Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X6: Tunnel Magnetoresistance: Yesterday, Today, and Tomorrow |
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Sponsoring Units: GMAG Chair: Igor Zutic, State University of New York at Buffalo Room: Morial Convention Center RO4 |
Friday, March 14, 2008 8:00AM - 8:36AM |
X6.00001: Tunnel Magnetoresistance, Spin Accumulation, and Spin Hall Effect Invited Speaker: In a tunnel junction consisting of two ferromagnetic (F) electrodes, the tunneling current depends on the relative orientation of the magnetization of F electrodes. This so-called tunnel magnetoresistance (TMR) has been extensively studied for various device applications. When a non-magnetic metal or semiconductor (N) is introduced in such a tunnel junction, the spin-polarized current is injected into N, and the spin current and spin accumulation occur in the region of the order of the spin diffusion length ($\lambda )$, which is in the range of a few 10 $\mu $m $\sim $ a few $\mu $m depending on materials. Therefore, in a device with the size of the order of $\lambda $, the spin current and spin accumulation give size to a variety of novel spin dependent phenomena [1]. The spin current is scattered by the spin-orbit interaction and induces the charge current in the perpendicular direction. It is also possible that the charge current is converted into the spin current in the perpendicular direction by the spin-orbit interaction. These conversions are the Spin Hall effects. If N is superconducting in the tunnel device, the situation is more dramatic. The effects of spin current and spin accumulation are strongly enhanced in the superconducting state. Some of the unique experiments are proposed. \newline [1] ``Concepts in Spin Electronics,'' ed. S. Maekawa (Oxford University Press, 2006). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X6.00002: Spin-Dependent Tunnelling Phenomena in Magnetic Tunnel Junctions with MgO Tunnel Invited Speaker: |
Friday, March 14, 2008 9:12AM - 9:48AM |
X6.00003: Electron tunneling in epitaxial magnetic tunnel junctions Invited Speaker: The remarkable progress in the performance of tunneling magnetoresistance (TMR) junctions using MgO as the barrier layer, has confirmed the theoretical prediction of a very high TMR ratio from first-principles calculations [1,2]. The theoretical prediction was based on the understanding of the band filtering effect by a class of barrier materials, including ZnSe, and even vacuum, in addition to MgO, along particular crystalline orientations. The combination of the preferential filtering for the electrons with the $\Delta_1$ symmetry by the barrier layer, and the presence of the $\Delta_1$ band only in the majority spin channel of Fe, FeCo and Co electrodes, results in the high TMR ratio previously believed only achievable with half-metallic electrodes. Here we show that the remarkable agreement between the experiment and the first-principles theory goes beyond the TMR. The effects of barrier thickness, interface resonance states, and quantum confinement calculated from the first-principles for MgO based magnetic tunnel junctions are compared with experimental data. In addition to excellent agreement for each case, additional insights are obtained from the first-principles calculations that are otherwise not directly available from the experiments. We also discuss other candidate materials for high TMR junctions. \newline \newline [1] W. H. Butler, X.-G. Zhang, T. C. Schulthess et al., Phys. Rev. B 63, 054416 (2001). \newline [2] J. Mathon and A. Umerski, Phys. Rev. B 63, 220403 (2001). [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X6.00004: TMR-related effects in structures involving semiconductors Invited Speaker: The growth of semiconductor heterostructures incorporating ferromagnetic material is a challenge for today's spintronic. We will report on GaMnAs/III-V and MnAs/III-V tunnel junctions that fulfill this condition. In the case of GaMnAs related junctions, the complexity of the transport mechanisms associated with spin-orbit coupled states make this material a powerful means for finding novel effects and provides new challenges for theoretical understandings. This includes tunnel magnetoresistance (TMR) across single and double barriers [1] and tunnel anisotropic magnetoresistance (TAMR). As an illustration, the resonant TAMR on a GaAs quantum well can be used as a probe of the GAMnAs valence band anisotropy [2]. In the case of MnAs, the necessary low temperature growth mode to avoid intermixing at the interfaces favors the insertion of As anti-site in the III-V spacer layer. We will show how the tunneling magnetoresistance is then modified and how the major role of the defects in the conduction can be taken into account. For this we have proposed an analytical model of spin-dependent resonant tunneling through a 3D assembly of localized states (spread out in energy and in space) in a barrier [3]. An inhomogeneous distribution of localized states leads to resonant tunneling magnetoresistance inversion and asymmetric bias dependence as evidenced with a set of experiments with MnAs$/$GaAs(7--10 nm)$/$MnAs tunnel junctions. By extension, we show the possibility of using such spectroscopic measurements to probe the spin asymmetry of ferromagnetic electrodes sandwiching an inhomogeneous tunnel barrier. \newline [1] R. Mattana et al. Phys. Rev. Lett. 90, 166601 (2003) \newline [2] M. Elsen et al. Phys. Rev. Lett. 99, 127203 (2007) \newline [3] V. Garcia et al. Phys. Rev. Lett. 97, 246802 (2006) [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X6.00005: Influence of spin polarized current on superconductivity and spin transport in organic semiconductors Invited Speaker: Spin conserved tunneling from a ferromagnetic (FM) electrode through an insulating barrier with a superconductor as the spin detector, was discovered by Meservey and Tedrow three decades ago. This phenomenon of spin polarized tunneling has been successfully utilized to understand many aspects of magnetism and superconductivity. In recent years enormous success is seen in magnetic tunnel junctions both from the fundamental viewpoint as well as due to their application in information technology. In general, BCS superconductivity with Cooper pairs formed at the Fermi level with electrons of opposite spins is not compatible with ferromagnetism which have parallel spin configuration. The superconducting state can be influenced by injecting spin polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance (MR) of over 1100{\%} for a F/I/S/I/F multilayer system (I - insulator) or even infinite MR can be achieved in epitaxial metallic systems. Due to the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T$_{C})$ in the spin parallel configuration is shifted below that of the spin anti-parallel configuration. This T$_{C}$ shift can much larger than that predicted by theories. Oscillation in Tc is also observed. The field is the spin transport properties in organic semiconductors (OSs), a least explored area. Because OSs are composed of mostly light elements (i.e. C, H, N, O) and thus have a weaker spin-orbit interaction, low hyperfine interactions, spin coherence lengths can be long in these materials. Large spin decay length was observed in amorphous Rubrene. Recent developments in electron spin polarized tunneling through ultrathin layers of the molecular organic semiconductors such as Alq3 and Rubrene will be discussed. [Preview Abstract] |
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