Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A14: Focus Session: Spin Dependent Tunneling I |
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Sponsoring Units: GMAG DMP FIAP Chair: Jinke Tang, University of New Orleans Room: Colorado Convention Center Korbel 4D |
Monday, March 5, 2007 8:00AM - 8:12AM |
A14.00001: Spectroscopic Studies on epitaxially grown Fe/MgO/Fe magnetic tunnel junctions on W(100) Tae-Young Khim, Jun-Sik Lee, Kee-Jeong Rho, Hoyoung Jang, Byeong-Gyu Park, Jae-Hoon Park, Jae-Young Kim, Hangil Lee In these days, there is a big interest in epitaxial Fe/MgO/Fe MTJ systems in the TMR issues. We investigated electronic states and magnetic behaviors of epitaxially grown Fe/MgO/Fe on W(100) at different MgO thicknesses using XAS, MCD, XPS, SRPES, and discussed a few noticeable phenomena in this system. First, XAS and MCD spectra at Fe $L_{2,3}$-, O $K-$, and Mg $K-$edges varies as a function of MgO thickness. Second, in MgO/Fe/W(100), the magnetic hysteresis curve suddenly changes at a certain MgO layer thickness, probably due to a developed strain at the MgO/Fe interface. Finally, in Fe/MgO/Fe/W(100), an antiferromagnetic and a ferromagnetic interlayer coupling between two Fe ferromagnetic layers were observed for different MgO layer thicknesses, and we determined the spin polarization of the density of states. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A14.00002: Electronic structure of sputter deposited MgO(100) tunnel barriers in magnetic tunnel junction structures exhibiting giant tunneling magnetoresistance See-Hun Yang, Mahesh Samant, Stuart Parkin Giant tunneling magnetoresistance (TMR) in magnetic tunnel junctions formed with crystalline MgO tunnel barriers [1] have potential applications in a wide variety of spintronic devices. However, the relationship of the TMR to the detailed chemical and electronic structure of the MgO barrier and its interfaces with the ferromagnetic electrodes is not yet fully understood. We have carried out valence band photoemission spectroscopy and x-ray absorption spectroscopy to characterize the chemical state and electronic structure of sputter deposited, highly oriented, MgO (001) barriers and its interfaces with ferromagnetic electrodes. A large band gap of $\sim $7.5 eV is found even for ultrathin MgO layers. This is consistent with barrier heights found from fitting current versus voltage curves providing that very small effective electron masses are used. We discuss the role of thin Mg interface layers that we have used to reduce oxidation of the underlying ferromagnetic layer during the MgO layer formation [1]. [1] S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, S.-H. Yang, Nature Materials 3, 862 (2004). [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A14.00003: Influence of disorder on tunnel magnetoresistance V. Karpan, I. Marushchenko, A. Starikov, P.X. Xu, K. Xia, M. Zwierzycki, P.J. Kelly In spite of recent success in observing large values of tunnelling magnetoresistance (TMR) in epitaxial FeCo$|$MgO$|$FeCo magnetic tunnel junctions (MTJ's), the values reported are still two orders of magnitude lower than those predicted by first-principles transport calculations for ideal, defect-free MTJ's. In this talk, we present results of a systematic study of the influence of roughness and leads disorder on TMR in a FeCo$|$vacuum$|$FeCo model system. Our study is based upon a tight-binding muffin-tin orbital (TB-MTO) implementation of the Landauer-B\"{u}ttiker scattering theoretical formulation of transport. Disorder is included in the transport calculation using large lateral supercells. In our study we found that in case of ideal, perfectly ordered systems, the values of TMR comparable in size to those predicted by others in which the mechanism responsible is the very effective transmission through resonant states close to the Fermi level for the minority spin channel. Roughness is found to quench these resonances leading to a drastic reduction of TMR to values comparable to those seen in experiment. Leads disorder is found to quench the TMR but less strongly than roughness. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A14.00004: First-principles prediction of high Curie temperature for ferromagnetic bcc-Co and its relation to Co/MgO/Co magnetic tunnel junctions Marjana Lezaic, Phivos Mavropoulos, Stefan Blugel We determine from first principles the Curie temperature of bulk Co in the ground state hcp phase and the metastable fcc and bcc phases. For fcc-Co we found a Curie temperature of $T_{\mathrm{C}}(\mbox{fcc-Co})=1280$~K, in reasonable agreement with experimental results. For bcc-Co, a Curie temperature of $T_{\mathrm{C}}(\mbox{bcc-Co})=1400$~K is predicted. This suggests that bcc-Co/MgO/bcc-Co tunnel junctions offer high tunneling magnetoresistance ratios even at elevated temperatures, giving them an advantage over Fe/MgO/Fe junctions. $T_{\mathrm{C}}(\mbox{bcc-Co})$ appears robust under tetragonalization upon epitaxial growth on MgO, in contrast to Fe for which $T_{\mathrm{C}}(\mbox{bcc-Fe})$ is found to drop by more than 20\% (from 970~K to 750~K) upon such a tetragonalization. We find that FeCo alloys have an even higher $T_{\mathrm{C}}$, as high as 1660~K for ordered FeCo. We discuss the origin of these effects in terms of the electronic structure and densities of states. The Curie temperatures are calculated by mapping {\it ab initio} results to a Heisenberg model, which is solved by a Monte Carlo method. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A14.00005: Theory of the spin-orbit induced anisotropy in the tunneling magnetoresistance of magnetic tunnel junctions. Alex Matos-Abiague, Jaroslav Fabian We investigate the effects of the spin-orbit interaction on the tunneling magnetoresistance of magnetic tunnel junctions. We propose a theoretical model in which the two-fold symmetry of the tunneling anisotropic magnetoresistance (TAMR) effect, observed in Fe/GaAs/Au tunnel junctions, originates from the interference between Dresselhaus and Bychkov-Rashba spin-orbit couplings at the interface between the ferromagnetic (Fe) region and the GaAs tunnel barrier. Bias induced changes of the Bychkov-Rashba spin-orbit coupling can result in a flipping of the axis of the two-fold symmetry of the TAMR. The theoretical calculations are in good agreement with recent experiments [1]. \newline \newline [1] J. Moser, A. Matos-Abiague, D. Schuh, W. Wegscheider, J. Fabian, and D. Weiss, cond-mat/0611406. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A14.00006: Bias induced inversion of the tunneling magnetoresistance Andrej Sokolov, Renat Sabirianov, Ildar Sabirianov, Bernard Doudin Demand for high density at low cost two-terminal nonvolatile memory devises has boosted research interest in electroresistive phenomena where conductivity exhibits voltage--induced resistance jump up to several orders in magnitude. NiO based junctions are particularly promising because of its high ON/OFF ratio and simple constituents. We report low temperature transport properties of electrochemically synthesized Ni/NiO/CoNi/NiO/Co magnetic double barrier tunneling junction (MTJ) in nanowires with diameter of 70nm, and NiO barrier thickness of about 2 nm. Resistance bi-stability of double NiO nanojunctions is observed and reaches 100{\%}. We observe the \textit{sign inversion }of the tunnel magnetoresistance upon resistance switching from low-resistance (LR) to high-resistance (HR) state, indicating a new resonant tunneling path promoted by an applied voltage bias. Thus our MTJ shows multifunctional properties with four resistance states which can be manipulated by applied electric and magnetic fields. This device can be used as a four-state logic gate or memory cell with multifunctional properties. The interpretation in terms of occupation-driven metal-insulator transition in one of the two junctions is proposed, explaining switching of the resistance and the magnetoresistance. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:48AM |
A14.00007: Evidence for WKB Failure in Contemporary Magnetic Tunnel Junctions Invited Speaker: This work describes evidence for the failure of the WKB approximation in state-of-the-art magnetic tunnel junctions. Surprisingly, the tunneling conductance of three varieties of CoFeB/MgO/CoFeB magnetic tunnel junctions depends quadratically on the applied voltage to anomalously high biases: the parabolic conductance persists to 2~V, greater than half the theoretical MgO barrier height. Within the framework of WKB, these data imply unphysical barrier parameters. We show that the origin of this breakdown is a distribution of barrier thicknesses consistent with experimentally feasible interfacial roughness, possibly in conjunction with the tunneling electron sensing the MgO band structure. Additionally, well defined and reproducible bias-dependent conductance oscillations are observed in CoFeB/MgO/NiFe devices. These oscillations are mediated by the reflection of tunneling electrons from the sharp MgO/NiFe interface, which allows electron standing waves to form within the MgO barrier. The oscillation amplitude is enhanced in the antiparallel state, which gives rise to oscillations of the tunneling magnetoresistance. A model employing spin-split free electron bands and the exact solution to the Schr\"{o}dinger equation demonstrates qualitative agreement with the data. This work implies that using existing WKB-based models may lead to physically incorrect barrier parameters for contemporary tunnel junctions, which may underscore the imminent necessity for first principles analyses of contemporary tunneling devices with textured or epitaxial barriers, MgO or otherwise. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A14.00008: Highly charged ion modified magnetic structures Holger Grube, Joshua Pomeroy, Andrew Perrella Highly charged ions (HCIs) deposit large amounts of energy very locally in small areas of only a few square nanometers per HCI impact. This allows for the modification of nanometer sized areas and the creation of nanosized features on impact surfaces. We have used highly charged ions such as Xe$^{44+}$ to modify ultrathin oxide barriers in magnetic tunnel junctions (MTJs) in order to locally change their electrical properties. We have been able to drastically reduce the resistance area (RA) product of our Co/Al-Ox/Co MTJs. While the magnetoresistance of HCI modified MTJ is also reduced, we created a new hybrid magnetic field sensor composed of tunnel and metallic junctions. We have analyzed the properties of individual HCI created conduction channels through ensemble measurements. Generalizing this approach, HCIs can be used to create hybrid materials through the introduction of nanometer sized electric or magnetic channels. This could be a useful tool to probe materials properties and physics on the nanometer scale. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A14.00009: Tunneling anisotropic magnetoresistance in Ni break junctions J. D. Burton, E. Y. Tsymbal, O. N. Mryasov Anisotropic magnetoresistance (AMR) is the difference in resistance as the magnetization direction is changed with respect to the direction of current flow. We will present results of first-principles calculations of AMR in Ni nanowires. It is known that in the ballistic regime the conductance of a magnetic nanowire changes in steps of $e^{2}$/$h$ as the angle of the magnetization changes with respect to the axis of the wire.[1] This ballistic AMR (BAMR) effect originates from the spin-orbit coupling which can change the number of bands crossing the Fermi energy ( $E_{F}$ ) as the magnetization direction is changed. We extend this consideration to the case of a break junction, where transport occurs via tunneling. We find a significant dependence of the tunneling conductance on the magnetization direction, an effect known as tunneling AMR (TAMR). We find that states localized at the electrode tips near the break are broadened by the spin-orbit interaction and contribute significantly to the tunneling. The position with respect to $E_{F}$ and broadening of these states depend strongly on the orientation of magnetization. Our results bear a striking resemblance to recent experimental results [2], clearly indicating an origin different from the one proposed previously.[2] This work is supported by Seagate Research and Nebraska NSF-MRSEC. [1] J. Velev et al. PRL \textbf{94}, 127203 (2005), [2] K. Bolotin et al. PRL \textbf{97}, 127202 (2006). [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A14.00010: Magnetoresistance in Boron Carbide junctions Ellen Day, A. Sokolov, A. Baruth, B.W. Robertson, S. Adenwalla The properties of thin insulator layers are crucial to the performance of magnetic tunnel junctions. Commercial requirements are a device with a high tunnel magnetoresistance (TMR) with low cost and high stability. At present the vast majority of barriers are made from amorphous Al$_{2}$O$_{3}$ and crystalline MgO. The TMR value depends not only on the spin-dependent electronic structure of the electrodes, but on the metal-insulator interface. Oxide-type barriers may suffer from local vacancies and other type of defects, resulting in oxygen diffusion, making the TMR value unstable with time. We present TMR results obtained on a non-oxide barrier, boron carbide (B$_{10}$C$_{2})$ for applications in magnetic tunnel junctions. This low $Z $inorganic material can be grown by plasma enhanced chemical vapor deposition (PECVD) without pinholes in the ultra thin film regime. PECVD grown boron carbide is an excellent dielectric with resistivities in the range of 10$^{7}$ ohm-cm, with a band gap that can be adjusted from 0.7 eV to 1.9 eV by altering the boron to carbon ratio and to band gap values well above 2.7 eV by adding phosphorus. This creates a unique opportunity for experimental study of a broad spectrum of phenomena, related to the dielectric properties of the barrier. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A14.00011: Inelastic Electron Tunneling Spectroscopy of a Molecular Magnetic Tunnel Junction. Wenyong Wang, Curt Richter We present the results of systematic measurements of molecular magnetic tunnel junctions (MTJs). In this study, we fabricated molecular-monolayer based MTJs and show that inelastic electron tunneling spectroscopy (IETS) can be utilized to characterize such junctions to investigate the existence of desired molecular species in the device area and to study the reported bias-dependence of junction tunneling magnetoresistance (TMR). Temperature-dependent current-voltage characterizations have been performed on the fabricated molecular MTJ with octanethiol as the molecular tunnel barrier. Tunneling transport has been observed at T $<$ 50K. IETS measurement at T = 4.2 K revealed spectra signatures due to $\nu $(Ni-S), $\nu $(C-S), and $\delta _{s}$(CH$_{2})$ vibrational modes, thus confirming the presence of the molecular species confined inside the ferromagnetic-octanethiol magnetic tunnel junctions. TMR measurements have been carried out and spin-dependent tunneling transport has been observed. A bias-dependence of the tunneling resistance has been observed. IETS measurements at different magnetic field suggest that the like cause of the TMR bias-dependence is inelastic scattering due to molecular vibrations. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A14.00012: Towards Fully Organic Tunnel Junctions Bridger Anderson, Tony Caruso, Munir Kaderbhai Quantum mechanical tunneling through ultrathin (25 A) insulating materials has been experimentally verified since the 1950's. Only recently, has tunneling through organic based materials and single molecules occurred from inorganic metallic injectors. We present here our effort focused on fabrication and characterization of an all organic tunnel junction where the injector is a conducting polymer and discuss the subtle difference which stem from polaronic metal. [Preview Abstract] |
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