electric field effect.References :[1] J. Takaobushi, H. Tanaka, T. Kawai et al, Appl. Phys. Lett., 89 (2006) 242507. [2] J. Takaobushi, S. Ueda, H. Tanaka, T. Kawai et al, Phys. Rev. B, 76 (2007) 205108 , [3] S. Ueda,J. Takaobushi, H. Tanaka, T. Kawai et al, Appl. Phys. Express,(2008) in press, [4] A. Rosencwaig, Phys. Rev. 181 (1969) 946, [5] J. Loos and P. Novák, Phys. Rev. B 66 (2002) 132403
10:45 AM - L1.4/C4.4
Charge-mediated Magnetoelectric Coupling in Complex Oxides with Competing Ground States.
Jason Hoffman 1 2 , Carlos Vaz 1 2 , Hajo Molegraaf 3 , Xia Hong 4 , Jean-Marc Triscone 3 , Charles Ahn 1 2
1 Applied Physics, Yale University, New Haven, Connecticut, United States, 2 Center for Research on Interface Structures and Phenomenon, Yale University, New Haven, Connecticut, United States, 3 DPMC, Université de Genève, Genève Switzerland, 4 Physics, Penn State University, State College, Pennsylvania, United States
Show Abstract Current efforts to exploit materials with multifunctional capabilities have rekindled interest in multiferroics, which display a coupling between magnetic and electric order parameters. Combining dissimilar magnetic and ferroelectric systems is one approach to optimize this coupling. In particular, large magnetoelectric coupling may be expected in strongly correlated magnetic materials exhibiting competing ground states. Here, we demonstrate charge-mediated magnetoelectric coupling in ferroelectric / lanthanum manganite heterostructures, including the electric field-controlled on/off switching of magnetism. In this work, off-axis RF magnetron sputtering was used to grow epitaxial ferroelectric Pb(Zr,Ti)O3 (PZT) / La0.8Sr0.2MnO3 (LSMO) heterostructures with high crystalline quality, atomically smooth surfaces, and low leakage current density. X-ray diffraction reveals c-axis oriented growth of PZT, with a root-mean-square (RMS) surface roughness of ~5Å. We employ magneto-optic Kerr effect (MOKE) magnetometry to directly probe the magnetic response as a function of the applied electric field, showing a large carrier-mediated magnetoelectric coupling, which can be understood within the double-exchange model. This experiment demonstrates a charge-based mechanism for the coupling between magnetic and electric order parameters in ferroelectric / Sr-doped lanthanum manganite heterostructures, which may permit the development of room temperature magnetoelectric devices with large coupling between electric and magnetic degrees of freedom.
11:00 AM - L1/C4:joint
BREAK
11:30 AM - L1.5/C4.5
Hybrid Multiferroic System: Non-Volatile Control of Ferromagnetism in (Ga,Mn)As Gated by Ferroelectric Polymer.
Igor Stolichnov 1 , Sebastian Riester 1 , Nava Setter 1 , Joe Trodahl 1 2 , Andrew Rushforth 3 , Kevin Edmonds 3 , Richard Campion 3 , Thomas Foxon 3 , Bryan Gallagher 3 , Tomas Jungwirth 4 3
1 , Ceramics Laboratory, EPFL-Swiss Federal Institute of Technology, Lausanne Switzerland, 2 , MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University, Wellington New Zealand, 3 , School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 4 , Institute of Physics ASCR, Praha Czechia
Show AbstractA new multicomponent multiferroic system where the ferromagnetism in the (Ga,Mn)As diluted magnetic semiconductor is modulated by polarization reversal in the ferroelectric gate has been successfully implemented. To overcome the apparent processing incompatibility between the magnetic semiconductor and gate ferroelectric we apply a ferroelectric co-polymer polyvinylidene fluoride with trifluoroethylene P(VDF-TrFE) characterized by relatively benign processing demands, in particular a low thermal budget. Use of (Ga,Mn)As in a multiferroic system is of particular interest, since this thoroughly investigated material is considered among the chief candidates for spintronic applications. The origin of the electric-field control of the ferromagnetism in this material is well understood in terms of the Mn-Mn exchange interaction by the strongly spin-orbit coupled valence band holes. The ferromagnetic Curie temperature Tc in (Ga,Mn)As is a significant function of the hole density, which offers the potential for altering the ferromagnetic response by the electric-field[1]. The ferroelectric gate control of ferromagnetism demonstrated in this work adds to this multiferroic FET device concept a qualitatively new feature of non-volatile operation. In the first proof-of-concept study performed on the 7 nm (Ga,Mn)As layer gated with the 200 nm P(VDF-TrFE) film we demonstrated that the polarization reversal by 30V pulses results in a persistent modulation of the ferromagnetic properties in the channel[2]. Magnetotransport and extraordinary Hall effect measurements allowed for inferring a Tc shift associated with ferroelectric polarization switching of 3.8K (4.7%), which is in a good agreement with the established understanding of hole-mediated exchange in (Ga,Mn)As. The magnitude of the gate effect and strength of the ferromagnetism modulation was considerably lower than the estimation based on the switching polarization charge. Study of the polarization stability using piezo-force microscopy in combination with the switching dynamics suggest that the gate effect is limited by a partial screening of the ferroelectric polarization. In this work we explore the possibilities to maximize the gate effect via the processing optimization of the ferroelectric polymer gate and appropriate selection of the polymer composition. The presented concept of the hybrid multiferroic FET suggests that the functionalities of ferroelectric field effect devices extend much further beyond the conventional non-volatile memories and may be of interest for spintronic logic elements.1. Dietl, T. & Ohno, H. Materials Today 9, 18(2006).2. Stolichnov, I., Riester, S.W.E., Trodahl, H.J., Setter, N., Rushforth, A.W., Edmonds, K.W., Campion, R.P., Foxon, C.T., Gallagher, B.L. & Jungwirth, T. Nature Materials 7, 464 (2008).
11:45 AM - L1.6/C4.6
Novel FeGaB Thin Films and Giant Microwave Tunability in FeGaB/PMN-PT Multiferroic Composites.
Jing Lou 1 2 , David Reed 1 2 , Carl Pettiford 1 2 , Ming Liu 1 2 , Nian Sun 1 2
1 Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Center for Microwave Magnetic Materials and Integrated Circuits, Northeastern University, Boston, Massachusetts, United States
Show Abstract Multiferroic composite materials with strong magnetoelectric (ME) coupling have led to many novel devices such as pico-Tesla sensitivity magnetoelectric magnetometers, electro-statically tunable microwave magnetic signal processing devices, etc. In order to achieve strong ME coupling with large frequency tunability at microwave frequencies, the magnetic phase in microwave multiferroic composites needs to have low ferromagnetic resonance (FMR) linewidth, large saturation magnetostriction constant (λs), high permeability and low saturation magnetic field. Single crystal yttrium iron garnet (YIG) has been the magnetic material of choice for tunable multiferroic microwave devices due to its narrow FMR linewidth. However, its low λs (~1.4 ppm) and low magnetization has severely limited the frequency tunability to be Δf ≤ 120 MHz and Δf/f ≤ 2.5%. In this work, we report on a new class of FeGaB magnetic films with high λs and their applications in multiferroic materials. With the increase of boron content, the λs of the FeGaB alloys showed a nonlinear behavior. A maximum λs of 72 ppm was achieved at 12 at. % of boron. Meanwhile, the FMR linewidth measured at X-band got dramatic reduced from ~700 Oe for binary FeGa film to < 20 Oe with boron content larger than 9 at. %, which is desired for ME tunable microwave devices. Multiferroic composites of FeGaB/lead magnesium niobate-lead titanate (PMN-PT) were made and their ME coupling was studied. The FeGaB/PMN-PT multiferroic composite show giant ME coupling at microwave and DC frequencies. A record-high microwave frequency tunability of Δf = 900 MHz or Δf/f = 58% was achieved in the composite with the change of external electric field from -6 kV/cm to +2 kV/cm. A strong electric field dependence of magnetic hysteresis loops was observed as well, which led to switched magnetic easy and hard axes. These FeGaB films have potential applications in RF/microwave multiferroic composite materials and devices.References:[1] J. Lou, R. E. Insignares, Z. Cai, K. S. Ziemer, M. Liu, and N. X. Sun, "Soft magnetism, magnetostriction, and microwave properties of FeGaB thin films", Appl. Phys. Lett. 91, 18254 (2007).[2] J. Lou. D. Reed, C. Pettiford, M. Liu, P. Han, S. Dong, and N. X. Sun, "Giant microwave tunability in FeGaB/lead magnesium niobate-lead titanate multiferroic composites", Appl. Phys. Lett. in press.
12:00 PM - L1.7/C4.7
Magnetoelectric Effect in Ferroelectric-antiperovskite Heterostructure.
Pavel Lukashev 1 , Renat Sabirianov 1
1 Physics, University of Nebraska, Omaha, Omaha, Nebraska, United States
Show AbstractWe report theoretical prediction of linear magnetoelectric effect (ME) in ferroelectric-antiperovskite PbTiO3/Mn3GaN heterostructure. The origin of the ME effect arises from the sensitivity of the Mn3GaN's magnetization to structural deformations which lowers its symmetry, i.e. piezomagnetic effect. Besides the recently proposed strain mediated coupling between magnetic and electric components of heterostructure, we observe a propagation of the soft mode atomic displacements from ferroelectric to the antiperovskite phase. Such deformations further lower the symmetry and increase the induced magnetization. The observed effect is linear, i.e. magnetization induced due to the soft mode propagation is reversed upon reversal of polarization. The first-principles calculations based on 25 atoms unit cell show that net magnetization of about 2 µB per 25 atom cell is induced. The magnetic symmetry is very sensitive to the nature of the interface. Free surface of Mn3GaN shows noticeable relaxation of surface layers and the rotations of local magnetic moments associated with these surface deformations. We find that this does not lead to the larger values of the induced net magnetization in the slab geometry. The calculations were performed by projector augmented wave method.
12:15 PM - **L1.8/C4.8
Unanticipated Consequences of Spin-Orbit Coupling in Half Metallic Ferromagnets.
Warren Pickett 1 , Kwan-woo Lee 1
1 Department of Physics, University of California-Davis, Davis, California, United States
Show AbstractTypically spin-orbit coupling (SOC) has been expected to degrade half metallic ferromagnetism slightly by mixing the spins, but usually with negligible consequences. In compounds containing 5d transition metal elements, however, the large spin-orbit coupling leads to unanticipated consequences, which are illustrated by two double perovskite compounds. In Ba2NaOsO6, the unconnected OsO6 octahedra based on heptavalent Os have a momentless ground state due to strong SOC. Hybridization between these clusters tends toward quenching of the orbital moment, leading to the emergence of magnetism from the nonmagnetic reference state. The double perovskite Sr2CrOsO6, based on pentavalent 5d Os and trivalent 3d Cr, is calculated to be a compensated ferrimagnetic ("half metallic antiferromagnet") if SOC is neglected. The large SOC leads to spin and orbital moments and net ferrimagnetic order. Surprisingly, in the isovalent compound Sr2CrRuO6, the (smaller) SOC also leads to changes in the spin and orbital moments, but these changes cancel leaving the compensated ferrimagnetic character. These density functional based results will be put in the context of the experimental information that is available.
12:45 PM - L1.9/C4.9
Magnetoelectric Coupling Through Exchange Bias at La0.7Sr0.3MnO3/BiFeO3 Interfaces.
Mark Huijben 1 2 , Ying-Hao Chu 2 3 , Lane W. Martin 2 4 , Martin Couillard 5 , Hajo Molegraaf 1 6 , Jan Seidel 2 4 , Nina Balke 2 , Pu Yu 2 , Micky B. Holcomb 2 , Guus Rijnders 1 , Jean-Marc Triscone 6 , David A. Muller 5 , Silvia Picozzi 7 , Elbio Dagotto 8 9 , Dave Blank 1 , R. Ramesh 2 4
1 Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands, 2 Department of Physics & Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Department of Materials Science and Engineering, National Chiao Tung Unviersity, HsinChu Taiwan, 4 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 5 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 6 DPMC, University of Geneva, Geneva Switzerland, 7 , CNR INFM, L’Aquila Italy, 8 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, United States, 9 Division of Materials Science & Technology, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractMultiferroics exhibiting simultaneously multiple order parameters, such as magnetism and ferroelectricity, offer an exciting way to explore coupled phenomena in solids. These investigations are driven by the prospect of magnetoelectric coupling in which charges are controlled by applied magnetic fields and spins by applied voltages. The recent availability of high-quality thin-film samples of hexagonal manganites and Bi-based perovskites, has improved the ability to accurately characterize multiferroic behavior, and has opened the door to the fabrication of practical devices based on magnetoelectric coupling. Currently, bismuth ferrite BiFeO3 (BFO) is being intensely explored since both ferroelectric (~820 oC) and antiferromagnetic (~370 oC) ordering temperatures are much higher than room temperature, which make it appealing for ambient applications. Recent studies have demonstrated the existence of strong coupling between ferroelectricity and antiferromagnetism. Thus, rotation of the ferroelectric order by 71 and/or 109 degrees leads to a corresponding rotation of the antiferromagnetic order. In order to manifest this intrinsic coupling into applications, it would be highly desirable to be able to control ferromagnetism. Since the intrinsic canted ferromagnetism in BFO is too small in magnitude to be useful, current approaches have focused on heterostructures consisting of a ferromagnet in intimate contact with the multiferroic. These studies have used a conventional metallic ferromagnet such as Co0.9Fe0.1 to couple to the BFO through exchange coupling at the interface. Due to the significant differences in chemistry between Co0.9Fe0.1 and BFO, the conventional exchange bias thermal annealing process cannot be used; instead, the ferromagnet is grown on the BFO in an applied field, typically ~200 Oe. The existence of double exchange coupled ferromagnets such as La0.7Sr0.3MnO3 (LSMO) provides us with an alternative approach to probe magnetic coupling at interfaces. Within this framework, we report the first observation of exchange bias coupling between the ferroelectric/antiferromagnet (multiferroic) BFO and the ferromagnet (LSMO) in high quality heterostructures. We will provide a suggestion for the cause of this interesting interface effect based on results from an extensive amount of magnetic measurements, structural analysis measurements (such as STEM-EELS) and first principles calculations. Finally, we will show the first indications of magnetoelectric coupling at these interfaces by magneto-optical Kerr effect measurements, while switching the polarization direction.
L2: New Half-metallic Compounds, Heusler Compounds,
Magnetic Tunnel Junctions with Half-metallic Ferromagnets
Session Chairs
Tuesday PM, December 02, 2008
Room 204 (Hynes)
2:30 PM - L2.1
Tunnel Magnetoresistance Characteristics of Post-Deposition-annealed Co2MnGe/MgO/CoFe Tunnel Junctions.
Tomoyuki Taira 1 , Takayuki Ishikawa 1 , Ken-ichi Matsuda 1 , Tetsuya Uemura 1 , Masafumi Yamamoto 1
1 Division of Electronics for Informatics, Hokkaido University, Sapporo Japan
Show Abstract2:45 PM - L2.2
Anomalous Hall Effect in Co2MnSi and Co2FeSi Thin Films.
Horst Schneider 1 , Enrique Vilanova 1 , Gerhard Jakob 1
1 Institute of Physics, Johannes Gutenberg-University, Mainz Germany
Show AbstractDue to their predicted halfmetallicity the Heusler compounds Co2MnSi and Co2FeSi are prime candidates for the use in spintronic applications. Recently, magnetic tunneling junctions showing high magnetoresistive effects at low temperatures have been fabricated. However, the effect is strongly temperature dependent. As a possible cause for this behavior the unfortunate position of the Fermi energy close to the border of the halfmetallic gap has been proposed.The use of pulsed laser deposition allowed us to succesfully grow epitaxial and L21 ordered thin films of Co2MnSi and Co2FeSi directly on insulating substrates. The absence of a metallic buffer layer combined with a standard photolithographic process allows the precise measurement of anisotropic transport properties of the films. A film thickness below 100 nm is very advantageous for the investigation of the Hall effect.At liquid helium temperature the sign of the anomalous Hall voltage depends on the investigated compound. By comparison with data from amorphous samples, this behavior can be attributed to the predicted shift of the Fermi energy. The high temperature increase of the anomalous Hall signal is identical for both materials. Theory of the anomalous Hall effect attributes this effect to impurity scattering.
3:30 PM - **L2.5
Tunnel Magneto-resistance Effect in Magnetic Tunnel Junctions using Half-metallic Heusler Alloy Electrodes and an MgO Tunneling Barrier.
Mikihiko Oogane 1 , Sumito Tsunegi 1 , Yuya Sakuraba 2 , Koki Takanashi 2 , Yasuo Ando 1
1 , Tohoku University, Sendai Japan, 2 , Tohoku University, IMR, Sendai Japan
Show AbstractSome of full-Heusler alloys (Co2MnSi, Co2MnGe, etc.) are expected to be an ideal ferromagnetic material in the spintronics field, because they have been predicted to possess half-metallic electronic structure and have a high Curie temperature. Previously, we have demonstrated large tunnel magneto-resistance (TMR) ratio of 570% at 2 K in magnetic tunnel junctions (MTJs) with Co2MnSi/Al-O/Co2MnSi structure [1]. However, the TMR ratio decreased drastically with the increasing of temperature and TMR ratio was only 70% at RT. In this work, we have succeeded to fabricate high-quality Co2MnSi/MgO/CoFe MTJs and investigated systematically their temperature dependence of TMR and conductance-voltage (G-V) curves.The MTJs with structure of MgO(100)-sub./Cr(40)/Co2 MnSi(30)/MgO(2)/CoFe(5)/IrMn(10)/Ta(5) (unit : nm) were fabricated using an ultrahigh vacuum magnetron sputtering system. The MgO tunnel barrier was prepared by EB evaporation. The MTJs were formed using photo-lithography and Ar ion etching. Magnetoresistance (MR) measurements were performed using a standard dc four-probe method, and the G-V curve was measured using an ac lock-in amplifier technique.We have achieved a very large TMR ratio of 756% at 2 K in a Co2MnSi/MgO/CoFe MTJ fabricated by optimized condition. This TMR ratio is the largest to date in MTJs using Heusler alloy electrode. We have also observed a large TMR ratio of 217% at RT. This TMR at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. Moreover, a peculiar dip structure was observed at 400mV in the G-V curves. It is considered that the large TMR effect at RT and the dip structure in the G-V curve are originated from the coherent tunneling through the (100)-oriented crystalline MgO barrier.This research was supported by Research and Development of for Next-Generation Information Technology by MEXT.[1] Y. Sakuraba, M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, and T. Miyazaki: Appl. Phys. Lett. 88 (2006) 192508.
4:00 PM - L2:Compounds
BREAK
4:30 PM - **L2.6
Spin-polarized Tunneling in Fully Epitaxial Magnetic Tunnel Junctions with Heusler Alloy Thin Films and a MgO Barrier.
Masafumi Yamamoto 1 , Takayuki Ishikawa 1 , Ken-ichi Matsuda 1 , Tetsuya Uemura 1
1 Division of Electronics for Informatics, Hokkaido University, Sapporo Japan
Show AbstractCo-based full-Heusler alloys (Co2YZ) have attracted much interest as a preferable ferromagnetic electrode material for spintronic devices. This is because of the half-metallic ferromagnetic nature theoretically predicted for many of these alloys, and because of their high Curie temperatures, which are well above room temperature (RT). We recently developed fully epitaxial magnetic tunnel junctions (MTJs) with a Co2YZ thin film as a lower electrode or Co2YZ thin films as both lower and upper electrodes, and a MgO (001) tunnel barrier [1-3]. The relatively small lattice mismatch between Co2YZ and MgO for a 45° in-plane rotation (e.g. about −3.7 % for Co2Cr0.6Fe0.4Al (CCFA) and −5.1 % for Co2MnSi (CMS)) enabled us to successfully fabricate fully epitaxial MTJ trilayers featuring highly smooth and abrupt interfaces. We have demonstrated a relatively high TMR ratio of 109 % at RT (317 % at 4.2 K) for CCFA/MgO/Co50Fe50 MTJs [1] and a TMR ratio of 179% at RT (683% at 4.2 K) for CMS/MgO/CMS MTJs (CMS-MTJs) [3].The MTJ layer structure was grown on a MgO(001) single-crystal substrate and, from the substrate side, was as follows (for CMS-MTJs): MgO buffer (10 nm)/CMS (50 nm)/MgO barrier (0.8-3.4 nm)/CMS (5 nm)/Ru (0.8 nm)/Co90Fe10 (2 nm)/IrMn (10 nm)/Ru cap (5 nm). All layers in these MTJs were successively deposited in an ultrahigh vacuum chamber (with a base pressure of about 6 × 10−8 Pa) through the combined use of magnetron sputtering for Co2YZ thin films and electron beam evaporation for MgO.The differential conductance (G=dI/dV) vs. V and d2I/dV2 vs. V characteristics of CMS-MTJs indicated the following spin-dependent electronic structures of CMS thin films incorporated into MTJs and dominant factors that determine spin-dependent tunneling characteristics in these MTJs. First, both the lower and upper CMS electrodes possessed an energy gap of about 0.36 eV for the minority spins and the Fermi level position appeared to be near the middle of the half-metal gap. Second, the CMS lower electrode had a half-metal gap with only interface states, while the CMS upper electrode had defect-induced states in the half-metal gap. Third, a dominant tunneling process for the antiparallel alignment for the bias voltage range of |V| < 100 mV is spin-flip scattering in the collector electrode from the interface states in the minority-spin half-metal gap to the majority-spin band via magnon excitation by hot electrons. In summary, the demonstrated high TMR ratios at RT suggest that an epitaxial Co-based full-Heusler alloy thin film in combination with an epitaxial MgO tunnel barrier is highly promising for ferromagnetic electrodes used in spintronic devices.[1] T. Marukame et al., Appl. Phys. Lett. 90, 012508 (2007).[2] M. Yamamoto et al., Advances in Solid State Physics 47, pp. 105-116, R. Haug (Ed.), Springer Berlin/Heidelberg, 2008.[3] T. Ishikawa et al., J. Appl. Phys. 103, 07A919 (2008).
5:00 PM - L2.7
New Half-metals:Half-metallic Completely Compensated Ferrimagnets.
Benjamin Balke 1 , Gerhard Fecher 2
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , University of Mainz, Mainz Germany
Show AbstractRecently Wurmehl et al [1] proposed among others the Heusler compound (MnFe)2(MnV)Al to be a half-metallic compensated-ferrimagnet with complete spin polarization. These kind of materials are supposed to have a potential advantage over half-metallic ferromagnets for some technical applications because they have no stray field andare much less affected by external magnetic fields. They could be used as sensors in spin polarised scanning tunnelling microscopes for measuring the spin polarisationof samples without perturbing the domain structures [2].They were also predicted as a base for a new type of superconductor that has only one superconducting spin channel, the so-called single spin superconductor [3].In this work we present the first and promising experimental results of this compound. Additionally these results are discussed and compared with results of compounds which consist of different mixtures from the two Heusler compounds Fe2MnAl and Mn2VAl. Electronic structure calculations support the experimental work.[1] S. Wurmehl, H. C. Kandpal, G. H. Fecher, and C. Felser J. Phys.: Condens. Matter 18 6171-6181 (2006)[2] R. A. de Groot Physica 172B 45 (1991)[3] W. E. Pickett Phys. Rev. Lett. 77 3185 (1996)
5:15 PM - L2.8
Structural Properties of Heusler Alloy Thin Films as a Function of Target Preparation, Substrate Conditions and Sputtering Parameters.
Subhadra Gupta 1 2 , Zeenath Tadisina 1 2 , Xiao Li 2 1 , Yuki Inaba 2 1 , Karen Torres 1 2 , Gregory Thompson 1 2 , Tim Mewes 3 2 , Patrick LeClair 3 2
1 Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, Alabama, United States, 2 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States, 3 , The University of Alabama, Tuscaloosa, Alabama, United States
Show Abstract5:30 PM - L2.9
Interface Dependence of the Spin Polarization and Magnetization of Co2Fe1-xCrxAl Heusler Compounds.
Martin Jourdan 1 , Christian Herbort 1 , Elena Arbelo 1 , Michael Kallmayer 1 , Hans-Joachim Elmers 1
1 Physics, Gutenberg University Mainz, Mainz Germany
Show AbstractMany Heusler compounds are predicted by LSDA(+U) band structure calculations to be highly spin polarized at the Fermi energy. In the simple quasi classical model of tunneling (Julliere model) this is expected to result in a large magnetoresistance of planar Heusler – AlOx – ferromagnet junctions. However, at least possible deviations of the density of states in the bulk and at the barrier interface, which is probed by the tunneling process, should be considered. Supercell band structure calculations are demonstrating a strong influence of the crystallographic orientation and of the terminating atomic layer of Heusler compounds on the local electronic properties at the interface with a tunneling barrier.We investigate experimentally the tunneling magnetoresistance of Co2Fe1-xCrxAl–AlOx-Co junctions. The epitaxial Heusler thin films are grown by a combined MBE and sputtering process in (100) and (110) orientation. In situ scanning tunneling microscopy demonstrates very smooth surfaces (rms roughness < 1nm) for both growth directions. The Co2Fe1-xCrxAl(100) surface consists of terraces with unit cell sized steps indicating a well defined termination. Co2Fe1-xCrxAl(110) surfaces are less defined, but shown an even reduced mean roughness. The surfaces are atomically well ordered for both growth directions as demonstrated by electron diffraction (RHEED and LEED).Different local magnetizations at the interfaces of Co2Fe1-xCrxAl films with the tunneling barriers indicate variations of the electronic properties. With x-ray magnetic circular dichroism (XMCD) experiments we investigate the magnetisation of a 2-3nm interface region of the Heusler electrode in the total electron yield (TEY) mode. For comparison the bulk magnetisation is determined by XMCD as well (in transmission mode). Although the bulk magnetisation of (100) and (110) oriented Co2Fe1-xCrxAl films is similar, the interface magnetisation is reduced at the (110) surface compared (100) grown samples.The tunneling magnetoresistance (TMR) of Co2Fe1-xCrxAl – AlOx-Co junctions with the Heusler electrode grown in the two different directions is compared and related to the results of the various characterization methods described above. Typically a reduced TMR is obtained if the Heusler electrode is (110) oriented, which is consistent with the reduced interface magnetisation of this orientation.Finally the termination of the well defined Co2Fe1-xCrxAl (100) surfaces is modified by the deposition of Cr monolayers on the Heusler films. The epitaxial growth of very thin Cr layers is demonstrated by RHEED investigations. The influence of the Cr layers on the TMR and the bias dependent junction conductivity (tunneling spectroscopy) is discussed.
5:45 PM - L2.10
Tunneling Conductance Characteristics for Co2Cr0.6Fe0.4Al/MgO/Co2Cr0.6Fe0.4Al Magnetic Tunnel Junctions.
Naoki Itabashi 1 , Takayuki Ishikawa 1 , Kazuki Yonemura 1 , Ken-ichi Matsuda 1 , Tetsuya Uemura 1 , Masafumi Yamamoto 1
1 Division of Electronics for Informatics, Hokkaido University, Sapporo Japan
Show Abstract
Symposium Organizers
Claudia Felser Johannes Gutenberg University of Mainz
Arunava Gupta The University of Alabama
Burkard Hillebrands Technische Universitaet Kaiserslautern
Sabine Wurmehl Eindhoven University of Technology
L3: Theoretical Studies of Half-metallic Behavior in Bulk Materials and Interfaces
Session Chairs
Wednesday AM, December 03, 2008
Room 204 (Hynes)
10:00 AM - L3.1
Variational Solution for the Rashba Splitting in AlGaAs/GaAs Heterojunction.
Antonio Ferreira da Silva 1 , Marcelo Toloza Sandeval 1 , Erasmo de Andrada e Silva 2 , Giuseppe La Rocca 3
1 Instituto de Física, Universidade Federal da Bahia, Salvador, Bahia, Brazil, 2 LAS, Instituto Nacional de Pesquisas Espaciais, São José dos Campos, São Paulo, Brazil, 3 , Scuola Normale Superiore, Pisa Italy
Show AbstractThe control of the spin orbit splitting for a 2DEGconfined in the conduction subband of III-V semiconductorheterojunctions is highly desirable for the spin electronicproposal [1]. To achieve such a control we propose an investigation of the barrier penetration and boundary effects in the Rashba splitting for AlGaAs/GaAs heterojunction.Starting from the eight bands Kane model for the bulk, theeffective mass equation for electrons of the conduction subbandsare obtained and spin dependent boundary conditions are derived[2]. By using the envelope function framework with the modifiedFang-Howard variational solution we obtain analyticaldispersion relations for spin-split subbands. The spin orbitsplitting is calculated for the heterojunction and perfectlyreproduced in the infinite barrier approximation [3]. Thedifferent contributions of the effective Hamiltonian to thesplitting are estimated. The results are compared to recent experimental data [4].[1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).[2] E. A. de Andrada e Silva, G. C. La Rocca and F. Bassani, Phys.Rev. B 55, 16293 (1997).[3] E. A. de Andrada e Silva, G. C. La Rocca and F. Bassani, Phys.Rev. B 50, 8523 (1994).[4] Hyonkwan Cho et al., Physica E (2008), doi:10.1016/j.physe.2008.01.010
10:15 AM - L3.2
Theoretical Design of Highly Spin-polarized Interfaces between Half-metallic Heusler Alloys and Semiconductors.
Masafumi Shirai 1 , Yoshio Miura 1 , Kazutaka Abe 1
1 Reseach Institute of Electrical Communication, Tohoku University, Sendai Japan
Show AbstractHalf-metallic ferromagnets are expected to improve the performance of various spintronic devices, such as magnetic tunnel junctions, spin light-emitting diodes (LED), and sorts of spin transistors. According to the theory of diffusive transport phenomena, the conductivity mismatch between metallic ferromagnets and semiconductors prevents efficient spin injection into semiconductors, except for ferromagnetic injectors with high spin polarization. The observed efficiency of spin injection from a half-metallic Heusler alloy Co2MnGe into AlGaAs/GaAs LED structure is, however, much lower than that expected. The low efficiency can be attributed to interfacial states which appear in the minority-spin band gap and degrade half-metallic characteristics. Therefore, high spin polarization at the ferromagnet/semiconductor interface is crucial for improving the efficiency of spin injection into semiconductors. We have carried out theoretical design of highly spin-polarized interfaces between half-metallic Heusler alloys and semiconductors on the basis of first-principles density-functional calculations. The half-metallic features are degraded by the interfacial states for all Heusler-alloy/semiconductor (001) interfaces studied so far. The interfacial states are originated from the symmetry lowering at each atomic site as well as reunited dangling-bond orbitals at the hetero-junction. On the other hand, high spin polarization is almost preserved at specific (110) interfaces between Heusler alloys and semiconductors, e.g. Co2CrAl/GaAs(110), Co2MnSi/GaAs(110), Co2FeSi/Si(110), and so on. Note that the electronic structure at the (110) surface of these Heusler alloys is far from half-metallic. The high spin polarization at the (110) interfaces owe a lot to the local coordinate around each atomic site as well as the bonding characteristics at the hetero-junction. Indeed, the optimization of atomic positions forces the interfacial states away from the Fermi level, liading to the minority-spin band gap retained. The highly spin-polarized (110) interfaces turn out to have the lowest energy among the (110) interfacial patterns studied here. So far, the epitaxial Heusler-alloy/semiconductor junctions have been fabricated only in the (001) orientation. The measurements of the spin-injection efficiency via the (110) junctions are highly desired since the significant improvement can be expected for the (110) junctions.
10:30 AM - L3.3
The Electronic Band Structure and Surface Stability of CoS2(100).
Ning Wu 1 , Yaroslav Losovyj 1 2 , David Wisbey 1 , Renat Sabirianov 3 , Wai-Ning Mei 3 , Chun-gang Duan 1 4 , Zhaoxian Yu 5 , Kirill Belashchenko 1 , Lan Wang 6 , Michael Manno 6 , Chris Leighton 6 , Peter Dowben 1
1 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States, 3 Department of Physics, University of Nebraska - Omaha, Omaha, Nebraska, United States, 4 Key Laboratory of Polarized Materials and Devices, Ministry of Education, East China Normal University, Shanghai China, 5 School of Physics & Engineering, Zhongshan University, Guangzhou China, 6 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe pyrite-type transition metal compound CoS2 is an itinerant electron ferromagnet. In ground state band structure calculations, CoS2 is predicted to be highly spin polarized and at least close to being half-metallic. We have undertaken angle resolved photoemission studies of CoS2(100) in order to map out the surface and bulk band structure. The experimental band structure indicates that CoS2 has a narrow band width, consistent with the theoretical expectations. A strongly dispersing band of CoS2(100), with both sulfur and cobalt weight, along the Γ-X direction of the bulk Brillouin zone, has been identified from photon energy dependent resonant photoemission studies. The Fermi level crossings are seen to be very sensitive to the separation in the S–S dimer, and it may be that the nominally half-metallic gap in CoS2 may be controlled by the bonding-antibonding splitting in this dimer, rather than by exchange splitting on the Co atoms. Surface structure studies investigated by LEED IV analysis that the favored surface structural model is one in which the S-S dimmers remain intact and keep a complete S-Co-S sandwich structure. The stability of various possible terminations of the CoS2 (1x1) surface have been explored and theoretical expectations are found to agree with experiment. The stability of surface structure has also been explored. This is important in terms of the implications of the surface termination on the spin asymmetry.
10:45 AM - L3:Theory
BREAK
11:15 AM - **L3.4
Rational Design of Half-Metallic Heterostructures.
William Butler 1 , Claudia Mewes 1 , Chunsheng Liu 1
1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States
Show Abstract11:45 AM - L3.5
Calculation of Gilbert Damping in Half Metals.
Claudia Mewes 1 , Chunsheng Liu 1 , Mairbek Chshiev 1 , Tim Mewes 1 , William Butler 1
1 , Center for Materials for Information Technology, Tuscaloosa, Alabama, United States
Show AbstractBased on Kambersky's torque correlation model of Gilbert damping [1,2] we calculated the precessional magnetization relaxation in half-metallic systems. In Kambersky’s model damping occurs through a combination of spin-flip excitations and orbital excitations. Because of the absence of spin-flip scattering in half-metallic systems, Gilbert damping is expected to be reduced. This makes half-metals interesting potential candidates for information storage technologies especially for use in CPP/GMR read head devices and spin-torque MRAM. Our calculations utilize a combination of first principle calculations to predict the band structure for the half-metal of interest and an extended Huckel tight binding model to calculate the Gilbert damping. Using this approach we calculated the damping predicted by the spin torque correlation model for different half-metallic structures, including the Heusler alloys Co2MnSi, Co2MnGe confirming the expected low damping.
References:[1] V. Kambersky, Czech. J. Phys. B 26, 1366 (1976).
[2] B. Heinrich, D. Fraitova and V. Kambersky, Phys. Stat. Sol. 23, 501 (1967).
12:00 PM - **L3.6
Many-body Correlations in Heusler Compounds.
Jan Thoene 1 , Stanislav Chadov 1 , Gerhard Fecher 1 , Claudia Felser 1
1 , Johannes Gutenberg University, Mainz Germany
Show AbstractWednesday, 12/3New Presenter *L3.6 @ 11:00 AMMany-body Correlations in Heusler Compounds. Jan Thoene, Johannes Guttenberg University
L4: Applications of Half-metallic Ferromagnets, Including Shape Memory, Nanoparticles, CPP-GMR, Spin filter, and Spin Dynamics
Session Chairs
Wednesday PM, December 03, 2008
Room 204 (Hynes)
2:30 PM - L4.1
Magnetic and Electrical Properties of Atomically-Controlled Fe3Si/Ge Heterostructure for Group IV-Based Spin Transistor.
Taizoh Sadoh 1 , Koji Ueda 1 , Yuichiro Ando 1 , Kohei Hamaya 1 , Yukio Nozaki 1 , Kimihide Matsuyama 1 , Yoshihito Maeda 2 , Masanobu Miyao 1
1 , Kyushu University, Fukuoka Japan, 2 , Kyoto University, Kyoto Japan
Show AbstractA Heusler alloy Fe3Si (Tc=840K) has three types (A2, B2, DO3), where DO3-type Fe3Si is expected to be spin-polarized at the Fermi level. Moreover, since the lattice constant (0.565nm) of Fe3Si completely matches with that of Ge, epitaxial growth of the high quality Fe3Si on Ge should be possible. Thus, it is expected that spin-polarized electrons can be injected with a high efficiency through the Fe3Si/Ge interfaces at room temperature. This will be a powerful tool to realize Ge-channel spin transistors with ultrahigh speed operation and ultralow power consumption. This paper reports atomically-controlled epitaxial growth of a Heusler alloy (Fe3Si) on Ge, and its magnetic and electrical properties.In the experiment, Fe and Si were co-deposited (Fe:Si=3:1) on Ge(111) substrates by the MBE system. Effects of the growth temperature on quality of Fe3Si/Ge were investigated in a wide temperature range (130-400oC). Cross sectional TEM observation demonstrated that Fe3Si/Ge(111) with an atomically flat interface was achieved at 130oC. In addition, its electron diffraction pattern indicated super-lattice reflection spots, which indicated the existence of ordered DO3-type Fe3Si. Intensities of the super-lattice reflection spots increased with increasing growth temperature (>200oC); however, atomic mixing occurred at the interfaces. To solve this problem, the two-step growth technique, i.e., low temperature MBE growth (130oC) and subsequent high temperature annealing (200oC), was examined. TEM observations clearly demonstrated the high quality Fe3Si/Ge structure with an atomically flat interface and strong super-lattice reflection spots. Magnetic properties of Fe3Si/Ge structures grown at 130oC were measured by VSM, where magnetic fields were applied in-plane to the sample surfaces. A very small coercivity (0.8 Oe) was obtained, indicating high crystallinity of the Fe3Si layers. Experiments of the shape anisotropy induced by patterning (line and space with 5um) indicated that the anisotropy energy was enhanced from 1.1x104 to 3.7x104 erg/cc. This means that the magnetic domain structures can be controlled by the shape anisotropy.Electrical properties were evaluated by using I-V and C-V measurements, which indicated good Schottky characteristics with a barrier height of 0.5 eV. The ratio of the on-current to the off-current was the order of 104. Annealing experiments after growth (i.e. post-annealing) were also performed. Results guaranteed the thermal stability of such good electrical properties up to 400oC. These results will be a powerful tool to realize group IV-based spin-transistors with Fe3Si/Ge structures for spin injection.
2:45 PM - L4.2
Electronic Properties and CPP-GMR Sensor Fabrication Based on Co2FeSi and Co2MnSi Heusler Alloy.
Zeenath Tadisina 1 , Courtney Guenther 1 , Tianyi Xu 1 , Xiao Li 1 , Subhadra Gupta 1 , William Butler 1 , Patrick LeClair 1 , Claudia Felser 2
1 MINT Center , The University of Alabama, Tuscaloosa, Alabama, United States, 2 Chemistry, Johannes-Gutenberg University, Mainz Germany
Show Abstract3:00 PM - **L4.3
Application of New Ferromagnetic Alloys to CPP-GMR Recording Heads.
Jeffrey Childress 1
1 San Jose Research Center, Hitachi GST, San Jose, California, United States
Show AbstractAll-metal current-perpendicular-to-the-plane (CPP) giant magnetoresistive (GMR) sensors are an alternative to CPP tunnel-magnetoresistance (TMR) sensors as the device dimensions are reduced below 40nm. With typical RA products in the range 0.03-0.10 Ω-μm2, CPP-GMR sensors have the potential to deliver low sensor impedance at the smallest conceivable dimensions, and therefore lower noise and higher bandwith performance. Among the challenges that CPP-GMR sensors face are low signal levels due to their low resistance, low ΔR/R for thin magnetic layers, as well as current-induced noise and instability due to the spin-torque effect. Several paths are available to increase signal and reduce spin-torque effects in CPP-GMR sensors. For example, the use of alternative ferromagnetic alloys such as CoFeAl and Heusler alloys allows sizeable ΔR/R to be achieved. In this context I will discuss progress achieved in developing CPP-GMR sensors for ultra-narrow magnetic recording head sensors with dimensions << 50nm.Typical test structures of single- and dual-SV's have antiparallel (AP)-coupled pinned layer structures with IrMn antiferromagnets, CoFe-based pinned layers (2-3 nm) and various reference magnetic layers (3-4 nm) based on either CoFe alloys or magnetic Heusler alloys with higher electron spin scattering. The free layer has a magnetic thickness equivalent to 4-6 nm of Ni80Fe20 and is separated from the pinned layer structure by a Cu spacer layer. Compared to standard GMR and TMR sensors, the ΔR/R of CPP-GMR sensors benefits from thicker magnetic layers due to spin-diffusion length effects, but thickness is limited by the desired shield-to-shield spacing (linear density). Optimized film-level ΔR/R of ~ 10% have been obtained, even for relatively thin sensors stacks < 45nm. To test these CPP-GMR films under recording conditions, shielded read head sensors were defined by a combination of optical lithography and e-beam lithography to achieve physical widths between 20 and 60 nm and sensor resistances of 30-60 Ω. Magnetic recording readback tests were conducted using perpendicular recording media. Substantial operating bias voltages of 50-100mV could be applied before the onset of spin-torque induced instabilities, yielding reasonable signal amplitudes over 1mV (peak-to-peak) as well as head & electronics SNR around 30db. The onset of spin-torque instabilities above a threshold voltage results in a sudden and dramatic increase in read error rate. Thus the reduction of spin-torque excitations, along with further increases in ΔR/R, are key components to the application of CPP-GMR to recording heads at ever-increasing densities. I will discuss the opportunities and challenges of integrating new high-polarization ferromagnetic alloys in future CPP-GMR sensors to further improve signal-to-noise ratio and read-head performance.
3:30 PM - **L4.4
The Heusler Alloys Co2MnSi, Co2FeSi and Co2Mn0.5Fe0.5Si in Magnetic Tunnel Junctions.
Guenter Reiss 1 , Andreas Huetten 1 , Jan Schmalhorst 1 , Andy Thomas 1 , Alexander Weddemann 1 , Daniel Ebke 1
1 Department of Physics, Bielefeld University, Bielefeld Germany
Show AbstractThe Heusler alloys Co2MnSi and Co2FeSi are among the best canidates for highly spin polarized ferromagnetic electrodes in magnetic tunnel junctions. We investigated the magnetic moments and the chemical states of these alloys both for single films as well as for the interface to aluminum oxide using X-ray magnetic circular dichroism and X-ray absorbtion. The atomically ordered bulk Co2MnSi showed characteristic differences in the x-ray absorption spectra in comparison with the disordered samples: in the ordered state of the bulk both, Co and Mn, showed additional shoulders about 4 eV above the L2,3 resonances, a large circular dichroism, and a pronounced x-ray absorption near edge fine structure. The same was found for interfacial Co, whereas for interfacial Mn these features were masked by the characteristic absorption multiplet structure of MnO right at the interface which also resulted in a strong reduction of the interfacial Mn moment in annealed samples.Additionally, the transport properties of Co2MnSi/AlOx/Co-Fe magnetic tunnel junctions are discussed with respect to temperature-dependent magnetic moments at the Co2MnSi/AlOx interface and electronic band structure effects. The junctions show a considerably larger temperature and bias voltage dependence of the tunneling magnetoresistance compared to Co-Fe-B/AlOx /Co-Fe-B junctions. With increasing atomic disorder of the interfacial Co2MnSi its magnetic moments decrease and show a stronger temperature dependence. Even for the best atomic ordering achieved the corresponding spin-wave parameters of Mn and Co at the Co2MnSi/AlOx interface are significantly larger than expected for Co2MnSi bulk. The influence of enhanced interfacial magnon excitation in the Co2MnSi/AlOx /Co-Fe junctions on their transport properties will be discussed as well as possible origins for the negative tunnel magnetoresistance at high bias voltage.For the temperature dependence of the TMR, the exact location of the fermi level is also a crcial parameter. In order to investigate this, we integrated the compounds Co2MnSi and Co2FeSi in magnetic tunnel junctions. An inversion of the tunnel magnetoresistance in the Co2FeSi as well as in the Co2MnSi case was found with alumina barriers at certain bias voltages at room and low temperatures. We present calculations of the density of states of these compounds based on fully relativistic spin-polarized KKR methods in order to elucidate the inversion and the location of the fermi level.
4:30 PM - **L4.5
Half-metallic Co2FeAl0.5Si0.5 Heusler Alloys for Spintronics Devices.
Koichiro Inomata 1 , Hiroaki Sukegawa 1 , Wenhong Wang 1 , Rong Shan 1 , Takao Furubayashi 1 , Yukiko Takahashi 1 , Kazuhiro Hono 1
1 , National Institute for Materials Science, Tsukuba Japan
Show AbstractThe performance of spintronics depends on spin polarization of the current. Therefore, highly spin polarized current source is strongly desired in spintronics. The use of half-metallic ferromagnets (HMFs) is a typical approach for this purpose. HMFs are characterized by a metallic density of states at Fermi level (EF) for one spin channel, while the states for the other spin channel display a gap at EF, leading to 100% spin polarization. Among some kinds of HMFs Co-based Heusler alloys with a chemical form of Co2YZ and L21 structure have been growingly investigated since a large tunneling magnetoresistance (TMR) observation at RT using a Co2Cr0.6Fe0.4Al (CCFA) electrode in magnetic tunnel junctions (MTJs)1). Recently, we have reported very large TMR up to 220% at RT for an epitaxial Co2FeAl0.5Si0.5(CFAS)/MgO/CFAS spin-valve-type MTJ2). In this MTJ the bottom CFAS electrode was fabricated on a Cr-buffered MgO (001) substrate. Unfortunately Cr buffer impedes a highly L21-ordered CFAS structure due to the Cr interdiffusion by the post-annealing at a high temperature and thus limits to achieve higher TMR. In this work we tried to the way to obtain a highly L21-ordered CFAS thin film using an MgO buffer instead of the Cr buffer. The films were prepared on MgO (001) single crystal substrates at an ambient substrate temperature using ultra-high vacuum magnetron sputtering system with the base pressure of below 8×10−8 Pa. The MgO buffer layer was deposited by rf sputtering from a sintered MgO target under an Ar pressure of 10 m Torr. The MTJs were patterned into 10 x 10 mm2 by using photolithography and Ar ion etching. The patterned MTJs were post-annealed at various temperatures in a vacuum by applying a 5 kOe magnetic field. We have successfully grown fully epitaxial MTJs on MgO (001) substrates with an MgO buffer layer and found that this method greatly improves L21-ordered structure and surface roughness due to without Cr interdiffusion after high temperature annealing. We have observed a half-metallic behavior for L21-CFAS from the bias voltage dependence of the differential conductance of the MTJs, which exhibited a band gap and Fermi level at the middle of the minority spin gap, consistent with the recent calculation3). We will report structural, magnetic and transport properties of CFAS films and TMR and CPP-GMR devices using CFAS films. References1) K. Inomata et al., Jpn. J. Appl. Phys. 42, L419 (2003). 2) N. Tezuka et al., Appl. Phys. Lett. 89, 112514 (2006), Jpn. J. Appl. Phys. 46, L454 (2007).3) Z. Gercsi and K. Hono, J. Phys.: Condens. Matter, 19, 326216 (2007).
5:00 PM - L4.6
Ferromagnetism and Transport in Mn-doped GaAs1-yPy and GaAs1-yNy: TC Trends and Metal-insulator Transition.
Peter Stone 1 2 , Kirstin Alberi 1 2 , Samuel Tardif 2 , Jeffrey Beeman 2 , Kin Yu 2 , Mark Ridgway 3 , Wladek Walukiewicz 2 , Oscar Dubon 1 2
1 Materials Science and Engineering, University of California-Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , The Australian National Univeristy, Canberra, Australian Capital Territory, Australia
Show AbstractWhile Curie temperatures (TCs) as high as 173 K have been reported for films of the canonical ferromagnetic semiconductor Ga1-xMnxAs [Wang et al., Proceedings of the 27th ICPS (Springer, New York, 2005), p. 333] further progress towards room temperature ferromagnetism in this and other Mn-doped III-V ferromagnetic semiconductors has been stifled by challenges in materials synthesis—namely, increasing x while avoiding the formation of second phases. Another proposed, though significantly less explored, route to raise TC is to partially substitute As in the host semiconductor with an isovalent anion of smaller atomic radius. It is believed that alloying GaAs with GaP may yield a host in which the itinerancy of the mediating holes is maintained while p-d exchange is enhanced due to the shorter average Mn-anion bond length. Recent calculations have predicted an enhancement of TC by a factor of 1.5 due to this effect in Mn-doped GaAs1-yPy [Masek et al., Phys. Rev. B 75, 045202 (2007)].We report the synthesis and magneto-electronic properties of Ga1-xMnxAs1-yPy and Ga1-xMnxAs1-yNy with y<0.04. Even in this dilute limit TC is reduced from 110 K to 60 K (100 K to 65 K) upon the introduction of 3.1 % P (1.4% N) into the As sublattice at constant Mn doping (x=0.046). The decrease in TC in the quaternary systems results from the reduced efficacy of holes in mediating inter-Mn exchange due to anion disorder, which reduces the mobility and, therefore, the mean free path of holes. Indeed, substitution of only 2.4% of As by P induces a metal-insulator transition (MIT) at a constant Mn doping of x=0.046 while the replacement of 0.4 % As with N results in the crossover from metal to insulator for x=0.037. The occurrence of a MIT at these dramatically low anion alloying levels is understood within the context of alloy disorder scattering of holes in an impurity band. This simple model reproduces well the experimentally determined values for the onset of the MIT. Furthermore, a transition from the metallic to insulating state results in a reduction of the saturation moment of 4.2±0.2 μB/MnGa for films with 0≤y≤0.016to ~3 μB/MnGa for y=0.031. The random distributions of P (substituting As) and MnGa lead to regions in the film where stronger hole scattering by alloy disorder decouples MnGa moments from the global ferromagnetic exchange. Finally, we discuss results in Ga1-xMnxAs1-yPy films with y>0.97. Analogous trends are observed in these films to those with y<0.04; TC and conductivity are both reduced in going from y=1 to y=0.97 due to the random potential fluctuations on the anion sublattice. Collectively these findings indicate that improvement of TC in Ga1-xMnxAs through isovalent anion substitution is fundamentally limited by alloy disorder scattering and that models of ferromagnetism in III1-xMnxV materials must take into account this effect as well as hole localization in general to more accurately predict TC.
5:15 PM - L4.7
Molecular Beam Epitaxial Growth of Iron and Iron Nitrides on Wurtzite Gallium Nitride (0001)*.
Wenzhi Lin 1 , Jeongihm Pak 1 , David Ingram 1 , Arthur Smith 1
1 Department of Physics and Astronomy, Nanoscale and Quantum Phenomena Institute, Athens, Ohio, United States
Show Abstract5:30 PM - L4.8
Variation of Growth Mode with Epitaxial Orientation for Half-Metallic CrO2 Films.
K. Chetry 1 , M. Pathak 1 , H. Sims 1 , P. LeClair 1 , W. Butler 1 , A. Gupta 1
1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractThe half-metallic properties of chromium dioxide (CrO2) have the potential for applications in magnetoresistive spin valve and tunnel junction devices. As a step towards fabricating spin valve and tunnel junction structures, we have studied the thickness-dependent magnetic and electrical properties of (110) and (100)-oriented epitaxial CrO2 films grown on TiO2 substrates. While the magnetic properties of (100)-oriented CrO2 films are strongly dependent on thickness because of the effect of strain, the (110) films grow strain-free and show no significant influence of thickness on magnetic properties. The (100) CrO2 films grow in a layer-by-layer mode, while an island growth mode is observed for (110) CrO2 films, as confirmed from atomic force microscopy (AFM) studies. To better understand the differences in the growth mode, we have performed first principles-based calculations using density functional theory implemented within the VASP code to study the surface and interface energies of (100) and (110) CrO2, (100) and (110) TiO2 systems. We find that for (110) orientation the surface energy (σ) of TiO2 is less than the sum of the surface energy of CrO2 and the CrO2-TiO2 interface energy (γ). This result is consistent with the island growth mode observed experimentally. However, for the case of (100) orientation, we also find that σ(TiO2) < σ (CrO2) + γ, which does not match with our experimental observation of layer-by-layer growth. We speculate that formation of some oxygen deficient phase of chromium oxide in the very first monolayer, which then gets converted to CrO2 by accepting oxygen from the second layer, favors the layer-by-layer growth.
5:45 PM - L4.9
Preparation and Characterization of (001) and (110)-Oriented Fe1.4Ti0.6O3 Films.
Tatsuo Fujii 1 , Tatsuya Sugano 1 , Yusuke Takada 1 , Makoto Nakanishi 1 , Jun Takada 1
1 Department of Applied Chemistry, Okayama university, Okayama Japan
Show AbstractL5: Poster Session
Session Chairs
Thursday AM, December 04, 2008
Exhibition Hall D (Hynes)
9:00 PM - L5.1
Tunneling Spectroscopy of Fully Epitaxial Co2MnSi/MgO/Co2MnSi Magnetic Tunnel Junctions.
Takayuki Ishikawa 1 , Naoki Itabashi 1 , Tomoyuki Taira 1 , Ken-ichi Matsuda 1 , Tetsuya Uemura 1 , Masafumi Yamamoto 1
1 Division of Electronics for Informatics, Hokkaido University, Sapporo Japan
Show Abstract9:00 PM - L5.10
Magnetic and Optical Properties of 3d Transition-metals Doped SnO2.
Luisa Scolfaro 1 , P. Borges 1 , H. Leite Alves 2 , J. A. Alves 2 , E. da Silva 3
1 DFMT, Instituto de Fisica, Universidade de Sao Paulo, Sao Paulo, SP, Brazil, 2 Depart. Ciencias Naturais, Universidade de Sao Joao del Rei, São João del Rei, MG, Brazil, 3 Depart. de Fisica, Universidade Federal de Pernambuco, Recife, PE, Brazil
Show AbstractIn the last years, the field of magnetic semiconductors has substantially expanded on a search for new materials of potential applications in spintronics. Diluted magnetic semiconductors (DMS), where nonmagnetic semiconductors are doped with a few percent of transition metal (TM) elements, have been intensively investigated by many research groups aiming to achieve ferromagnetism at Curie temperatures well above the room temperature. Among them, magnetic ions substituted wide band gap semiconducting oxides, such as the SnO2, have attracted increasing attention due to its transparency. As a transparent n-type conductor, SnO2 has been widely used in device applications as gas sensors and solar cells.Above room temperature ferromagnetism has been reported for Co- and Fe-doped SnO2 (Sn1-xCoxO2 and Sn1-xCoxO2) with considerably high values of magnetic moment for TM contents of x ≤ 0.05 [1, 2]. On the other hand, other works report ferromagnetic (FM) behavior in Mn and Fe [3] and Mn [4] doped SnO2, but the observed magnetism is believed not to be originated in the magnetic ions. Although there haa been considerable experimental effort devoted to DMS based on these wide gap semiconducting oxides, very few attempts to theoretically study the series of 3d TM elements doped SnO2 have been reported so far. Here, we present the results of electronic structure calculations performed through the first principles state-of-the-art Projector Augmented Wave method as implemented in the Vienna Ab-initio Simulation Package code (VASP-PAW ) for 3d TMs as substitutional impurities in SnO2. In this method, the super-cell approach is adopted, and we have chosen the percentages of 4.17 and 8.33 % for the TM ion replacing Sn in the rutile structure of SnO2. The magnetic properties such the occurrence of FM behavior is analyzed through full-spin polarized calculations. In an attempt to shed light towards the understanding of the origin of the magnetism present in the TM-doped systems, we also carried out spin polarized band structure calculations for some configurations in which the 3d element is next neighbor to an oxygen vacancy in the host matrix. Our results indicate an increase of the magnetic moment per atom, when a vacancy approaches the TM ion. Finally, it is also shown for the doped systems some optical properties as obtained from the calculated complex dielectric function. [1] S.B. Ogale et al., Phys. Rev. Lett. 91, 077205 (2003).[2] J.M.D. Coey et al., Appl. Phys. Lett. 84, 1332 (2004).[3] C. B. Fitzgerald et al., J. Appl. Phys. 95, 7390 (2004).[4] N. H. Hong et al., Phys. Rev. B 77, 033205 (2008).
9:00 PM - L5.11
Giant TMR Effect in Granular-structured C60-Co Hybrid Films.
Seiji Sakai 1 , Yoshihiro Matsumoto 1 , Isamu Sugai 2 , Seiji Mitani 2 , Koki Takanashi 2 1 , Yasumasa Takagi 3 , Tsuyoshi Nakagawa 3 , Toshihiko Yokoyama 3 , Hiroshi Naramoto 1 , Yoshihito Maeda 4 1
1 Advanced Science Research Center, Japan Atomic Energy Agency, Tokia, Naka, Ibaraki, Japan, 2 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, 3 Institute for Molecular Science, Japan Atomic Energy Agency, Myodaiji, Okazaki, Aichi, Japan, 4 Department of Energy Science and Technology, Kyoto University, Kyoto Japan
Show Abstract9:00 PM - L5.12
Electronic and Spin States of C60-Co Thin Films Studied by XPS and X-ray Magnetic Circular Dichroism.
Yoshihiro Matsumoto 1 , Seiji Sakai 1 , Hiroshi Naramoto 1 , Takeshi Nakagawa 2 , Yasumasa Takagi 2 , Toshihiko Yokoyama 2 , Toshihiro Shimada 3 , Norie Hirao 4 , Yuji Baba 4 , Yoshihito Maeda 1 5
1 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki, Japan, 2 , Institue for Molecular Science, Okazaki, Aichi, Japan, 3 Department of Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan, 4 Synchrotron Radiation Center, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki, Japan, 5 Department of Energy Science and Technology, Kyoto University, Sakyo-ku, Kyoto, Japan
Show Abstract
We have found that fullerene (C60) and cobalt (Co) mixture films where Co nanoparticles are dispaersed into the matrix of C60-Co compounds show the large tunnel magnetoresistance (TMR) effects at low temperatures. The observed TMR effects cannot be explained only by a spin transport among the Co nanoparticles, so it is expected that the C60-Co compounds play an important role to spin-dependent transport in the films. Therefore, in the present study, we investigated the local electronic and magnetic structures of the C60-Co films by X-ray photoelectron spectroscopy (XPS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. The XPS and XMCD measurements were performed using soft x-ray synchrotron radiation of KEK-PF and UVSOR in Japan, respectively. The different Co content films of C60Cox (x: the number of Co atoms per a C60 molecule) were prepared by a co-deposition method under the UHV condition (<10-7Pa) and then transferred into the experimental chamber without breaking the vacuum. The Co L-edge MCD spectra were taken with the polarization factor of about 0.8.
In the case of the Co dense films (x>10), the peak profiles of MCD spectra are similar to that of the pure Co film. This result indicates that Co particles dispersed into the film have the same magnetic states as crystalline Co. On the other hand, in the case of the Co dilute films (x<10), the MCD spectra show several absorption structures, which have different energy-positions to that of the pure Co film. These structures can be assigned to the formation of hybrid molecular orbitals (MOs) between the Co 3d and C60 π orbitals in the C60-Co compounds from our previous XPS and NEAXFS measurements. In addition, intensities of the MCD signals for the Co dilute films show the strong temperature dependences, which are reasonably consistent with the change of the magnetoresistance ratios observed in the C60-Co films. These findings indicate that there are spin-polarized hybrid MOs in the C60-Co compounds and they can influence the abundance of the MR ratios in the mixture films.
9:00 PM - L5.13
Effect of Growth Temperature and Annealing on Structural and Magneto-transport Properties of Co-doped In2O3 Diluted Magnetic Semiconductors.
A. Ghosh 1 , R. Gupta 1 , P. Kahol 1 , K. Ghosh 1
1 Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri, United States
Show Abstract Recently, Diluted magnetic semiconductors (DMS) based on transition metal oxides have attracted considerable attention due to their potential applications in spintronic devices. In2O3 is a wideband gap semiconductor with unique optical and electrical properties. The high conductivity of In2O3 is due to presence of interstitial oxygen vacancy. By doping with the transition metal such as cobalt, the possibility of room temperature ferromagnetism is expected. Thin films of Co-doped In2O3 diluted magnetic semiconductor having different concentration of cobalt have been grown on c-plane sapphire single crystals using pulsed laser deposition technique. Different characterizations such as X-ray Diffraction, Atomic Force Microscopy, and magneto-transport have been carried out to study the effect of cobalt doping concentration, growth temperature, and annealing on the structural, electrical, and magnetic properties of these films. The films grown at high temperature have preferred orientation along (222) direction, while films grown at low temperature show amorphous nature. It is observed that electrical properties of the films strongly depend on growth parameters as well as oxygen annealing. The resistivity of the films decreases with increase in growth temperature while increases with annealing. On the other hand, mobility of the films increases with increases in growth temperature and annealing, this could be due to improvement in crystallinity of the films. Detailed temperature dependent resistivity, magneto-resistance, and Hall Effect data will be presented.
9:00 PM - L5.14
Magnetotransport and Magnetic Interactions in Cobalt-doped ZnO.
Mathew Ivill 1 , Patrick Sadik 1 , Lii-Cherng Leu 1 , Stephen Pearton 1 , Ritesh Das 2 , Art Hebard 2 , Matthew Chisholm 3 , John Budai 3 , David Norton 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Physics, University of Florida, Gainesville, Florida, United States, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Gainesville, Tennessee, United States
Show AbstractIn recent years, ZnO doped with transition metal cations has been researched as a possible candidate for room temperature spin-electronic (or Spintronic) materials. In particular, cobalt doping has shown promising results in realizing ferromagnetic interactions in ZnO that persist up to room temperatures. This talk examines the structural, magnetic, and magneto-transport properties of cobalt-doped ZnO thin films in relation to growth conditions and cobalt concentration. The films are deposited onto sapphire substrates by pulsed laser deposition, and the cobalt concentration is varied from 2 to 30 atomic%. The magnetoresistance shows a strong dependence on both temperature and carrier concentration, and switches from positive to negative at low temperature as the carrier concentration crosses a metal-to-insulator transition. Evidence for Anomalous Hall Effect (AHE) is also observed in heavily cobalt doped films. Depositions in the presence of oxygen gas cause the magnetization to decrease, suggesting that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Films doped with cobalt concentrations of a few atomic% appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger ferromagnetic signal at room temperature with clear hysteretic shape. Heavily doped films grown at low vacuum pressures are plagued by the precipitation of metallic cobalt nanoparticles that are well-aligned with the ZnO lattice, but can be reduced by tuning the growth pressure while still retaining a ferromagnetic signature of ~0.08 µB/Co. The prospects of using the magnetic moments in device constructs will be discussed.This work is supported by the National Science Foundation and Air Force Office of Scientific Research. ORNL research is sponsored by the Division of Materials Science, US Department of Energy, under contract with UT-Battelle, LLC. The authors would like to acknowledge the staff and facilities of the Major Analytical Instrumentation Center (MAIC), Dept. of Materials Science and Engineering, University of Florida.
9:00 PM - L5.15
Gd-doped ZnO Thin Films for Spintronic Applications.
Deepayan Chakraborti 1 , Sudhakar Nori 1 , John Prater 2 1 , Jagdish Narayan 1
1 Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science Division, Army Research Office, Durham, North Carolina, United States
Show Abstract9:00 PM - L5.16
Structural and Magneto-transport Properties of Cr-doped In2O3 Diluted Magnetic Semiconductors.
N. Ukah 1 , R. Gupta 1 , K. Ghosh 1 , P. Kahol 1 , R. Giedd 2
1 Physics, Astronomy and Materials Science, Missouri State University, Springfield, Missouri, United States, 2 2Roy Blunt Jordan Valley Innovation Center, Missouri State University, Springfield, Missouri, United States
Show Abstract Diluted magnetic semiconductors (DMS), which are formed by doping a transition-metal ion into a semiconductor host lattice, have attracted numerous attention in recent times due to their potential applications in spintronic devices. In2O3, a transparent opto-electronic material, has been shown to be critical in the realization of DMS due to its unique optical and transport properties attributable to the inherent oxygen vacancies in this material. High quality Cr-doped In2O3 diluted magnetic semiconductor thin films have been grown on c-plane sapphire single crystals using pulsed laser deposition technique. A systematic investigation of the effect of oxygen growth pressure on the microstructural, optical, and electro-magnetic properties of these films using various techniques such as X-ray Diffraction, Raman Spectroscopy, Optical Transmission Spectroscopy, and Atomic Force Microscopy, have been carried out in detail. The films exhibit cubic In2O3 single phase with (222) preferred orientation, and characteristic Raman peaks at 308.6, 366.3, 496.3, and 630.2. The observed contraction of the lattice with increasing particle size is attributable to the oxygen vacancies in these films. Also observed is the strong dependence of the electrical parameters such as resistivity, carrier concentration, and mobility on the oxygen partial pressure. The films are highly transparent (> 85 %), with the optical transparency increasing with oxygen pressure. Detailed temperature and magnetic field dependent resistivity, magneto-resistance, and Hall Effect data will be presented.
9:00 PM - L5.17
Novel Nanostructured Magnetic Materials: Self-assembled Epitaxial Nanostructures.
Jagdish Narayan 1 , Gopinath Trichy 1
1 Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractPulsed laser deposition has been used to create novel nanostructured materials either as layered or nanodot structure. By controlling thin-film growth kinetics during island growth, we are able to create three-dimensional self-assembled nanodot structures of Ni and ordered L10 FePt in a given matrix. Epitaxial growth and Integration of Ni and FePt on Si(100) substrate was achieved via domain matching epitaxy which facilitated epitaxial growth across the misfit scale. Magnetic properties can be varied by controlling the orientation and coercivity higher than 1.2 Tesla achieved. These results on ordered L10 FePt will be compared with those Ni with practical implications of information storage1,2.1H. Zhou, D. Kumar, A. Kvit, A. Tiwari, J. Narayan, J. Appl. Phys. 94, 4841 (2003).2 G.R. Trichy, D. Chakraborti, J. Narayan, J. T. Prater, J. Phys. D: Appl. Phys. 40, 7273 (2007).
9:00 PM - L5.19
Charge Transport in Mn Doped Polymers.
Quincy Williams 1 , Osei Amponsah 1 , Rakhim Rakhimov 1 , Alexey Aleksandrov 2 , Natalia Noginova 1
1 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States, 2 , Institute of Synthetic Polymer Materials, Moscow Russian Federation
Show AbstractCoordination polymers exhibiting magnetic properties are of great interest for magnetic and spintronic applications. According to the electron spin resonance studies, the polymers doped with binuclear Mn complexes exhibit magnetic transitions related to disproportionation process Mn3+–O–Mn3+ to Mn2+–O–Mn4+ in the binuclear complex. To better understand the mechanisms of charge transfer and charge transport and characterize materials for spintronic applications, the voltage-current dependences are studied for different temperatures and concentrations of Mn ions. Thermo-activated character of the conductivity is demonstrated and discussed in terms of the small polaron hopping model with the activation energy in the order of 160 – 250 meV.
9:00 PM - L5.2
Site Preference of Atoms in Heusler Alloys Fe3Si and Fe2MnSi Grown on Ge(111) Toward Realization of Ge Channel Spin Transistors.
Yusuke Hiraiwa 1 , Kazumasa Narumi 2 , Atsuo Kawasuso 2 , Yoshikazu Terai 3 , Yuichiro Ando 4 , Kouji Ueda 4 , Taizoh Sadoh 4 , Kohei Hamaya 4 , Masanobu Miyao 4 , Yoshihito Maeda 1 2
1 Department of Energy Science and Technology, Kyoto University, Kyoto Japan, 2 Advanced Science Research Center, Japan Atomic Energy Agency, Takasaki Japan, 3 Department of Engineering, Osaka University, Suita Japan, 4 Department of Electronics, Kyushu University, Fukuoka Japan
Show AbstractA Heusler alloy Fe3Si with a DO3 structure is a base material to synthesize some ternary Heusler alloys such as Fe2TSi (T: Mn, V, Ti, Co) with an L21 structure. These ordered structures can be regarded as consisting of four interpenetration fcc lattices shifted along the body diagonal which originates at sites of A (0,0,0), B(1/4,1/4,1/4), C(1/2,1/2,1/2), and D(3/4,3/4,3/4). In the perfect DO3-Fe3Si, iron atoms distribute at A, B and C sites, whereas silicon atoms are located at the D site. In Fe2MnSi, the site preference occupation of manganese at the B site was confirmed in the wide Mn/Fe ratio. These site occupations of atoms have a great influence on magnetization and are the most important issue to control electronic structures such as spin densities of states and half-metallic properties which dominate performance of spin injection or filtering through the interface between ferromagnetic Heusler-alloy electrodes and semiconductors (Ge in our case). The perfect atomic rows along the <111> direction consists of periodic interval of Fe(A)Fe(B)Fe(C)Si(D). Using Rutherford backscattering spectrometry (RBS), we have examined quality of their axial orientation along the Ge<111> direction in order to find dominant factors for an epitaxy control of these Heusler alloy layers on Ge(111), then obtained χmin=1.5% and the half-angle ψ1/2=0.9 degrees for stoichiometric Fe3Si, χmin=7.5% and ψ1/2=0.82 degrees for off-stoichiometric Fe3Si, and χmin=2.5% and ψ1/2=0.82 degrees for Fe2MnSi from 2.0MeV-4He+ axial channeling experiments at room temperature. The measured values of χmin and ψ1/2 showed good agreement to the values computed from the Debye theory of thermal vibrations. The values of the one-dimensional root-mean-square (rms) thermal vibration amplitude u1 deduced from the experiment were 0.006 nm for stoichiometric Fe3Si, 0.013 nm for off-stoichiometric Fe3Si and 0.0077 nm for Fe2MnSi. A comparison between u1 in this study and the values measured by X-ray absorption studies teaches us that the stoichiometric Fe3Si layer may be close on a perfect crystal, whereas off-stoichiometric Fe3Si may include structural imperfection which has also been suggested by our positron annihilation experiments. The u1 value obtained in Fe2MnSi indicates a small contribution of Mn-Si pairs (with the large u1=0.014-0.017 nm in a disorder of Si atom locations) to increase of χmin, therefore, the presence of Mn atoms replacing at Fe(A) or Fe(C) sites in the first shell of neighbor configurations. The u1 of perfect crystalline Fe2MnSi, where almost of Mn atoms (~0.93 experimentally near x=1.0) occupy the B site, can be determined by Fe-Mn pairs with the small u1 value of 0.005 nm. In this case, we can expect χmin smaller than 2.5% measured in this study. We conclude that the evaluation of thermal vibration by channeling measurements is a powerful technique to investigate actual site preference of each atom in Heusler alloys on a semiconductor channel in spin transistors.
9:00 PM - L5.3
Structure and Influence of the Barrier on Heusler based Tunneling Junctions.
Christian Herbort 1 , Elena Arbelo Jorge 1 , Martin Jourdan 1
1 Institute of Physics, Johannes Gutenberg University, Mainz Germany
Show AbstractTunneling magneto resistance (TMR) measurements are a method to get access to the spin polarization of ferromagnets. However, in general the TMR is related to the spin polarization in a more complicated way as suggested by the simple Julliere model. Smooth and epitaxial barriers like MgO between Fe - layers can increase the TMR drastically due to matching effects of the wave functions. AlOx barriers are considered to be amorphous with rough interfaces. With these assumptions the application of the Julliere model is reasonable. Here we are investigating experimentally the structure and morphology of AlOx - barriers on a high quality Heusler surface. Atomically flat surfaces of the Heusler compounds Co2Cr0.6Fe0.4Al (CCFA) and Co2FeAl (CFA) are deposited by RF - sputtering.We are depositing AlOx barriers by several methods: MBE and sputter deposition with subsequent plasma oxidation as well as by sputter deposition from an Al2O3 target. Electron diffraction (RHEED) shows the epitaxial growth of the metallic Al on top of the Heusler compound. STM investigations demonstrate Stranski-Krastanov morphology of the Al. These results in a 1nm closed layer but strongly increased roughness if additional material is added. Thus a very thin Al2O3 barrier should rather be considered as crystalline and smooth rendering the application of the Julliere model doubtful.However, the plasma oxidation of an only 1nm thick Al barrier results in the oxidation of the highly reactive Heusler electrode. We investigate different oxidation methods including sequential deposition process.The results of the barrier characterization, including Transmission Electron Microscopy (TEM), are related to the tunneling magneto resistance of the junctions.
9:00 PM - L5.4
An Epitaxial Full-Heusler Alloy Fe2MnSi for Group-IV-Semiconductor Spintronic Applications.
Kohei Hamaya 1 , Kenji Yamamoto 1 , Koji Ueda 1 , Yuichiro Ando 1 , Hiroyoshi Itoh 2 , Yoshihito Maeda 3 , Masanobu Miyao 1
1 Department of Electronics, Kyushu University, Fukuoka Japan, 2 Department of Pure and Applied Physics, Kansai University, Suita Japan, 3 Department of Energy Science and Technology, Kyoto University, Kyoto Japan
Show AbstractAn electrical injection of spin-polarized carriers from ferromagnetic contacts into a group-IV-semiconductor is required for combinations of the silicon electronics and future spintronics. In general, the spin injection and detection efficiencies are affected by the interfaces between ferromagnetic metals (FM) and semiconductors (SC) due to the issue of the conductivity mismatch [1]. To solve this fundamental obstacle, an introduction of spin-dependent tunneling via the interfaces has been suggested and demonstrated [2]. If half-metallic ferromagnets (HMFs), which have fully spin-polarized electrons at the Fermi level, can be utilized as the source and drain contacts in group-IV-semiconductor devices, the spin injection and detection efficiencies will be further enhanced. To realize these requirements, we here demonstrate an epitaxial growth of a full-Heusler alloy Fe2MnSi, which has been expected to show half-metallic properties [3], on Ge substrates by using low-temperature molecular beam epitaxy (LT-MBE). Also, we control the doped Mn concentration to modify its magnetic properties and explore high-quality epilayers with room-temperature ferromagnetism. The epitaxial Fe2MnSi layers with a thickness of ~ 100 nm were grown by LT-MBE on n-type Ge(111) substrates at 200oC [4]. The three elements of Fe, Mn, and Si were co-evaporated using Knudsen cells. After the growth, the Fe2MnSi/Ge(111) structures were characterized by x-ray diffraction, Rutherford backscattering spectroscopy (RBS), and transmission electron microscopy (TEM). Cross-sectional TEM images and electron diffraction patterns of the grown Fe2MnSi/Ge(111) clearly showed the formation of an atomically flat FM/SC interface and the ordered L21 phase. For this sample, we measured magnetic field- and temperature-dependent magnetization curves. The saturation magnetization (MS) of ~ 2.17 μB/f.u. and the Curie temperature (TC) of ~ 230 K were found, being similar to the magnetic properties reported in the previous study of bulk Fe1.95Mn1.05Si sample [5]. We further tried to grow the epitaxial Fe3-xMnxSi layers with x ~ 0.6 and ~ 1.4. Significant variations in the MS and TC were observed with keeping highly axial orientation, which was revealed by RBS measurements. We note that, for the sample with x ~ 0.6, the ferromagnetic features were also seen above 300 K. The half-metallicity of this sample is now being discussed based on the theoretical calculation. The LT-MBE technique can expand the growth on Si substrates, so that this study will pave the way for future group-IV-semiconductor spintronics with epitaxial HMFs. [1] G. Schmidt et al., Phys. Rev. B 62, R4790 (2000). [2] E. I. Rashba, Phys. Rev. B 62, R16267 (2000); A. T. Hanbicki et al., Appl. Phys. Lett. 80, 1240 (2002).[3] S. Fujii et al, J. Phys. Soc. Jpn. 64, 185 (1995).[4] K. Ueda et al, submitted.[5] S. Yoon and J. G. Booth, J. Phys. F : Metal Phys. 7, 1079 (1977).
9:00 PM - L5.5
Formation of Nanocrystalline Film of Sr2FeMoO6 on Si(100) by Pulsed Laser Deposition: Observation of Preferential Oriented Growth.
Helia Jalili 1 , Nina Heinig 1 , Tong Leung 1
1 Physics and Chemistry, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractNanocrystalline Sr2FeMoO6 films have been grown on a Si(100) substrate by Pulsed Laser Deposition under different growth conditions, including deposition temperature and time. A nanocrystalline, single-phase Sr2FeMoO6 film was obtained at a temperature as low as 600 C. This high-quality ferromagnetic film was found to have a saturation magnetic moment of 3.4 μB per formula unit and a coercive field of 1.5 kOe at 77 K, with micrometer-sized magnetic domains. By using glancing-incidence X-ray diffraction with different incident beam angles, the crystal structure of the film was sampled as a function of depth. For the as-grown Sr2FeMoO6 films thicker than 60 nm, a preferential orientation of the nanocrystals in the film was observed, despite the lack of good lattice matching with the Si substrate. At a higher deposition temperature of 800 C, the as-grown film exhibited the same saturation magnetic moment but with a discernibly lower coercive field of 0.8 kOe, consistent with the larger grain size obtained at a higher growth temperature.
9:00 PM - L5.6
X-ray Photoemission Study of Sr2FeMoO6 and SrMoO4 Films Epitaxially Grown on MgO(100): Near-surface Chemical-state Composition Analysis.
Helia Jalili 1 , Nina Heinig 1 , Tong Leung 1
1 Physics and Chemistry Depts, University if Waterloo, Waterloo, Ontario, Canada
Show AbstractThin films of the double perovskite ferromagnet, Sr2FeMoO6, have been grown on MgO(100) substrates by pulsed laser deposition at a temperature as low as 600°C . High resolution X-ray diffraction studies were carried out and revealed single-phase Sr2FeMoO6 material epitaxially grown on the MgO. These films were subsequently post-annealed in oxygen, producing epitaxial films of SrMoO4. The magnetization data showed that the post annealing treatment lowered the saturation magnetic moment from 3.4 µB/f.u to 1.4 µB/f.u. To study surface/interface quality and chemical-state composition of grain boundaries, X-ray photoemission as a function of sputtering depth have been conducted on the as-grown and post-annealed samples. By applying appropriate fitting parameters to the photoemission spectra, notable spectral differences can be found in the O1s and Mo 3d regions of these films. The depth profile of the Mo 3d feature indicated that the ratio between different oxidation states of Mo becomes unchanged after a certain depth, which supports the hypothesis that the observed secondary phase do not exist in the grain boundaries and only on the surface the as-grown film.
9:00 PM - L5.7
Magnetic Transitions in the Double Perovskite Sr2FeRe1-xFexO6.
Haitao Gao 1 , Martin Panthoefer 1 , Vadim Ksenofotov 1 , Joachim Barth 1 , Claudia Felser 1 , Wolfgang Tremel 1
1 , Institute fuer anorganische und analytische Chemie,Johannes Guttenberg-Universitaet Mainz, Mainz Germany
Show AbstractThe double perovskites Sr2FeReO6 and Sr2FeMoO6 reveal half-metallic ferrimagnetic behavior at room temperature. Both compounds obey a relatively high Curie temperature (401 K and 415 K, respectively) which is an important condition for a high degree of spin polarization at room temperature. The conceptual basis for half – metallic ferromagnetism is a large density of states of one spin direction at the Fermi level whilst the other spin direction is insulating. In this study we explore the effect of replacing the electronic 5d buffer element Re with variable valency by the 3d transition metal Fe.We present the preparation, crystal structure and physical characterization using magnetism susceptibility measurements, conductivity studies and Mössbauer spectroscopy for the double perovskite (DP) solid solution of Sr2FeRe1-xFexO6 (0.1 ≤ x ≤ 0.5). We observed a phase transition of the solid solution from a double perovskite to a simple perovskite by Rietveld refinements based on X-ray diffraction data. The intensity of the 111 and 002 reflections, indicative of the double perovskite structure, decrease with increasing doping level x. The structure gradually transforms to the simple perovskite SrFeO3 while Re is replaced by Fe. The results obtained for Sr2FeRe1-xFexO6 lead to the conclusion that antiferromagnetic interactions along the Fe-O-Fe units reduce the saturation moment greatly, while the Curie temperature Tc remains almost unaffected. We observed a magnetic transition with temperature when x = 0.3 and x = 0.4. To elucidate this phenomenon, we also prepared Sr2FeRe1-xGaxO6 (0.1 ≤ x ≤ 0.5). In contrast, we did not observe a similar behavior for the Ga analogues Sr2FeRe1-xGaxO6. Therefore we ascribe this magnetic anomaly to a complex interaction between helix coupling in the substructure of SrFeO3 and double exchange coupling in the Fe-O-Re units of the double perovskites.Mössbauer spectra were performed at room temperature. The sharp lines in the parent compounds are indicative of a high degree of Fe-Re order in the structure. With increasing x, the Mössbauer signals broaden due to the Fe/Re disorder, which is consistent with the results obtained from X-ray refinements. Smaller isomer shifts and larger hyperfine fields in solid solutions compared to the non-substituted parent compounds imply that the valence state of Fe increases from +2.7 in the parent compound to +4 in SrFeO3. All doublets, indicative of a paramagnetic behavior, might be due to oxygen deficiency produced by the sample preparation (annealing in evacuated ampoules).Temperature-dependent conductivity measurements suggest semiconducting behavior for both series of compounds. We did not find solid evidence that due to the preferred formation of the simple perovskite SrFeO3 the conductivity within the solid solution series is enhanced because SrFeO3 is metallic.
9:00 PM - L5.8
Magnetic Resonance in Magnetic Nanoparticles: From Quantum to Classical Behavior.
Adrian Radocea 1 , Aleksandr Andreyev 2 , Vadim Atsarkin 3 , Natalia Noginova 4
1 , Cornell University, Ithaca, New York, United States, 2 , Virginia Tech, Blacksburg, Virginia, United States, 3 , Institute for Radio Engineering and Electronics, Moscow Russian Federation, 4 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States
Show AbstractIn order to better understand the transition from quantum to classical behavior in high spin system, electron magnetic resonance (EMR) is studied in suspensions of magnetic iron oxide nanoparticles for various particle sizes (5- 40 nm). The special quantum-related features such as the temperature-dependent narrow spectral component and low-field signals related to multiple-quantum transitions, were clearly observed in the systems of small particles, while the systems with larger particles (40 nm) demonstrate typical ferromagnetic resonance behavior. The detailed studies of the low-field signals in the dependence on temperature, concentration and orientation of the texturized samples were performed and discussed in terms of giant spin model.
9:00 PM - L5.9
Gate Control of Single-electron Spins in Realistic Asymmetric Confining Potentials in III-V Semiconductor Quantum Dots.
Sanjay Prabhakar 1 , James Raynolds 1
1 College of Nanoscale Science and Engineering, State University of New York University at Albany, Albany, New York, United States
Show AbstractAmong recent proposals for next-generation, non-charge-based logic is the notion that a single electron can be trapped and its spin can be manipulated through the application of gate voltages (Rev. Mod. Phys.79, 1217 (2007)). In this talk we present numerical simulations of such Spins in Single Electron Transistors (SSET) for asymmetric confining potentials in support of experimental work at the University at Albany, State University of New York aimed at the practical development of post-CMOS concepts and devices. We use a finite element based simulation strategy to solve Schrödinger-Poisson equations self-consistently (with and without exchange-correlation effects) to obtain realistic confining gate potentials for realistic device geometries. We will discuss the calculation of the gate-tuned ``g-factor" for electrons (Phys. Rev. B 68, 155330 (2003)) in electro-statically defined quantum dots including the Rashba and Dresselhaus spin-orbit interactions computed numerically from realistic wave functions in asymmetric confining potentials. The new simulation results open the possibilities for the design of quantum dot single electron transistors where the electron g-factor can be manipulated through the spin orbit coupling in asymmetric confining potentials by applied external gate voltages. This work is supported through funding from the DARPA/NRI INDEX center.
Symposium Organizers
Claudia Felser Johannes Gutenberg University of Mainz
Arunava Gupta The University of Alabama
Burkard Hillebrands Technische Universitaet Kaiserslautern
Sabine Wurmehl Eindhoven University of Technology
L6: Half-metallic Oxides and New Ferromagnetic Semiconductors, Such as e.g., Double perovskites, Doped ZnO, Spinels, etc.
Session Chairs
Thursday AM, December 04, 2008
Room 204 (Hynes)
10:00 AM - **L6.1
Spin Polarization in Oxides and Chalcogenides.
John M.D. Coey 1 , K. Oguz 1 , J. Alaria 1
1 Physics Department and CRANN, Trinity College, Dublin Ireland
Show Abstract10:45 AM - L6.3
Surface Ferromagnetism in Antiferromagnetic Cr2O3 Thin Films.
Xi He 1 , Yi Wang 1 , N. Wu 1 , Sarbeswar Sahoo 1 , Kirill Belashchenko 1 , A. Caruso 2 , E. Vescovo 3 , Peter Dowben 1 , Alexei Gruverman 1 , Christian Binek 1
1 Physics & Astronomy, University of Nebraska, Lincoln, Lincoln, Nebraska, United States, 2 Department of Physics , University of Missouri, Kansas City KS 64110, Missouri, United States, 3 , National Sychrotron Light Source, Upton, New York, United States
Show AbstractCr2O3 thin films are grown on top of a single crystalline (110) Cr seed layer by Molecular Beam Epitaxy. Metallic Cr is evaporated and transformed into the stable Cr2O3 oxide using an oxygen partial pressure of 2.2*10-6 mbar during deposition while the substrate is maintained at T= 573K. The resulting films of ≈100 nm thickness are structurally characterized by small and large angle X-ray diffraction and atomic force microscopy evidencing (111) texture with pronounced surface roughness of 4 nm. Our preliminary studies of the energetics and thermodynamics of the ordering of the Cr atoms at the Cr2O3 (111) surface predict a very unusual stability of an uncompensated magnetic surface moment when the film is prepared in its antiferromagnetic single domain state. The roughness insensitivity of the uncompensated surface moment is understood as a consequence of a specific Cr-termination. The latter stabilizes the structure by avoiding a charged surface. Its magnetization is carried by the S=3/2 spins of the Cr3+ ions. The uncompensated surface magnetization on mesoscopic lateral length scales is experimentally evidenced by SQUID and polar Kerr magnetometry as well as spin resolved photo emission data. The Kerr and photo emission experiments in particular allow comparing the surface magnetism of films in a multi domain state and those specifically prepared in the antiferromagnetic single domain state. The single domain state is selected in a magnetoelectric annealing process. The latter describes a field cooling protocol in the simultaneous presence of axial magnetic and electric fields. On cooling to below the Néel temperature the magnetoelectric effect of Cr2O3 gives rise to an electrically induced magnetic moment which in turn selects and stabilizes one of the two antiferromagnetic 180° domains via the minimization of Zeeman energy. A prominent spin asymmetry is observed in the spin resolved photo emission data illustrating the uncompensated surface magnetization after magnetoelectric annealing. With magnetic field cooling only, the spin asymmetry is much reduced and with either electric or magnetic field cooling, the surface remains in the multi domain state and shows no spin up/spin down intensity contrast and, hence, implies a zero net surface moment. The results are in agreement with our SQUID and in particular Kerr magnetometry data. The temperature dependent Kerr magnetometry evidences in addition that the surface magnetization undergoes a sequence of magnetic phase transitions which are very likely associated with structural surface transitions in the 100-300K temperature range. The electrically controlled surface and interface magnetism of Cr2O3 and its unusual robustness against roughness make Cr2O3 a prototypical material for potential spintronic applications.
Financial support by NSF through Career DMR-0547887, MRSEC DMR-0213808 and NRI is gratefully acknowledged.
11:00 AM - L6: Oxides
BREAK
11:30 AM - L6.4
Induced Half-metallic State in Cr-based Chalcospinels: CuCr2S(Se)4-xEx (E=F, Cl, Br), CuCr2S(Se)4-x and CdCr2S(Se)4-xE’x (E’=N, P, As).
Yu-Hsiang Wang 1 3 , Mairbek Chshiev 3 , William Butler 2 3 , Arunava Gupta 1 3
1 Chemistry Dept., Uinversity of Alabama, Tuscaloosa, Alabama, United States, 3 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States, 2 Physics Dept., University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractThe metal chalcogenides represent a large class of materials that display a varied range of crystal structure and properties [1]. Recently, the unusual transport properties in the presence of an applied magnetic field, referred to as colossal magnetoresistance and relaxor ferroelectricity and colossal magnetocapacitance has also been observed in the ferromagnetic semiconductor CdCr2S4 [2,3]. Moreover, a dissipationless anomalous Hall current has been observed in the ferromagnetic spinel CuCr2Se4-xBrx [4]. These observations have provoked strong interest in this class of materials for possible application in spintronics. Previous studies suggest that a number of chalcospinels are highly spin-polarized [5,6], and detailed electronic structure calculations of mixed CdxCu1-xCr2X4 (X = S, Se, Te) chalcogenides have been reported [7]. Here we present the electronic structure calculation of CuCr2S(Se)4-xEx (E=F, Cl, Br), CuCr2S(Se)4-x and CdCr2S(Se)4-xE’x (E’=N, P, As). Our results indicate that a number of these mixed composition compounds can also be half-metallic over a range of concentration. The calculations used the Vienna ab-initio simulation package (VASP) with generalized gradient approximation (GGA) for the exchange correlation potential and projector augmented wave (PAW) pseudopotentials. Full structural relaxation both in shape and volume has been performed. The magnetic moment is found to scale linearly with adding or withdrawing electrons in the majority band and varies between 5.1 and 6 μB/f.u. Our results provide a better understanding of the magnetic and transport properties of the Cr-based chalcospinels.[1] Wells, A. F. Structural Inorganic Chemistry: Oxford University Press, Oxford, 1986. [2] Ramirez, A. P. et al. Nature 1997, 386, 156. [3] Hemberger, J. et al. Nature 2005, 434, 364. [4] Lee, W.-L. et al. Science 2004, 303, 1647. [5] Ramesha, K.; Sheshadri, R. Solid State Sci. 2004, 6, 841. [6] Shanthi, S. et al. J. Solid State Chem. 2000, 155, 198. [7] Wang, Y.-H. A. et al. Appl. Phys. Lett. 2008, 92, 062507.
11:45 AM - L6.5
Effect of Particle Size and Molecular Motion on NMR and Spin Relaxation in Systems with Magnetic Nanoparticles.
Tracee Weaver 1 , Roselyn Obasi 1 , Aleksandr Andreyev 2 , Adrian Radocea 3 , Natalia Noginova 1
1 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States, 2 , Virginia Tech University, Blacksburg, Virginia, United States, 3 , Cornell University, Ithaca, New York, United States
Show AbstractProton NMR spectra and spin relaxation times are studied in systems with iron oxide nanoparticles as a function of particles size and solvent viscosity. Transverse and longitudinal relaxation times of solvent protons depend on the both particle concentration and particle size in the liquid systems. Practically no changes in the relaxation times are observed over solid systems of varying particle sizes and concentration, however strong inhomogeneous broadening of the NMR spectrum is seen. In the transitory range from solid to liquid, a specific multi-exponential character of the relaxation kinetics is observed. In liquid systems, the ratio of T1/T2 is determined primarily by the particle size, changing by almost two orders of magnitudes with increase in the nanoparticle size from 5 nm to 40 nm. We discuss a possible model based on diffusion related mechanism with some modifications including account for small aggregates.
12:00 PM - **L6.6
Spin Injection and Giant Magnetoresistance in Ferromagnet–Superconductor Heterostructures*.
Ramesh Budhani 1 , Soumen Mandal 1
1 Condensed Matter Low-dimensional Systems Laboratory, Department of Physics, Indian Institute of Technology Kanpur, Kanpur - 208016 India
Show AbstractElectron transport and magnetic ordering in ferromagnet (FM) - superconductor (SC) heterostructures display a plethora of novel phenomena which acquire increasing richness in systems where the FM electrode has a high degree of spin polarization and superconductivity is exotic. Heterostructures of manganites and high temperature superconducting cuprates offer such systems. We have examined the relevance of pair breaking by dipolar and exchange fields and injected spins into and perpendicular to CuO2 superconducting planes in YBa2Cu3O7 and its low carrier density alloys Y1-xPrxBa2Cu3O7 and also in NbN; is a conventional low Tc superconductor. We further address interlayer exchange coupling (IEC) and giant magnetoresistance in manganite - cuprate – manganite trilayers. The high c-axis resistivity of YBCO makes the IEC decay exponentially with its thickness. Exceedingly large MR is seen in the flux flow regime of the superconducting spacer. We address the MR in three distinctly illuminating ways which involve; i) current density dependence of MR over a broad range of temperature below Tc, ii) field dependence of MR when the magnetizations of La2/3Sr1/3MnO3 (LSMO) layers M1 and M2 are parallel and fully saturated and, iii), dependence of MR on the angle between current and field below and above the superconducting transition temperature. These measurements permit disentanglement of the contributions of flux flow and pair breaking effects in the superconductor, and the intrinsic anisotropic MR of LSMO layers to GMR in FM-SC-FM trilayers, and establish a fundamental theorem which warrants diverging MR in the limit of infinitely conducting spacer. *This research has been supported by the Government of India agencies - Department of Information Technology, Board of Research in Nuclear Sciences and Defence Research and Development organization.
12:30 PM - L6.7
Ferromagnetism in MgO Doped with Ni and Co.
Jagdish Narayan 1 , Sudhakar Nori 1 , Dinesh Pandya 2
1 Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina, United States, 2 Physics Department, Indian Institute of Technology Delhi, New Delhi India
Show AbstractRecently considerable attention has been focused on obtaining room temperature ferromagnetism (RTFM) in transition-metal ion doped transparent metal oxides of Zn, Sn, In and Ti with a view to obtain ferromagnetic semiconductors exhibiting spin polarized current transport for applications in spintronic devices. A large number of systems exhibiting RTFM have been reported by various research groups with varying degree of magnetic moment. Mostly the systems are devoid of any extrinsic magnetic impurities, but have varying degree of conductivity leading the explanation of the observed RTFM to presence of defects or free charge carriers. There have been deliberate attempts to introduce extra free charge carriers in the matrix by doping at cationic or anionic sites by suitable non-magnetic impurities. In general due to use of metal oxides like ZnO, SnO2, In2O3 and TiO2, which have intrinsic presence of oxygen vacancies, it is quite difficult to isolate the role of free carriers and those of oxygen vacancies on the magnetism induced by them. MgO provides a unique host matrix that is by large free of any oxygen vacancies under given experimental conditions. We have carried out experiments using (1) MgO crystals doped with 1- 4 at% Ni impurity, which are devoid of any kind of defects or vacancies and have Ni sitting in Mg substitution sites; (2) MgO 4at% Ni thin films containing vacancy defects deposited by PLD; and (3) MgO bulk crystals doped with 5at.% Co which have been irradiated with heavy ions to create defects/ vacancies. There is no ferromagnetism when there are no defects, only second and third samples show ferromagnetism. Results of our optical absorption, EPR, M-H and M-T data will be presented and discussed. These results establish a strong correlation between RTFM and presence of vacancies which provide the three dimensional ferromagnetic ordering.
12:45 PM - L6.8
Magnetic and Electronic Characters of Photo-induced Magnet (Al,Ru,Fe)3O4 Spinel Ferrite Thin Films.
Teruo Kanki 1 , Yasushi Hotta 1 , Naoki Asakawa 1 , Munetoshi Seki 2 , Hitoshi Tabata 2 , Hidekazu Tanaka 1 , Tomoji Kawai 1
1 Institute of Scentific and Industrial Research, Osaka University, Osaka Japan, 2 Graduate School of Engineering, University of Tokyo, Hongo, Tokyo, Japan
Show AbstractOptical control of magnetism has been investigated as an attractive target for fundamental science and the development of information storage or switching devices. In particular, photoinduced magnetization (PIM) phenomena have been reported for various sorts of cobalt-iron Prussian blue analogs [1]. In magnetic hetero-junctions including magnetic oxide materials [2], photocarrier injections through interfaces changes in magnetic properties. However, the operation of devices at room temperature remains problematic. As promising materials for the generation of high temperature PIM, on the other hand, spinel ferrite oxides with a spin-cluster-glass system have the sufficient potential [3]. Here, we report that room-temperature PIM was achieved in the spinel ferrite (Al,Ru,Fe)3O4 thin films by appropriate dopant-frictions of Ru and Al ions. The material studied was Fe3O4-based compounds partially substituted with Ru and Al ions [4]. Ru3+ or 4+ and Al3+ ions preferentially occupy only the octahedral B-site, comprising a (Fe3+)A-site(Al3+,Ru3+ or 4+,Fe2+ or 3+)B-siteO4 structure. Key points for realizing PIM at high temperature and with high efficiency in this material include selection of the doped transition-metal ions and control of the spin-freezing temperature (Tf), defined as the maximum point of magnetization in a zero-field-cooling (ZFC) process, by moderate doping with non-magnetic ions. Additionally, the films exhibited significant properties as spintronic materials, showing a low saturation magnetization under 0.6 μb/unit cell and good conductivity with a high spin polarized electron level of over 75 %. A combination of high-temperature PIM and the electronic properties associated with spintronics would generate an area of research and development that utilize the degrees of freedom offered by optical systems in the field of spintronics.[1] O. Sato et al, Science 272 (1996)704. [2] S. Koshihara et al, Phys. Rev. Lett. 78 (1997) 4617, H. Katsu et al, Appl. Phys. Lett. 76 (2000) 3245. [3] Y. Muraoka et al, Appl. Phys. Lett. 76 (2000) 1179. [4] T. Kanki et al, Appl. Phys. Lett. 92 (2008) 182505
L7: Advanced Characterization Methods, Including Spin Polarization Measurement
Session Chairs
Thursday PM, December 04, 2008
Room 204 (Hynes)
2:30 PM - **L7.1
Atomic Short-Range Disorder in Heusler Alloys Measured by NMR.
Marek Wojcik 1
1 Division of Physics of Magnetism, Institute of Physics, Polish Academy of Sciences, Warszawa Poland
Show AbstractHalf metallic ferromagnets are a very important group of materials because of their potential application as electrodes in the TMR based devices. For this application, full spin polarization and Curie temperature above RT are requested. Co based Heusler alloys (Co2YZ, Y=Fe, Z=Si, Al) are characterized by the highest Curie temperature among the materials which are theoretically predicted to display a full polarization at the Fermi level. However, the reported experimental results indicate spin polarization substantially below the expected 100%. To understand the correlation between limited transport and structural disorder, it is essential to obtain information on the short range atomic order in the studied samples. To this end, we have used Nuclear Magnetic Resonance (NMR) experiment, which is a sensitive probe of the local environment due to a strong dependence of hyperfine field at the studied nucleus to the atomic configuration in the neighbor shells.In this work the results of recent NMR studies on the Co based ternary and quaternary Heusler thin films as well as on the corresponding bulk reference samples will be summarized.59Co NMR studies in Co2FeAlxSi1-x , (0≤x≤1), Heusler bulk samples have revealed that the short range order strongly depends on composition. Co2FeSi turned out to be the most stable against disorder, showing only marginal amount of Fe antisites on Si sublattice in otherwise perfect L21 structure. Alloying Al with Si in Co2FeSi Heusler alloy introduces a clear tendency to disorder between Fe and Z sublattices lowering the symmetry from L21 towards B2. The key evidence of this disorder is the presence of well resolved satellites due to the Fe/Al antisites extending symmetrically on both sides of the main Co line (the line corresponding to atoms with non distorted environment). The concentration of Fe antisites grows almost proportionally to Al content reaching 5% of Fe/Al swapping for Co2FeAl0.5Si0.5 composition and growing faster to 16.5 % for the end composition Co2FeAl. For the quaternary alloys the 59Co hyperfine field corresponding to Co in the ordered environment depends strongly on Z (Si, Al) element. The variation of 59Co hyperfine field with Si/Al content is due to dependence of Co hyperfine field on valence s electron polarization and can be regarded as the evidence of the Fermi level shift due to the increased number of valence electrons when substituting Si for Al. This information was subsequently used to analyze the NMR data obtained on epitaxial Co2FeAlxSi1-x (x=0, 0.5, 1) thin film samples grown on MgO (001). In all studied thin film Heusler alloys two effects dominate the structural order. The first is the strong dependence of the degree of disorder on Si/Al composition, the second results from a tendency of the films to grow with a slightly Fe-rich stiochiometry. These effects strongly depend on the growth conditions such as deposition temperature, post annealing and the presence of Cr buffer layer.
3:00 PM - L7.2
Exchange Stiffness Constant and Magnetic Anisotropy in Co2-based Heusler Compounds.
Jaroslav Hamrle 1 , Oksana Gaier 1 , Burkard Hillebrands 1 , Horst Schneider 2 , Gerhard Jakob 2 , Martin Jourdan 3 , Takahide Kubota 3 , Yuya Sakuraba 3 , Mikihiko Oogane 3 , Yasuo Ando 3 , Claudia Felser 4
1 Fachbereich Physik and Research Center OPTIMAS, Technische Universität Kaiserslautern, Kaiserslautern Germany, 2 Institut für Physik, Johannes-Gutenberg-Universität, Mainz Germany, 3 Department of Applied Physics, Tohoku University, Sendai Japan, 4 Institute of Inorganic and Analytical Chemistry, Johannes-Gutenberg-Universität, Mainz Germany
Show AbstractCo2-based Heusler compounds are promising candidates for spintronics devices with the potential to provide 100% spin polarization. Here, we report on the determination of exchange interaction, saturation magnetization and magnetic anisotropy in Co2FeSi, Co2FeAl, Co2Cr0.6Fe0.4Al (CCFA), Co2MnSi and Co2MnAlxSi1-x Heusler compounds using Brillouin light scattering (BLS) spectroscopy. The samples are studied as a function of sample composition and sample ordering [1,2].An overview of the measured values of the exchange stiffness constant A, saturation magnetization MS and magnetic cubic volume anisotropy K1 is presented in the attached Table, together with literature values of Curie temperature TC. The values of A in Co2-based Heusler compounds were found to be about 4-times smaller than those of 3d-metals (i.e. Co, Fe, Ni), varying from 0.44 µerg/cm for Co2MnAl to 0.75 µerg/cm for Co2FeSi. The values of TC and MS are about 2-times smaller than those of 3d metals. It has been found, that A in Co2-based Heusler compounds scales linearly with the number of valence electrons. Furthermore, A scales linearly with TC. This behavior is analogous to the Slater-Pauli rule [3] where a linear dependence between MS and the number of valence electrons has been found for Co2-based Heusler compounds. The magnetic cubic volume anisotropy K1 is found to be small in Co2-based Heusler compounds, varying from 0 to 70 kerg/cm3, i.e. being about 10-times smaller than K1 of 3d-metals.Finally, by studying systems with different crystallographic order (as in the case of the B2 to L21 transition for Co2MnSi, where different orders were obtained by different annealing temperatures [1]) it has been found that the values of A depend weakly on the crystallographic order of the compound, whereas K1 depends strongly on the crystallographic order.The project was financially supported by the Research Unit 559 New materials with high spin polarization funded by the Deutsche Forschungsgemeinschaft, by the NEDO International Joint Research Grant Programm 2004/T093 and by the Stiftung Rheinland-Pfalz für Innovation.[1] O. Gaier, J. Hamrle, S. Hermsdoerfer, B. Hillebrands, Y. Sakuraba, and Y. Ando, J. Appl. Phys. 103, 103910 (2008)[2] J. Hamrle, S. Blomeier, O. Gaier, B. Hillebrands, H. Schneider, G. Jakob, K. Postava, C. Felser, J. Phys. D: Appl. Phys. 40, 1563 (2007)[3] G. Fecher, H.C. Kandpal, S. Wurmehl, C. FelserG. Schönhense, J. Appl. Phys. 99, 08J106 (2006)
3:15 PM - L7.3
X-ray Magnetic Circular Dichroism in CrO2-based Heterostructures.
Patrick LeClair 1 , Arunava Gupta 1 , Manjit Pathak 1 , Gary Mankey 1 , Krishna Chetry 1 , William Butler 1
1 Department of Physics and Astronomy and MINT Center, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractSoft x-ray magnetic circular dichroism (XMCD) spectra of half metallic CrO22 thin films epitaxially grown on TiO2 substrates has been investigated at 295 and 180K for both (100) and (110) orientations. Both Cr L edge and O K edge absorption and dichroism spectra have been recorded. The orbital character and magnetic polarization of both O-2p and Cr 3d states near EF has been determined for both orientations according to the sum rules in conjunction with moment analysis. Our results are interpreted in the framework of electronic structure calculations, and compared with extensive magnetometry and ferromagnetic resonance experiments.Our work supports the notion of CrO2 as a half-metallic ferromagnet. Additional experiments on CrO2/RuO2 bilayers will also be reported, motivated by a recent prediction of non-collinear interface spins and induced Ru magnetic moments.
3:30 PM - L7.4
Magnetic and ESR Studies of Nanoparticle Co and Ni in MgO System.
Sudhakar Nori 1 , Jagdish Narayan 1 , John Prater 3 1 , Dimitri Ponarin 2 , Alex Smirnov 2 , Bill Holton 4
1 Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina, United States, 3 Materials Science Division, Army Research Office, Durham, North Carolina, United States, 2 Department of Chemistry, North Carolina State University, Raleigh, North Carolina, United States, 4 Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractPresence of small ferromagnetic nanoparticles exhibit interesting magnetism below ~100 nm size due to the possible formation of single magnetic domains and are viewed as potential candidates for advanced data storage applications. The important role of defects and carriers in magnetic semiconductors and oxide based systems in general is a subject of intense investigation both from the perspective of basic understanding as well as from the technological application view points for they control the macroscopic physical properties, namely, thermal, electrical and magnetic properties1. These will have more pronounced effect when the system size approach nano scale regime. Fine tuning and control of these properties are essential in order to realize any of the next generation devices popularly known as spintronics devices. A typical spintronics-device exploits both the fundamental attributes of an electron the charge and also the spin degrees of freedom. The MgO, due to its high intrinsic purity and ~6 eV bandgap, is an excellent insulating host matrix. Doping with 3d divalent Fe, Co, Ni and Cu can bring in a plethora of changes in magnetic and other physical properties. The magnetic properties of Co and Ni doped MgO system were systematically studied using SQUID, VSM and ESR (both in X- & W-bands) in the range from room temperature to 4 K. Two types of samples were studied, namely, the paramagnetic samples in which Ni atoms occupy substitutional sites and samples with ferromagnetic nano precipitates or clusters with an average size of ~50 nm. A comparison of X- and W-bands for the case of paramagnetic samples shows that the line width is determined predominantly by dipole-dipole interaction. On the other hand the wide particle size distribution of Ni nanoclusters/precipitates in MgO host matrix gives rise to the observed superparamagnetic and ferromagnetic behavior. Decrease in particle size enhances the role of magnetization surface effects that show dominating behavior below a critical size. Thus, controlling the particle shape in addition to its size is becoming increasingly important. We have shown that high quality Co- and Ni-doped MgO crystals can be prepared that are essentially devoid of any point defects down to significantly low values, exhibit nearly ideal paramagnetic behavior when the divalent Ni ions occupy the substitutional sites in the host MgO matrix. When such a crystal is heated in a reducing atmosphere, the nickel dopant precipitates into magnetic nanoparticles. One of the requirements for magnetic memory application is that the Curie temperature of this magnetic array be above room temperature. Ideally, such crystals are supposed to have crystal imperfections and vacancies as low as possible. Further, the unpaired electronic spin states can be understood using electron spin resonance spectra uniquely suited for studying paramagnetic point defects.1S.Ramachandran, J.T.Prater and J.Narayan, Appl.Phys.Lett. 90, 132511 (2007).
3:45 PM - L7.5
Damping in Epitaxial CrO2(110).
Tim Mewes 1 , Hwachol Lee 1 , Krishna Chetry 2 , Claudia Mewes 1 , Arunava Gupta 2
1 Physics, University of Alabama, Tuscaloosa, Alabama, United States, 2 Chemistry, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractWe report on the magnetization dynamics of epitaxial CrO2 thin films grown on TiO2(110) substrates using chemical vapor deposition (CVD) using a CrO3 precursor as described elsewhere [1]. Angular dependent ferromagnetic resonance (FMR) measurements reveal an in-plane uniaxial anisotropy with the easy axis along the c-axis. Frequency dependent FMR measurements were carried out over a frequency range from 7-60 GHz along the in-plane easy axis of the film. The resonance field dependence on the microwave frequency is well described by the Kittel formula, enabling the determination of Meff and γ of the films. The ferromagnetic resonance linewidth depends only weekly on the microwave frequency: the linewidth has a minimum around 20 GHz and increases linearly for larger frequencies with a very small slope. Based on this we estimate the Gilbert damping constant (intrinsic) to be of the order 10-4, i.e. very small. We have confirmed this by performing frequency dependent measurements of the out-of plane FMR linewidth. Although the extrinsic linewidth contribution in this case is significantly smaller, it is in both cases the main contribution to the magnetization relaxation and can therefore be further optimized.
References: [1]: X. W. Li, A. Gupta, and G. Xiao, Appl. Phys. Lett. 75, 713 (1999).
4:00 PM - L7:Methods
BREAK
4:30 PM - **L7.6
Exploring the Electronic Structure of Heusler Compounds by Hard X-ray Photoemission Spectroscopy.
Gerhard Fecher 1
1 , Johannes Gutenberg - Universität, Mainz Germany
Show AbstractThe electronic structure plays an important role in determining the magnetic properties of materials with high spin polarization and in particular half-metallic ferromagnets. Photoemission is the most powerful tool to investigate the electronic structure of solids. In particular, it is one of the best suited techniques for studying the occupied electronic states of materials. Even though many reliable results were obtained by ultraviolet photoemission, the low kinetic energies result in a small electron mean free path of less than 0.5 nm at kinetic energies below 100 eV. In X-ray photoelectron spectroscopy (XPS) as it is usually used, the medium energies for excitation result in a mean free path of only about 2nm at photon energies of about 1.2 keV. The situation improves significantly at higher energies. In hard X-ray photoelectron spectroscopy (HAXPES) with excitation energies of about 8 keV, a high bulk sensitivity is attained with an escape depth larger than 10 nm, resulting in a probing depth of about 40 cubic cells for typical Heusler compounds. An other drawback of XPS is the imbalance of the partial cross section that overestimates the contribution of d-states. This can be overcome in the energy range from 3 keV to 8 keV where the partial cross sections of the partial waves (s, p, and d) are nearly equal and allow a direct comparison of the measured spectra to the calculated density of states.In the present contribution the effect of the electron mean free path and the partial cross sections in Heusler compounds as well as the barrier materials of magnetic tunneling junctions will be discussed. The measured spectra from Heusler compounds will be compared to the calculated electronic structure [1,2]. It will be shown that the spectra recover the electronic fingerprints of half-metallic ferromagnets already in spin integrated spectra. The analysis of the valence band spectra clearly hints on correlation effects in Co based Heusler compounds [3]. The magnitude of the electron-electron interaction in the Co planes will be determined from the position of prominent peaks in the density of states arising from localized states. It will also be demonstrated that the electronic structure of deeply buried Heusler compounds can be detected even under 20 nm thick MgO layers [4]. Finally, future developments and prospects of the method will be discussed.[1] B. Balke, G. H. Fecher, et al.; Phys. Rev. B 74: 104405 (2006).[2] G. H. Fecher, B. Balke, et al.; J. Phys. D: Appl. Phys. 40: 1576 (2007).[3] H. C. Kandpal, G. H. Fecher; Phys. Rev. B 73: 094422 (2006).[4] G. H. Fecher, B. Balke, et al.; Appl. Phys. Lett. 92: 193513 (2008).