Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N14: Focus Session: Current Induced Magnetization Dynamics and Spin Transfer |
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Sponsoring Units: GMAG DMP FIAP Chair: Tingyong Chen, The Johns Hopkins University Room: Colorado Convention Center Korbel 4D |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N14.00001: Temperature rise due to Joule heating in a spin transfer torque nano-pillar structure Chun-Yeol You, Seung-Seok Ha, Hyun-Woo Lee Considering that the spin-transfer-torque-induced magnetization dynamics in a nano-pillar structure usually requires a large current density of $10^{11}$ A/m$^2$, it is desired to have an accurate estimation of the temperature rise caused by the current-induced Joule heating. We investigate the current- induced heating effect in the nano-pillar by analytical and numerical methods. We employ the Green's function method to obtain analytic solution of the heat conduction equation. With proper approximations, we derive a simple analytic relation that expresses the temperature in term of the current density, the geometry of the nano-pillar, and material properties. The validity of the analytic expression is confirmed by the comparis on with commercial finite element method software. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N14.00002: Current-Hysteretic Low Frequency Oscillations in Spin-Transfer Nanocontacts Matthew Pufall, William Rippard, Michael Schneider, Thomas Silva, Stephen Russek We have observed spin-transfer-driven large amplitude, current hysteretic, low frequency ($<$ 500 MHz) oscillations in nanocontacts made to spin valve structures. The oscillations occur only for small ($<$50 Oe) in-plane applied fields, but persist in fields up to several kOe for out of plane fields. The frequency of oscillation is typically far below the uniform-mode ferromagnetic resonance frequency, and is only a weak function of applied field. Hysteresis in the presence/absence of the oscillations is observed with dc current, with oscillations first appearing at high currents with increasing current, but persisting to lower currents upon decreasing the current. We suggest that these observations are consistent with dynamics of a vortex-like state in the vicinity of the contact, one nucleated by the Oersted fields generated by the dc current, and with dynamics driven by the spin transfer torque. The electrical oscillation amplitudes are large, with the largest amplitudes approaching 1 mV, and are narrowband, with many devices exhibiting sub-megahertz linewidths. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N14.00003: Thermal effects in spin torque switching Michael Schneider, Mattheew Pufall, William Rippard, Stephen Russek, Jordan Katine We compare low temperature device behavior with room temperature behavior. We find agreement between our low temperature critical current measurements and Slonczewski theory$^{1}$. In addition, we find that the values extrapolated from the low temperature measurements were robust with respect to device size. At room temperature we find substantial variation in the hysteretic region from device to device for devices of the same nominal size. While this is not expected, it has been attributed to thermal effects having a strong influence on the response of the freelayer to applied field as well as the coercivity$^{2}$. We find that by reducing the temperature, and thus any thermal fluctuations, the device to device variations are drastically reduced. While we did observe indications of non-single domain behavior at 5 K, it is noteworthy that these did not seem to affect the critical switching current. Thus, we conclude that the room temperature device to device variations in the quasi-static switching behavior is dominated by thermal effects. 1. J. C. Slonczewski, J. Magn. Magn. Mater. \textbf{159,} L1-L7 (1996) 2. D. Lacour, J. A. Katine, N. Smith, M. J. Carey, and J. R. Childress, \textbf{85,} 4681-4683 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N14.00004: Nonlocal magnetization dynamics Invited Speaker: Recently, nonlocal properties of ferromagnetic dynamics in magnetic nanostructures, such as damping sensitive to the Ohmic environment and spin-wave transfer between exchange-decoupled ferromagnets, have attracted a considerable interest [1]. It is also becoming clear [2] that nonlocal dynamic effects are important for understanding intrinsic properties such as magnetic damping, noise, and spin-transfer torques in inhomogeneous ferromagnets, with consequences for phenomena ranging from spin- wave propagation and domain-wall motion to current-driven instabilities in the bulk. I will present a self-consistent mean-field approach for treating these properties in a unified and rather general fashion. \newline \newline [1] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, Rev. Mod. Phys. 77, 1375 (2005) \newline [2] Y. Tserkovnyak, H. J. Skadsem, A. Brataas, and G. E. W. Bauer, Phys. Rev. B 74, 144405 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N14.00005: Precession damping in itinerant ferromagnets Keith Gilmore, Mark Stiles, Yves Idzerda The damping of excited magnetic states has long been understood at a phenomenological level through the Landau-Lifshitz-Gilbert equation. Increased interest in nanoscale devices, the behavior of which can be strongly dependent on the damping, demands a more thorough understanding of the relaxation process. While magnetic alloy systems are used in most applications, we consider the simpler 3d transition metals (iron, nickel, and cobalt) in order to understand the most basic processes involved in damping before approaching the more complicated mechanisms expected in alloys. Resonance experiments for Co and Ni indicate low and high temperature regions for which the damping parameter is roughly proportional and inversely proportional, respectively, to the scattering time. We report and numerically test a model that produces both of these behaviors. As with all previous work, the calculations presented are given in terms of an unknown electronic scattering time. To make meaningful comparisons between the calculated and measured damping parameters we evaluate an expression for the conductivity, derived by similar methods, and compare also to transport experiments. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N14.00006: Fast Pulse Measurements and Temperature Variation of Enhanced Magnetic Damping of Spin-Transfer Excitation E. M. Ryan, A. G. F. Garcia, P. M. Braganca, G. D. Fuchs, N. C. Emley, J. C. Read, E. Tan, D. C. Ralph, R. A. Buhrman, J. A. Katine Recently, light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase the damping parameter $\alpha $ by several orders of magnitude [1]. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.004 um$^{2}$ Py/Cu/Py nanopillar spin valves with 0 and 2{\%} Tb in the free layer, and measured critical currents across a range of temperatures from 4.2 K to 295 K. We find that the critical currents for reversibly switching the free layer, generally expected to be proportional to $\alpha $, are several times larger on average in the 2{\%} Tb samples than in pure Py samples, and increase linearly with decreasing temperature. We will also discuss FMR data, and data for switching with fast pulses from 1 to 100 nsec at both 150 K and room temperature, along with matching simulations that allows us to extract $\alpha $ and other spin-torque parameters [2]. These results suggest one approach for controllably reducing the negative impact of spin-torque effects on nanoscale spin valve and read head sensors, and achieving a deeper understanding of these spin-torque devices. [1] W. Bailey, P. Kabos, F. Mancoff, and S. E. Russek, IEEE Trans. Magn. 37, 1749 (2001). [2] P. M. Braganca, et al. Appl. Phys. Lett. 87, 112507 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N14.00007: Modeling study on the self-consistent feedback between inhomogeneous magnetization and the spin torque Kyung-Jin Lee, Bernard Dieny The Slonczewski's spin torque terms were originally suggested within the context of homogeneous magnetic domain. Micromagnetic [1] and experimental [2] studies have revealed that the magnetizations excited by the spin torque could be inhomogeneous. Therefore we have to find a way of correcting the Slonczewski's terms in describing the magnetization dynamics. We show the self-consistent model to numerically solve the equations of motion of local magnetization and spin accumulation. The self-consistent model enables us to consider the feedback between inhomogeneous magnetization and the spin torque. We found the feedback is crucial in the magnetization dynamics induced by the spin torque. We will show the computational evidence of the importance of the feedback for the current-induced magnetic excitation in a single Co layer and a spin valve structure. [1] K. J. Lee et al. Nat. Mat. 3, 877 (2004); Appl. Phys. Lett. 88, 132506 (2006), [2] Y. Acremann et al. Phys. Rev. Lett. 96, 217201 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N14.00008: Planar Spin-Transfer Device with a Dynamic Polarizer. Yaroslaw Bazaliy, Debo Olaosebikan, Barbara Jones In planar nano-magnetic devices magnetization direction is kept close to a given plane by the large easy-plane magnetic anisotropy (e.g. by shape anisotropy in a thin film). In conventional micromagnetics it is known that in this case the magnetization motion is effectively in-plane with only one angle required for its description, and can become overdamped even for small values of the Gilbert damping. We extend the equations of the effective in-plane dynamics to include the spin-transfer torques. The simplifications achieved in the overdamped regime allow us to study systems with several dynamic magnetic pieces (``free layers''). A transition from a spin-transfer device with a static polarizer to a device with two equivalent magnets is observed: when the size difference between the magnets is less than critical, the device does not exhibit switching, but goes directly into the ``windmill'' precession state. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N14.00009: Role of Spin dependent Inelastic Scattering in Spin Torque Devices Sayeef Salahuddin, Supriyo Datta Spin torque devices are commonly modeled by looking only at the spin dependent transmission and reflection at the tunnel oxide-ferromagnet interface. Here, we describe a different approach where, in addition to barrier dependent phenomena, an inelastic spin-flip scattering is included at the interface. We show that such scattering events may have significant influence on the device behavior, specifically on the magnitude of TMR and on the efficiency of spin torque. We shall show that recent experiments provide evidence for this prediction. Our transport model is based on Non Equilibrium Green's Function (NEGF) formulation where the scattering is included through a self energy matrix. We also discuss$^{1}$ how the spin flip scattering may help to reduce the switching current necessary to flip the magnetization in penta-layer spin torque devices, a phenomenon demonstrated in recent experiments$^{2}$. \begin{enumerate} \item S.Salahuddin and S. Datta, Appl. Phys. Lett., 89,153504, 2006. \item G. D. Fuchs et. al., Appl. Phys. Lett. 86, 152509, 2005;M. Huai et. al., Appl. Phys. Lett., 87, 222510, 2005~;H. Meng et. al., Appl. Phys. Lett. 88, 082504, 2006. \end{enumerate} [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N14.00010: Spin-polarized scanning tunneling microscope and the Kondo effect Kelly Patton, Stefan Kettemann We calculate the tunneling current between a spin-polarized scanning tunneling microscope (SP-STM) and a Kondo impurity on a metallic substrate, including the effects of the spin-polarization of the SP-STM on the adsorbate. This spin-polarization breaks the spin symmetry of the Kondo system, similar to an applied magnetic field, which leads to a splitting of the Abrikosov-Suhl-Kondo resonance. The amount of splitting is controlled by the strength of the coupling between the impurity and the SP-STM tip. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N14.00011: Absence of persistent spin transport Noah Bray-Ali, Zohar Nussinov, Alexander Balatsky A system that is in its ground state does not transport charge. Spin transport also does not occur. We extend Bohm's argument for the absence of persistent charge transport to show the absence of persistent spin transport. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N14.00012: Effect of Annealing on Extraordinary Hall Effects in Sputtered Granular Cu$_{80}$Co$_{20}$ Thin Films Nam H. Wang, Jian-Qing Wang This work explores the microstructure dependence of extraordinary Hall effect (EHE) in Cu$_{80}$Co$_{20}$ granular thin films. Upon annealing, the Cu-Co films showed anomalous microstructure evolution into two-particle distribution, as evident in measured magnetic susceptibility versus temperature, showing existence of double peak structures in magnetic blocking for annealing temperature above 300 $^{o}$C. Such unusual nanostructure directly affected the magneto-transport properties, most noticeable in the extraordinary Hall effect (EHE). The measured EHE was compared with Co-Ag films series, with more uniform particle distribution, which was shown to inversely scale with the scattering length and average particle sizes. Such scaling relationship was absent in Cu-Co films. It was concluded that the EHE in Cu-Co annealed films primarily depends on the population of smaller-sized particles. This was evident in independence of EHE saturation field on the annealing temperature. The gradual decrease of EHE with the annealing is a result of two combined effects. The initial linear decrease below 250 $^{o}$C in EHE is a result of interface change of the Co particles in Cu matrix. As the larger particles began to emerge, further decline in EHE is due to the reducing smaller particle population, while the larger particles do not contribute significantly to the EHE. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N14.00013: Evidence for a disorder-dependent localization correction to the anomalous Hall conductance of ultrathin Fe films Rajiv Misra, Arthur F. Hebard We present an experimental study of quantum corrections to the conductivity tensor of thin ferromagnetic films when the disorder is systematically varied. Using the sheet resistance as a measure of disorder, \textit{in situ} magnetotransport studies were performed on a series of thin iron films deposited onto sapphire substrates having sheet resistance $R_{0 }\equiv $~$R_{xx}$(5K) varying over the range 140~$\Omega $ (60{\AA}) to 6250 $\Omega $ ($<$20 {\AA}). For temperatures $T$~$<$~20 K, a logarithmic temperature dependence of the longitudinal $R_{xx}$ and anomalous Hall resistances $R^{AH}_{xy}$ is observed. In the low disorder limit (R$_{0}$ $<$~150~$\Omega )$, we find that relative changes in the anomalous Hall conductivity $\delta $\textit{$\sigma $}$^{AH}_{xy}$/\textit{$\sigma $}$^{AH}_{xy }$exhibit a temperature independent behavior implying that there are no quantum corrections to \textit{$\sigma $}$^{AH}_{xy}$. As disorder increases, a finite logarithmic temperature dependence to $\delta $\textit{$\sigma $}$^{AH}_{xy}$/\textit{$\sigma $}$^{AH}_{xy}$ appears and then evolves toward a universal weak localization correction defined by the equality $\delta $\textit{$\sigma $}$^{AH}_{xy}$/\textit{$\sigma $}$^{AH}_{xy}=-\delta R^{AH}_{xy}$/$R^{AH}_{xy}$ [1]. Thus with increasing disorder, we see a crossover from a region where there are no quantum corrections to \textit{$\sigma $}$^{AH}_{xy}$ to a region dominated by weak localization corrections. These results for iron, where spin is carried by itinerant electrons, will be compared with data on thin films of gadolinium, a localized moment system. [1]. Mitra P. et al. \textit{cond-mat 0606215} (2006) [Preview Abstract] |
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