Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S32: Focus Session: Magnetic Tunneling |
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Sponsoring Units: GMAG DMP FIAP Chair: Jack Bass, Michigan State University Room: Morial Convention Center 225 |
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S32.00001: Bound states in electronic transport through Fe/MgO tunnel junctions Ivan Rungger, Nadjib Baadji, Stefano Sanvito Using the ab initio code Smeagol we calculate the electronic transport properties of Fe/MgO/Fe(100) tunnel junctions for applied bias up to 2 Volt. The correct bias-dependent occupation of the interface states (IS) in the Fe/MgO junction is crucial to obtain a physically meaningful potential drop. The coupling of the IS to the Fe electrodes varies strongly for different k-points, and bound states are found along the high symmetry lines and at those k-points where there are no open channels in the Fe leads. A general method for setting the occupation of both weakly-coupled and bound states, based on the notion of a local Fermi energy and a finite relaxation time, is presented. For parallel alignment of the Fe leads the current through the IS is quenched above 20 mV, whereas for the anti-parallel alignment the current flows mainly through IS up to about 0.4 V. In this bias range the TMR shows a pronounced bias dependence, at higher voltages it decays smoothly. If the transmission through the IS is suppressed, for example by adding a finite imaginary part to the energy, the TMR decays monotonically with bias even at low voltage. Finally we show that oxygen vacancies inside the MgO barrier quench the TMR if they are within the first few layers from the interface, and that a similar reduction is achieved by partially oxidizing the Fe interface layer. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S32.00002: Fabrication and characterization of fully epitaxial Fe/MgO/Fe magnetic tunnel junctions on Si(100) and the influence of MgO buffer layer G. X. Miao, M. V. Veenhuizen, M. Munzenberg, J. S. Moodera Spin injection into Si can integrate the spin degree of freedom into future semiconductor technology. Coherent tunneling through epitaxial MgO barrier due to symmetry filtering is expected to yield large spin poarization [1]. Here we report the MBE growth of fully epitaxial Fe/MgO/Fe MTJ stacks on top of Si (100) with epitaxial MgO buffer layer giving rise to a large TMR. Due to the large lattice mismatch between Si and MgO ($\sim $9.6{\%}), the crystalline structure of MgO is critically dependent on its thickness, and the influence is subsequently passed on to the MTJ structure. We observe a systematic change of TMR with the buffer layer thickness, which we attribute to the effect of dislocations. Coherent growth of MTJ on top of Si is a significant step spin injection into Si [2,3]. 1. W. H. Butler et al., Phys. Rev. B, 63, 054416 (2001). 2. Ian Appelbaum et al., Nature 447, 295 (2007). 3. B. T. Jonker et al., Nature Physics 3, 542 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S32.00003: Temperature and Angular Dependences of Dynamic Spin-Polarized Resonant Tunneling Casey W. Miller, Johan \AA kerman, Yan Zhou, Renu Dave, Jon Slaughter, Ivan K. Schuller The bias dependence of tunneling magnetoresistance oscillations due to dynamic resonant tunneling in CoFeB/MgO/NiFe magnetic tunnel junctions was studied as functions of temperature and the relative magnetization angle of the two magnetic layers. The effect of temperature is consistent with thermal smearing, while that of the relative magnetic orientation was typical of a spin valve. A model of tunneling between spin-split free electron bands using the exact solution of the Schr\"{o}dinger equation for a trapezoidal tunnel barrier agrees with experiment.\\ ~\\ C.~W.~Miller \textit{et al.}: J. Appl. Phys., In Press (2008); Phys. Rev. Lett. {\bf 99}, 047206 (2007); Appl. Phys. Lett. {\bf 90}, 043513 (2007); Phys. Rev. B {\bf 74}, 212404 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S32.00004: Transport and spin transfer torques in Fe/MgO/Fe tunnel barriers. Invited Speaker: The prediction of very high tunneling magnetoresistance (TMR) ratios in crystalline Fe/MgO/Fe [1,2] tunnel junctions has been verified by a number of experiments [3,4]. The high TMR can be understood in terms of the electronic structure of the system. In MgO the $\Delta_1$ states at the Brillouin zone center decay the most slowly and dominate the tunnelling current. For coherent interfaces, which are achievable due to the small lattice mismatch between Fe and MgO, these $\Delta_1$ states at the Brillouin zone center are half-metallic in the Fe layers. The dominance of the $\Delta_1$ states and their half-metallicity cause the high tunnelling magnetoresistance measured in Fe/MgO/Fe tunnel junctions [5]. For the spin transfer torque, we calculate the linear response for small currents and voltages. Our calculations show that the half metallicity of the Fe $\Delta_1$ states leads to a strong localization of the spin transfer torque to the interface. As a result, the linear current dependence of the torque in the plane of the two magnetizations is independent of the free layer thickness for more than three monolayers of Fe. For perfect samples we also find a linear current dependence of the out-of-plane component. However, this linear piece oscillates rapidly with thickness and averages to zero in the presence of structural imperfections like thickness fluctuation, in agreement with experiment [6]. In this talk I discuss the bias dependence of the TMR and spin transfer torque effects mentioned above and the influence on them of the following factors: the interface structure Fe/MgO, the barrier thickness, and the structure of the leads [7]. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement. [1] W. Butler, X.-G. Zhang, T. Schulthess, J. MacLaren, Phys. Rev. B 63 (2001) 054416. [2] J. Mathon, A. Umerski, Phys. Rev. B 63 (2001) 220403. [3] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nature Materials 3 (2004) 868. [4] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yang Nature Materials 3 (2004) 862. [5] C. Heiliger, P. Zahn, I. Mertig, Materials Today 9 (2006) 46. [6] J. C. Sankey, P. M. Braganca, A. G. F. Garcia, I. N. Krivorotov, R. A. Buhrman, and D. C. Ralph, Phys. Rev. Lett. 96 (2006) 227601. [7] C. Heiliger, M.Gradhand, P. Zahn, I. Mertig, Phys. Rev. Lett. 99 (2007) 066804. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S32.00005: Tunneling Magnetoresistance in MgO based double-barrier Magnetic Tunnel Junctions Weigang Wang, Chaoying Ni, Q. Wen, H.W. Zhang, Takahiro Moriyama, John Xiao Double-barrier magnetic tunnel junctions (DMTJs) have attracted much attention due to their fertile physics and promising applications in spintronics devices. Here we report the fabrication and characterization of DMTJs of Si/SiO$_{2}$/Ta 7/Ru 20 /Ta 7/CoFe 2/ IrMn 15/CoFe 2/Ru 1.7 /CoFeB 3/ MgO 2/ CoFeB 3 /MgO 2/CoFeB 3 /Ru 1.7/ CoFe 2/IrMn 15/ Ta 8/Ru 10, where the numbers are layer thickness in nanometers. Single barrier MTJs with similar structure were also fabricated. While the DMTJs exhibit the tunnel magnetoresistance (TMR) of 185 {\%} at room temperature, which is the highest value in DMTJs reported to date, the corresponding single barrier MTJ shows 300{\%} TMR. The reduction of TMR in DMTJs is understood in terms of sequential tunneling through two junctions in serials. The effects of annealing temperature and bias voltage on the TMR of DMTJs will also be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S32.00006: Influence of spin accumulation on superconducting properties of aluminum layers in magnetic double tunnel junction devices See-Hun Yang, Hyunsoo Yang, Saburo Takahashi, Sadamichi Maekawa, Stuart Parkin We discuss the influence of spin accumulation on the superconducting (SC) properties of thin aluminum layers in crystalline MgO barrier based magnetic double tunnel junction devices composed of ferromagnet-insulator-superconductor-insulator-ferromagnet (FISIF) structures. Below the Al SC transition temperature, when the magnetization directions of the two outer CoFe ferromagnetic layers are antiparallel, the SC energy gap of the Al layer is suppressed, as compared to the case for parallel orientation of these same layers. This is consistent with theoretical models in which spin polarized quasi-particles are accumulated in the SC layer. The accumulated spin depends on the rate at which spin polarized current is injected and leaves and, most importantly, on the spin relaxation rate of the injected quasi-particles. We discuss the dependence of the spin accumulation on the spin-injection rate, which can be varied at a fixed voltage, by varying the MgO barrier thickness. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S32.00007: Enhanced tunneling spin polarization by amorphizing usually crystalline CoFe alloys without any glass forming additives Li Gao, Xin Jiang, See-Hun Yang, Philip M. Rice, Stuart S.P. Parkin Ferromagnetic alloys of Co and Fe are particularly useful electrodes in magnetic tunnel junctions because they exhibit high spin polarization and high Curie temperatures as well as high thermal stability. These alloys are crystalline but they can be made amorphous by adding suitable glass-forming elements such as Boron. Here we show that films of pure CoFe alloys can be made amorphous, without the need of any additives, by sandwiching them between two amorphous layers, a tunnel barrier formed from amorphous Al$_{2}$O$_{3}$ and an amorphous overlayer. The films are amorphous when thinner than $\sim $25 {\AA} but are crystalline for thicker layers. We find that both the tunneling magnetoresistance and the tunneling spin polarization (measured using superconducting tunneling spectroscopy in related junctions) are significantly enhanced when the alloy is amorphous. However, by heating the alloy above its glass crystallization temperature the enhancement is observed to vanish. Possible reasons for this behavior are discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S32.00008: Tunneling Anisotropic Magnetoresistance in Co/AlO$_{x}$/Au Tunnel Junctions Ruisheng Liu, Lukasz Michalak, Carlo Canali, Lars Samuelson, Hakan Pettersson We observe spin-valve-like effects in nano-scaled thermally evaporated Co/AlO$_{x}$/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S32.00009: Transport Studies of reduced RA product MTJs produced by highly charged ion irradiation Joshua Pomeroy, Holger Grube As is commonly known, magnetic tunnel junctions (MTJs) are used for hard drive read heads and are actively being developed for MRAM. In both of these cases, the resistance-area (RA) product is a critical parameter for device speed and bandwidth as well as total power dissipation (particularly for current driven devices). We present transport studies of MTJs whose barrier oxide has been partially reduced by highly charged ions (HCIs). The bias, temperature, and applied field dependence of these devices will be discussed. Beyond magnetic memory applications, HCI modified MTJs provide a compelling new route for preparing order THz spin torque oscillators. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S32.00010: Large voltage from spin pumping in magnetic tunnel junctions Zhifang Lin, Siu Tat Chui We studied the response of a ferromagnet-insulator-normal metal tunnel structure under an external oscillating radio frequency (R.F.) magnetic field. The D. C. voltage across the junction is calculated and is found not to decrease despite the high resistance of the junction; instead, it is of the order of $\mu V$ to $100\mu V$, much larger than the experimentally observed value (100 nano-V) in the ``strong coupled'' ohmic ferromagnet-normal metal bilayers. This is consistent with recent experimental results in tunnel structures, where the voltage is larger than $\mu V$s. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S32.00011: Nature of Voltage Dependence of Spin Torque in Magnetic Tunnel Junctions M. Chshiev, I. Theodonis, A. Kalitsov, N. Kioussis, W. H. Butler Current-induced spin torque [1] is important because it may elucidate fundamental physics and because it may have useful applications. We have recently shown [2] that in magnetic tunnel junctions, both in-plane (Slonczewski) and perpendicular-to-the-plane (field-like) components of the spin torque behave nontrivially as a function of applied bias. Recent experiments [3] are in agreement with these predictions. Here we present a systematic study of voltage-induced spin torque in magnetic tunnel junctions and provide an insight into the nature of its voltage behavior. The explanation is given in terms of the spin and charge current dependence on the interplay between evanescent states in the insulator and the Fermi surfaces of the ferromagnetic electrodes comprising the junction. Calculations are based on the Keldysh formalism with non-equilibrium Green functions technique. [1] J. C. Slonczewski, J. Magn. Magn. Mat. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996) [2] I. Theodonis, N. Kioussis, A. Kalitsov, M. Chshiev, and W. H. Butler, Phys. Rev. Lett. 97, 237205 (2006) [3] J. C. Sankey et al, Nature Physics (2007); H. Kubota et al, Nature Physics, ibid. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S32.00012: Space-Charge Induced Pauli Blockade Effect in Tunnel Junctions with Half-Metallic Electrodes A. P. Li, T.-H. Kim, X.-G. Zhang, J. F. Feng, X. F. Han, Y. Wang, J. Zou, D. B. Yu, H. Yan A space-charge induced Pauli spin blockade effect has been observed in the magnetic tunnel junction consisting of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) electrodes and SrTiO$_{3}$ barrier at temperatures up to 100 K. The blockade voltage under zero magnetic field provides a direct measurement of the energy gap between the Fermi energy and the top of the minority spin valence band of the LSMO. Outside the spin blockade regime, the low temperature conductance oscillates with the bias voltage as the trap centers are charged by electrons. The spin blockade can be lifted when the trap levels are thermally activated or when a large magnetic field lowers the LSMO minority spin mobility edge to below the Fermi energy. A very large magnetoresistance up to 10000{\%} is observed and is correlated to the blockade effect. $^{\ast }$Email: apli@ornl.gov [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S32.00013: First-principles theory of nonequilibrium vertex correction:disordered magnetic tunneling junction Youqi Ke, Ke Xia, Hong Guo The Keldysh Non-equilibrium Green's Function (NEGF) formalism has been developed Within DFT to calculate the electronic structure of disordered system at finite bias based on TB-LMTO+ASA combined with Coherent Potential Approximation. The conditionally averaged NEGF is evaluated by including the vertex correction. The disordered Fe/Va/Fe tunneling junctions are investigated with present first principle method, the vertex correction shows important role in both the electronic structure and transport calculations. The bias dependence of Tunneling Magnetoresistance (TMR) in the disordered Junctions and the disordered dependence of TMR at a finite Bias were illustrated. [Preview Abstract] |
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