AVS2001 Session MI+EL-TuA: Spintronics II: Spin Injection & Transport

Tuesday, October 30, 2001 2:00 PM in Room 110

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	MI+EL-TuA-1 III-V Based Epitaxial Magnetic Heterostructures: Large Tunneling Magneto-Resistance M. Tanaka (University of Tokyo, Japan) Tunneling magnetoresistance (TMR) is one of the most important phenomena for future spin-electronics devices. Here, we present very large TMR (>70%) in all-semiconductor magnetic tunnel junctions (MTJs), having (GaMn)As ferromagnetic electrodes separated by an ultrathin AlAs tunnel barrier.¹ Trilayer heterostructures, (Ga_1-xMn_x)As(x=0.04, 50nm)/AlAs(d nm)/(Ga_1-xMn_x)As (x=0.033, 50nm), were grown on p⁺GaAs substrates by low-temperature MBE. Mesa etched MTJs with the barrier thickness d ranging from 1.3nm to 3.0nm were fabricated, and showed clear TMR due to the change from parallel to anti-parallel magnetization of the two ferromagnetic (GaMn)As layers. Very high TMR ratios up to 75 % were observed at 8K for the junction with d=1.5nm. For d≥1.6nm, the TMR ratio was found to decrease with the barrier thickness. This behavior can be explained by calculations assuming that the wavevector k// of carriers is conserved in tunneling. This means that conventional Julliere's model is not valid in such epitaxial MTJs. Also, we have found that the TMR behavior strongly depends on the applied magnetic field direction, which is well explained by the cubic magneto-crystalline anisotropy of GaMnAs.² Unlike the conventional MTJs made of polycrystalline ferromagnetic metals and an amorphous tunnel barrier, the present MTJs are all-epitaxial monocrystalline semiconductor-based junctions, which have the following advantages: (1) MTJs made of all-semiconductor heterostructures can be integrated with semiconductor circuitry. (2) Many parameters, such as the barrier height, barrier thickness, and Fermi energy of the electrodes, are controllable. (3) Introduction of quantum heterostructures, such as resonant tunneling structures, will be easier than any other material system. ¹ Y. Higo and M. Tanaka, Physica E (2001), in press. ² Y. Higo and M. Tanaka, J. Appl. Phys. (2001), in press.
2:40 PM		MI+EL-TuA-3 Spin Filtering and Tunneling Magnetoresistance in Double Barrier Magnetic Heterostructures A.G. Petukhov, D.O. Demchenko, A.N. Chantis (South Dakota School of Mines and Technology) We report the results of our theoretical studies of spin-dependent resonant tunneling of holes in GaMnAs-based double-barrier magnetic heterostructures. Our approach is based on the k.p Hamiltonian with exchange-field parameters obtained from first-principle calculations and on multi-band transfer matrix technique. Zeeman splittings of the light hole (LH1) and heavy hole (HH2) resonant peaks are the most striking features of the calculated I-V characteristics of the structures with magnetic emitters. This finding is in good agreement with experimental data by H. Ohno et al.¹ The splittings of other resonant channels are smeared due to various bandstructure effects. The resonant tunneling through magnetic quantum wells in GaAs/AlAs/GaMnAs/AlAs/GaAs resonant tunneling diodes displays even more pronounced Zeeman splittings of the resonant channels. These splittings strongly depend on the orientation of the magnetization. The spin polarization of the transmitted current is also quite significant and can be controlled by an external bias. This spin-filtering effect also leads to tremendous enhancement of tunneling magnetoresistance at small biases. ¹H. Ohno et al., Appl. Phys. Lett. 73, 363 (1998).
3:00 PM		MI+EL-TuA-4 Magnetotransport in Digital Ferromagnetic Heterostructures T.C. Kreutz, G. Zanelatto, R. Kawakami, E. Johnston-Halperin, E.G. Gwinn, A.C. Gossard, D.D. Awschalom (University of California, Santa Barbara) Recent studies of digital ferromagnetic heterostructures (DFH), in which fractional monolayers (ML) of MnAs alternate with interlayers of low temperature (LT) GaAs, have shown that the Curie temperature, Tc, is sensitive to the separation between MnAs sheets.¹ We report studies of in-plane magnetotransport in these structures, for Be-doped and nominally undoped LT GaAs interlayers with thicknesses from 10 to 40 ML. For undoped DFH grown at 260 C, structures with 10 ML interlayers show an anom alous Hall effect, while structures with 20 and 40 ML interlayers show only the ordinary Hall effect. The decrease in Tc with increasing interlayer thickness is accompanied by a decrease in the Hall carrier density and mobility. The magnetoresistance of the 10 ML sample has a similar field dependence to bulk GaMnAs. The 20 and 40 ML magnetoresistances are qualitatively different. Effects of Be doping are also considered for DFH samples.. ¹ R.K. Kawakami, et al APL 2379 (2000).
3:20 PM		MI+EL-TuA-5 Theoretical Band Offsets in Magnetic Semiconductor Heterostructures: CdCr₂Se₄ on Si and GaAs J.M. Sullivan, S.C. Erwin (Naval Research Laboratory) Ferromagnetic semiconductors grown on semiconductor substrates are being widely investigated as spin injection sources for spintronics applications. Of the many issues critical to the injection efficiency, the band offset plays a central role. In particular, band offsets provide a direct link between microscopic parameters which can be determined theoretically, and macroscopic properties which can be measured experimentally. Moreover, magnetic band offsets can be tuned by methods well known from traditional band-offset engineering, and thus will be important for efforts to optimize injection efficiencies. Here we present first-principles results for the magnetic band offsets in heterostructures consisting of CdCr₂Se₄, an n-type ferromagnetic semiconductor with a Curie temperature of 130 K, grown on Si and GaAs substrates. We first use density-functional total-energy methods to explore and identify stable and metastable interface structures, taking into consideration different interface terminations, intermixing, and polar vs. non-polar interfaces. For the thermodynamically favorable interfaces we then apply standard first-principles methods¹ for calculating the band offsets. Finally, we present detailed comparisons to recent experiments² for these heterojunctions. ¹ A. Franciosi and C. G. Van de Walle, Surf. Sci. Rep. 25, 1 (1996). ² D. Park et al., unpublished. .
3:40 PM		MI+EL-TuA-6 Effects of Interface Structure and Chemistry on Spin Injection Efficiency in Spin-LEDs R.M. Stroud, Y.D. Park, A.T. Hanbicki, B.R. Bennet, B.T. Jonker (Naval Research Laboratory); G. Itskos, M. Furis, G. Kioseoglou, A. Petrou (SUNY Buffalo) The efficiency of spin injection across a heterointerface can be strongly affected by the structure and chemistry of that interface. To quantify the relationship between interface quality and spin injection efficiency, spin-LEDs make an ideal test system. In a spin-LED, carriers with a net spin polarization are injected into a LED, where radiative recombination results in circularly polarized light emission. The optical circular polarization directly reflects the degree of the spin polarization of the injected current. By varying the growth conditions to vary the quality of the interface for ZnMnSe/AlGaAs/GaAs/AlGaAs spin-LEDs, injection efficiencies of 0 to 65% have been achieved. In this system, the primary structural defect, identified by cross sectional transmission electron microscopy, is a stacking fault that nucleates at the ZnMnSe/AlGaAs interface. Stacking fault densities ranged from 10¹⁰ cm^-2 for the lowest efficiency samples to < 10⁸ cm^-2 for the 65% efficiency sample., The optical polarization scales with the stacking fault density, which indicates that the spin injection efficiency is affected by the ZnMnSe/AlGaAs interface structure and chemistry. We compare results for growth on As- and Ga-terminated AlGaAs surfaces, and for structures grown with a Zn- and Se-initiated growth of the ZnMnSe polarized contact layer. These results demonstrate that although the spin injection efficiency is sensitive to interface quality, the spin injection effect is robust enough in all-semicondutor spin-LEDS to withstand moderately high defect densities, and can be produced using pre-grown LEDs. ¹ This work supported by ONR and the DARPA SpinS program.
4:00 PM	Invited	MI+EL-TuA-7 Recent Developments in Spin Electronics A. Fert (Université Paris-Sud, France) My talk reviews recent developments of the research in spin electronics at Unité Mixte de Physique in Orsay. I will focus on the three following topics: 1)Magnetization reversal by injection of a spin-polarized current: After an introduction on the effect predicted by Slonczewski (and also Berger), I will present experimental results obtained on pillar-shaped trilayers (collaboration with LPN-CNRS and University of Brest) and I will describe the pending problems in the understanding of the spin transfer mechanisms generating the reversal. Current-induced reversal could lead to general applications for the switching of spin electronic devices and I will present the perspective in this direction. 2)Magnetic tunnel junction (MTJ): Although applications of the magnetoresistance (TMR) of MTJ are already around the corner, physics of spin-polarized tunneling is still far from being clearly understood. I will present experimental results on epitaxial (single crystal) MTJ which shed light on the physical mechanisms of TMR. 3)Spin injection into semiconductors: the development of devices combining ferromagnets and semiconductors is an important challenge in spin electronics. I will present a model clearing up the conditions for efficient spin injection from a ferromagnetic metal into a semiconductor.
4:40 PM		MI+EL-TuA-9 Structural and Optical Characterization during Growth of Co on Ga_1-xIn_xAs(001) K. Lüdge (Technische Universität Berlin, Germany & University of Minnesota); P. Vogt (Technische Universität Berlin, Germany); B.D. Schultz (University of Minnesota); C.J. Palmstrom (University of Minnesota, United States); W. Braun (BESSY); N. Esser, W. Richter (Technische Universität Berlin, Germany) The growth of magnetic overlayers on semiconductors has received considerable interest due to their potential use in spintronic devices. The interface between the ferromagnet and semiconductor is critical to spin polarized transport across the interface. Thus, it is important to determine dependence of the interfacial structure and the crystalline quality of the ferromagnetic film on the substrate temperature and surface reconstruction. The initial growth of Co on GaAs(001) has been studied using STM, PES and reflectance anisotropy spectroscopy (RAS). The Co tends to be disordered when grown at room temperature. However, crystalline islands are observed at a substrate temperature of 430 K. STM-images taken during Co deposition show, that the substrate surface morphology does not change during deposition despite the change in surface reconstruction. The initial growth on the As-rich c(4x4) surface is different from the growth on the c(8x2) Ga-rich reconstruction. For growth on the c(8x2) surface two different growth modes can be distinguished. At first Co-atoms are adsorbed into rows to form one-dimensional chains. Further deposition results in epitaxial cubic islands. The PES data indicate two metallic components in the Ga3d core level. One is interpreted as resulting from CoxGay and the other from metallic Ga. The As3d core level contains two different components leading to the conclusion of As-Co bonds at the interface and access As on top. The influence of lattice mismatch on the structural and magnetic properties of the epitaxial Co-layer will be studied by Co growth on Ga_1-xIn_xAs(001).
5:00 PM		MI+EL-TuA-10 Structural and Magnetic Characterization of the Fe_xCo_1-x / GaAs(100) Interface B.D. Schultz, L.C. Chen, A. Isakovic, J. Strand, P.A. Crowell (University of Minnesota); M.M.R. Evans (University of Wisconsin-Eau Claire); C.J. Palmstrom (University of Minnesota) Two distinct surface contributions to the magnetic anisotropy can be used to control the magnetic properties of thin films of bcc Fe_xCo_1-x grown on GaAs (100). On bare GaAs (100), the sp³ bonding in the zincblende structure results in a two-fold surface symmetry of the gallium and arsenic bonding and a (2x4)/c(2x8) surface reconstruction for an arsenic surface coverage ~0.75 monolayers. This two-fold surface symmetry reduces the expected cubic four-fold magnetic anisotropy for Fe_1-xCo_x films to a strong uniaxial magnetic anisotropy. However, four-fold symmetry is restored in films grown with an interlayer of Sc_1-yEr_yAs(100), in which the rock-salt structure provides an unreconstructed surface. Initial STM images of 0.10 monolayer deposited Fe_xCo_1-x on GaAs(100) (2x4)/c(2x8) surface grown by MBE at 95°C indicate isolated clusters of atomic dimensions with preferential attachment along the arsenic dimer rows. The images also indicate that the (2x4)/c(2x8) reconstruction remains relatively undisturbed by the initial nucleation and growth at this coverage. The deposition of bcc Fe_xCo_1-x on Sc_yEr_1-yAs(100) indicates there is no preferred nucleation site for the Fe_xCo_1-x atoms on the unreconstructed surface. Control of the interfacial properties of ferromagnetic metals and semiconductors is important for optimizing spin dependent transport across these interfaces. Spin dependent ejection of photo generated carriers from GaAs(100) into Fe_xCo_1-x ferromagnetic metal contacts has recently been observed. This paper will emphasize the correlation between the structure and chemistry of the Fe_xCo_1-x/GaAs and Fe_xCo_1-x/Sc_1-xEr_xAs/GaAs interfaces, determined by STM, RHEED, LEED, XPS, RBS, XRD and TEM, and the magnetic and transport properties. Supported by: ONR-N/N00014-1-0233, DARPA N/N00014-99-1-1005, and NSF-MRSEC NSF/DMR-9809364.