Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B2: Spin Transfer-Driven Magnetic Excitations |
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Sponsoring Units: GMAG Chair: Dan Ralph, Cornell University Room: Colorado Convention Center Four Seasons 4 |
Monday, March 5, 2007 11:15AM - 11:51AM |
B2.00001: Spin Transfer Torque in Non-Uniform Magnetizations Invited Speaker: A spin polarized current exerts a torque when it passes through a region of non-uniform magnetization. In magnetic multilayers, this torque can reverse the magnetization of the layers or cause it to precess. In magnetic wires, it can move domain walls. These torques and their consequences can be largely understood from a series of simple models. However, experiments have become sophisticated enough to show that these simple models are not complete. In this talk, I will motivate the interest in these systems, describe the simple models that capture most of the physics, and highlight some the open questions that will be addressed in the later talks in this session. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B2.00002: Spin-Torque Diode Effect in Magnetic Tunnel Junctions Invited Speaker: Spin-injection magnetization switching (SIMS) technique [1] made it possible to control magnetization by a direct current. A discovery of spontaneous rf oscillation from CPP-GMR nano-pillars and a real time observation of the switching process have revealed essential amplification function of a precession in the magnetic nano-pillars under a direct current [2]. Beside of those progresses, developments of giant tunneling magneto-resistive (GTMR) effect using an MgO barrier [3] made it possible to utilize a very large resistance change according to the magnetization switching. In this talk, several attempts to utilize interplay between spin-torque and giant-TMR effect will be presented referring to a ``spin-torque diode effect'' [4] and other properties such like rf noise control and possible signal amplification using magnetic tunnel junctions (MTJs). \newline \newline [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) , L. Berger, Phys. Rev. B 54, 9353 (1996), and E. B. Myers, et al., Science 285, 867 (1999). \newline [2] S. I. Kiselev et al., Nature 425, 380 (2003), I. N. Krivorotov et al., Science, 307, 228 (2005). \newline [3] W. Wulfhekel, et al. Appl. Phys. Lett. 78, 509--511 (2001), M. Bowen, et al. Appl. Phys. Lett. 79, 1655--1657 (2001), J. Faure-Vincent, et al. Appl. Phys. Lett. 82, 4507--4509 (2003), S. Yuasa, et al., Jpn. J. Appl. Phys. Part 2, 43, L588 (2004), S. Yuasa, et al., Nature Mat. 3, 868 (2004), S. S. P. Parkin et al., Nature Mat. 3, 862 (2004), and D. D. Djayaprawira et al., Appl. Phys. Lett. 86, 092502 (2005). \newline [4] A. A. Tulapurkar, et al., Nature, 438, 339 (2005). [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B2.00003: Ferromagnetic resonance of individual nanomagnets driven by spin-polarized currents. Invited Speaker: Spin-polarized electrons passing through a nanoscale magnet can transfer their spin angular momentum to the magnetization, applying a torque far more efficiently than can be achieved by employing magnetic fields. We discuss the characterization of microwave-frequency magnetic excitations driven by these spin-transfer torques in metallic spin-valve devices and magnetic tunnel junctions using high-bandwidth electrical techniques. We first describe spontaneous magnetic oscillations that can be excited by DC currents, and then focus on a new form of ferromagnetic resonance (FMR) that uses high-frequency spin-polarized currents to excite resonance, instead of high-frequency magnetic fields. This technique allows measurements of individual magnetic samples orders of magnitude smaller than can be probed by traditional FMR, and should scale to much smaller devices as well. Using spin-transfer-driven FMR, we have been able to measure the spectra of normal modes for individual nanomagnets, including both the fundamental mode and higher-order more spatially-nonuniform modes. From the lineshapes, we can distinguish two different resonant regimes: simple FMR and phase-locking to a pre-existing DC-driven mode. The linewidths also enable efficient measurements of the magnetic damping parameter in single nanomagnets. We find that the FMR lineshapes differ between metallic spin valves and magnetic tunnel junctions when a DC current is applied, pointing to important differences in the fundamental mechanisms of the spin transfer torque in these two systems. This work was done in collaboration with P. M. Braganca, A. G. F. Garcia, I. N. Krivorotov, J. Z. Sun, J. C. Slonczewski, R. A. Buhrman, and D.C. Ralph. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B2.00004: X-Ray Imaging of Spin Transfer Induced Magnetization Reversal Invited Speaker: Time resolved x-ray microscopy allows one for the first time to image the magnetization switching process in a spin transfer structure. Instead of the coherent magnetization reversal, we observe switching by lateral motion of a magnetic vortex across a nanoscale element. The results of the first experiment demonstrates that the spin-switching can proceed in a new surprising mode that is quite different from uniform rotation. The switching can occur by lateral motion of a magnetic vortex across the magnetic film. The vortex structure is favored by the Oerstedt- field produced by the charge current while the spin current induces the lateral motion of the vortex leading to switching of M as soon as the center of the vortex moves out of the film and the resulting C-state relaxes into the uniform state. Our experiment also clearly shows that we have observed the effect of the torque induced by the Oerstedt field superimposed onto the torque produced by the spin current. The ratio of the torques from the Oersted field to spin transfer effects can be influenced by the size and thickness of the free layer, leading to different switching mechanisms. To obtain the spin torques without such bias, future experiments must eliminate or compensate the Oerstedt field. This appears to be possible for instance in lateral spin valve structures. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B2.00005: Temperature dependence of the spin torque effect in current-induced domain wall motion Invited Speaker: Rather than using conventional field-induced reversal, a promising approach for switching magnetic nanostructures is current-induced domain wall motion (CIDM), where due to a spin torque effect, electrons transfer angular momentum and thereby push a domain wall [1-4]. Since this interaction is strongly dependent on the wall spin structure, we have imaged domain walls in NiFe and Cobalt nanostructures and correlate the above mentioned effects with the imaged spin structure [1-4]. We find that both domain walls types can be moved due to the spin torque effect in the direction of the electron flow [2]. In addition to wall movement, changes in the wall spin structure have been predicted [2], and we have recently observed such wall type transformations using PEEM [3] and found that the velocity depends strongly on the wall type and the transformations occurring [3]. \newline Temperature dependent measurements of field- and current-induced wall motion have shown that the critical fields for field-induced wall motion decrease with increasing temperature, which can be attributed to thermal excitations. The critical current densities for current-induced motion though have been found to increase with increasing temperature, which is opposite to the behaviour due to thermal excitations [4], and might be due to the influence of thermally activated spin waves [4]. Using constrictions, we have been able to probe the interplay between current-induced motion and the attractive potential wells that the constrictions generate at variable temperature. We find that we can not only move domain walls with currents even into areas, where no current is flowing but the temperature dependence is also a sensitive probe separating the influence of thermal excitation vs. the intrinsic temperature dependence of the spin transfer torque. \newline \begin{enumerate} \item M. Kl\"{a}ui et al., PRL \textbf{94}, 106601 (2005); A. Yamaguchi et al., PRL \textbf{92}, 77205 (2004). \item A. Thiaville et al., EPL \textbf{69}, 990 (2005). \item M. Kl\"{a}ui et al., APL \textbf{88}, 232507 (2006). \item M. Laufenberg et al., PRL \textbf{97}, 46602 (2006). \end{enumerate} [Preview Abstract] |
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