Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D43: Focus Session: Magnetic Tunnel Junctions I |
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Sponsoring Units: GMAG DMP Chair: Pieter Visscher, Univ. of Alabama Room: LACC 150C |
Monday, March 21, 2005 2:30PM - 3:06PM |
D43.00001: Tunneling magnetoresistance oscillation effect in double-barrier magnetic tunnel junctions Invited Speaker: In this work, we present the fabrication of the double barrier magnetic tunnel junction (DBMTJ) of Ta (5 nm)/Cu (30 nm)/Ni$_{79}$Fe$_{21}$(10 nm)/Ir$_{22}$Mn$_{78}$(12 nm)/Co$_{75}$Fe$_{25}$(4 nm)/Ru(0.9 nm)/Co$_{75}$Fe$_{25}$ (4 nm) /Al(1 nm)-oxide/Co$_{75}$Fe$_{25}$ (1 nm)/ Ni$_{79}$Fe$_{21}$ (2 nm)/ Co$_{75}$Fe$_{25}$ (1 nm)/Al(1 nm)- oxide/ Co$_{75}$Fe$_{25}$ (4 nm)/Ru(0.9 nm)/Co$_{75}$Fe$_{25}$ (4 nm) / Ir$_{22}$Mn$_{78}$ (12 nm)/ Ni$_{79}$Fe$_{21}$ (10 nm)/Cu(30 nm)/Ta(5 nm) on Si/SiO$_{2}$ wafer using Magnetron Sputtering System. Lithographic technique combined with Ar ion-beam etching was adopted in the micro-fabrication processes. Active area of a patterned elliptic DBMTJ element was 3x6$\pi $ $\mu $m$^{2}$. Thus, TMR ratio of 27{\%} and 42.2{\%}, and resistance-area product RS of around 13.6 and 17.5 k$\Omega \mu $m$^{2}$ at 300 K and 4.2 K were obtained respectively. A tunneling magnetoresistance oscillation phenomenon with respect to the bias voltage was first observed in this experiment. Such an effect can be attributed to either the spin-polarized electron coherent and resonant tunneling or the quantum well states. It may open up the possibility of developing novel spintronic devices such as resonant-tunneling spin transistors, etc. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D43.00002: Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films Zhong-Yi Lu, Xiao-Guang Zhang, Sokrates T. Pantelides Quantum-well states in nonmagnetic metal films between magnetic layers are known to be important in spin-dependent transport, but quantum well states in magnetic films remains elusive. We report first-principles transport calculations for the junctions Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr and identify the conditions for resonant tunneling through quantum well states in magnetic films. We show that, at resonance, the current increases by one to two orders of magnitude. The tunneling magnetoresistance ratio is much larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as basis for novel spintronic devices. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D43.00003: Spin transport in nanowires with a double-junction Ildar Sabirianov, Andrei Sokolov, Jody Redepenning, Bernard Doudin Transport properties of ferromagnetic nanowires involving double insulating barriers are studied. Electrochemical synthesis of a Ni/NiO/CoNi/NiO/Co structure in shape of 70 nm diameter nanowires results in a CoNi ``island'', approximately 100 nm long, separated by barriers to Ni and Co cylinders of typically 1 micron length. Transition from high-resistance to low-resistance states were observed during IV curves measurements when high bias voltage is applied. This system exhibits magnetoresistance ratios reaching 20{\%} at low temperature, of sign changing after the high bias transition. This research is sponsored by NSF MRSEC, the Keck Foundations, and the Nebraska Research Initiative. University of Nebraska [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D43.00004: Bias voltage dependence of the tunneling magneto-resistance of SrTiO3 based magnetic tunnel junctions grown epitaxially on Si (100) Guenole Jan, Mahesh Samant, Andreas Ney, Kevin Roche, S.S.P Parkin We present data on the magneto-transport properties of epitaxial magnetic tunnel junctions fabricated with SrTiO$_{3}$. These junctions were deposited on silicon $<$100$>$ using pulsed laser and magnetron sputtering techniques. Various ferromagnetic electrode materials were studied. The structures were patterned using in-situ metal shadow masks. Structural and material characterization of these films was performed using x-ray diffraction, SQUID, Auger spectroscopy and transmission electron microscopy. The relationship of the sign of the tunneling magneto-resistance, whether positive or negative, to details of the structures will be presented. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D43.00005: Probing Spin-Polarized Tunneling at High Bias with a Magnetic Tunnel Transistor B.G. Park, T. Banerjee, B.C. Min, J.C. Lodder, R. Jansen The Magnetic Tunnel Transistor (MTT) is a three terminal hybrid device that consists of a tunnel emitter, a ferromagnetic (FM) base and a semiconductor collector. Since the magnetocurrent (MC) depends on the tunneling spin polarization, the MTT can be used to study the spin-polarization of ferromagnetic/insulator interfaces at high bias voltage. Using a standard photolithography process we have fabricated MTT's with the configuration Si/Au/Co/Al2O3/NiFe. We obtain a MC of 82{\%} at room temperature. This corresponds to a tunnel spin polarization of the NiFe/Al2O3 emitter interface of 29{\%}, demonstrating that the tunnel current is still spin-polarized at a high bias voltage of -900mV. The MC increases to 104{\%} at 100K, corresponding to a tunnel polarization of 34{\%} at -900mV. Tunnel spin polarization of other FM/insulator combinations using the MTT has been examined. Inserting SiO2 at the interface between the FM emitter and Al2O3 causes the MC to almost disappear, thus indicating that the tunnel spin polarization of NiFe is drastically reduced in contact with SiO2. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D43.00006: Spin polarized tunneling at finite bias Sergio O. Valenzuela, D.J. Monsma, C.M. Marcus, V. Narayanamurti, M. Tinkham We fabricated mesoscopic lateral-spin-valves [1] with spin transresistances as high as 2.5 Ohm. We employed these devices to measure the bias dependence of the polarization of the electrons tunneling \emph{from} and \emph{into} a ferromagnet. Our measurements show an intrinsic asymmetry between these two processes. Using a free-electron model, we found that the polarization is strongly suppressed for electrons tunneling \emph {into} the ferromagnet due to the reduced polarization for hot electron states and a spin-dependent wave-vector matching effect through the tunnel barrier. [1] M. Johnson and R. H. Silsbee, Phys. Rev. Lett. {\bf 55}, 1790 (1985); F.J. Jedema et al., Nature {\bf 416}, 713 (2002). [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D43.00007: Bias and temperature dependence on junction magnetoresistance in manganite/magnetite based magnetic tunnel junctions Rajesh Chopdekar, Guohan Hu, Yuri Suzuki Studies of the bias and temperature dependence of junction magnetoresistance (JMR) in manganite/magnetite trilayer junctions have revealed non-monotonic dependence on both quantities. Such junctions were constructed to probe the spin-polarization of magnetite using the well-established majority spin-polarized oxide (La,Sr)MnO$_{3}$ as a counter-electrode. We have found that using an isostructural oxide cobalt chromite tunnel barrier reduces disorder at the chromite-magnetite interface as compared to junctions with a rocksalt structure MgO barrier. An order of magnitude increase in JMR for these junctions is strongly peaked as a function of bias magnitude and sign, but has weak temperature dependence. The Verwey transition in magnetite, the paramagnetic-ferrimagnetic transition in cobalt chromite, and the interface roughness in the junctions all play a role in determining the temperature and bias dependence of the measured magnetoresistance. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D43.00008: Characterization of Magnetic Tunnel Junctions by IETS and STS Hyunsoo Yang, Christian Kaiser, See-Hun Yang, Stuart Parkin Inelastic electron tunneling spectroscopy (IETS) and superconducting tunneling spectroscopy (STS) have been employed to investigate spin-dependent tunneling in magnetic tunnel junctions (MTJs). MTJs were studied in which the ferromagnetic electrodes were formed from the 3d transition metals, Fe, Co and Ni and their alloys, and the tunnel barriers were formed from various nitrides and oxides including MgO. MTJs with MgO barriers exhibit more than 220{\%} tunneling magnetoresistance (TMR) at room temperature[1]. IETS was used to measure the contributions of defects and impurities, as well as phonons and magnons, to the tunneling current. These processes give rise to conductance peaks at characteristic voltages according to their excitation energies. STS was used to measure the spin polarization of the tunneling current as well as to explore the role of spin-flip scattering in the tunneling process. The goal of this research is a more complete understanding of the mechanisms which gives rise to the bias voltage dependence of the TMR as well as indirect tunneling through states in the barrier. [1] S. S. P. Parkin, C. Kaiser, A. Panchula, P. Rice, B. Hughes, M. Samant, and S.-H. Yang, \textit{Nature Materials}, vol. Published online: 31 October 2004, 2004. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D43.00009: Nanoscale magnetic tunnel junctions Kirill Bolotin, Abhay Pasupathy, Ferdinand Keummeth, Daniel C. Ralph We describe measurements of nm-sized ferromagnetic Ni and permalloy tunnel junctions. Electromigration is used to form a gap between the electrodes ranging in width from zero to several nanometers. Thus both the metallic-point-contact and tunneling regimes can be accessed. The source and drain electrodes are shaped differently to give them different coercive fields, so that the relative direction of their magnetic moments can be controlled by an external magnetic field. Magnetoresistances of 10-20 {\%} are observed for most tunnel junctions, comparable to Julliere estimates. The magnetoresistance is bias-dependent but does not vanish until hundreds of millivolts, indicating a high quality of the vacuum tunnel barrier. In some junctions we observe values of magnetoresistance larger than the Julliere value and of different sign. We have also incorporated a nonmagnetic metal island into the junction to form a F/N/F single-electron transistor. We will report the dependence of tunneling via single quantum states on the relative direction of electrode magnetizations. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D43.00010: Fluctuations, dissipation, and switching in tunneling spin-transfer devices Yaroslaw Bazaliy, Konstantin Matveev We propose a theory of tunneling spin-transfer devices which treats quantum-mechanically not only the spins of itinerant electrons, but also the magnetic moment of the free layer. A Fokker-Plank equation, describing an open spin-transfer system at finite temperatures and currents, is derived. It consistently accounts for both thermal and current-related fluctuations, and for bulk and contact contributions to the Gilbert damping constant. In the presence of fluctuations, switching is shown to be governed by current-dependent effective energy barriers. Our approach provides a unified treatment of escape probabilities from static and precession states of the device. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D43.00011: Perpendicularly magnetized exchange-biased magnetic tunnel junctions Andreas Ney, Sebastiaan van Dijken, Stuart Parkin Exchange biased magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have been studied. The ferromagnetic electrodes were fabricated from either Co/Pt or Co/Pd multilayers and the tunnel barriers were formed from Al$_{2}$O$_{3}$. In some cases one of the electrodes was exchange biased with either PtMn or IrMn. We discuss the dependence of the PMA and the exchange bias on the thickness of the Co, Pt and Pd layers. The properties of the MTJs are strongly influenced by the structural morphology of the Co/Pt and the Co/Pd multilayer electrodes, which appear to give rise to rough tunnel barriers with low resistance. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D43.00012: Negative TMR in Magnetic Tunneling Junctions with Zr oxide barrier Takahiro Moriyama, Weigang Wang, Xiaohai Xiang, John Xiao, Tao Zhu, Fei-fei Li, Jun Du, Ming-wen Xiao, Zheng-zhong Li, An Hu Negative Tunneling Magnetoresistance (TMR) was observed in Magnetic Tunnel Junctions (MTJs) with AlO$_{x}$/ZrO$_{x}$ and ZrO$_{x}$/AlO$_{x}$ hybrid barrier. The TMR shows a strong asymmetric bias dependence and changes from 8\% at around zero bias to -4\% at a high bias. The critical bias voltage ($V_{c}$) at which the TMR changes from positive to negative decreases with increasing the thickness of ZrO$_{x}$ layer. The MTJs were prepared by using magnetron sputtering system. The barriers were formed by two-step oxidation process. The structures of the MTJs are Si/FeNi/Cu/FeMn/Co/Barrier/Co/Cu (Barrier: AlO$_{x}$/ZrO$_{x} $, ZrO$_{x}$/AlO$_{x}$ or AlO$_{x}$). The observation of negative TMR at high bias was originally explained by using reversed majority and minority spin-DOS. However those explanations ignore the barrier shape such as barrier height and width which effectively contribute to the conductance at high bias. In this study, we point out that the negative TMR at high bias is due to not only the energy dependence of spin-DOS but also the barrier shape of the MTJs. Furthermore the mechanism for the negative TMR at high bias is quite different from that responsible for the negative TMR in low bias range. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D43.00013: Spin Transport via Nanoscale Ferromagnet/Superconductor Hybrid Tunnel Junctions Dawei Wang, Manjiang Zhang, Go Tateishi, Robert Mueller, Gerd Bergmann, Jia G. Lu Ferromagnetic single-electron transistors (FMSET) combine the recent advances in magnetic tunnel junctions and single- electron transistors. Various phenomena such as enhanced magnetoresistance, magnetoresistance oscillation, spin accumulation, and superconducting gap suppression are theoretically predicted. FMSETs are fabricated using ebeam lithography and shadow evaporation techniques. The devices consist of multiple tunnel junctions in orthogonal geometry, with ferromagnetic leads (Co) of different widths perpendicular to the island (Al). They exhibit typical single electron tunneling behavior including Coulomb blockade and current oscillation vs. gate voltage. Measurements performed in applied magnetic field show bell shaped $I-H$ curves as a result of the direct influence of the magnetic field on the superconducting gap. In addition, the magnetic moment switching in the Co electrodes from parallel to antiparallel configurations give rises to a rich phenomenon of spin transport. Different transport mechanisms based on the spin accumulation effect and the Meservey-Tedrov effect are presented. [Preview Abstract] |
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