Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J14: Focus Session: Domain Wall Motion and Spin Dynamics |
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Sponsoring Units: GMAG DMP FIAP Chair: Mark Stiles, National Institute of Standards and Technology Room: Colorado Convention Center Korbel 4D |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J14.00001: Effects of domain wall width on current- and field-driven wall motion G.S.D. Beach, C. Knutson, M. Tsoi, J.L. Erskine Magnetic domain wall motion can be driven by a magnetic field or by a spin-polarized electric current traversing the wall. The velocity of field-driven wall motion [1] depends on the details of the domain wall structure and varies in direct proportion to the wall width. By contrast, a current is predicted to augment the velocity of a domain wall by an amount that is \textit{independent} of its structure. Using high-bandwidth scanning Kerr polarimetry, we have studied field- [1] and current-driven [2] wall dynamics in Permalloy nanowires whose widths span a broad range. Wall width is a function of the wire cross-sectional geometry, and the field-driven wall mobility varies in proportion to the calculated wall width. However, the mobility of current-driven motion also depends on wire geometry and is strongly correlated with the field-driven wall mobility. The results will be discussed in relation to available spin-torque models. [1] G. S. D. Beach, et al., Nature Mater. 4, 741 (2005) [2] G. S. D. Beach, et al., Phys. Rev. Lett. 97, 057203 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J14.00002: Current induced resonance of vortex domain wall in permalloy nanowires Rai Moriya, Luc Thomas, Masamitsu Hayashi, Xin Jiang, Charles Rettner, Stuart S.P. Parkin We explore the current induced resonant excitation of magnetic domain walls in permalloy nanowires which have vortex structures. Domain walls (DWs) are injected and pinned at artificial pinning sites or notches patterned by electron beam lithography in nanowires 300 nm wide and 20 nm thick. Ac current excitation of vortex DW has been measured using RF rectification method where ac-current is applied to nanowire by using bias-tee and at the same time dc-voltage difference across the nanowire has been measured with voltmeter. The ac current results in a translational motion of the DW vortex core which accompanies the resistance change of sample due to the anisotropic magnetoresistance effect at the DW. This resistance change gives rise to a small but measurable dc voltage along the nanowire and allows us to detect its resonance. The resonance frequency is very sensitive to external magnetic field applied along the wire. We interpret the resonance to be due to a magnetic field induced motion of the DW within the pinning potential arising from the notch. Using the DW energy profile calculated from micromagnetic simulations and a 1d analytical model we obtain good agreement with the experimentally observed field dependence of the resonant frequency. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J14.00003: Thermally-Assisted Current-Driven Domain Wall Motion Rembert Duine, Alvaro Nunez, Allan MacDonald Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall $\langle \dot{r}_{\rm dw} \rangle$ as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero temperature $\langle \dot{r}_{\rm dw} \rangle$ initially varies linearly with current, even in the absence of non-adiabatic spin torques. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:27PM |
J14.00004: Oscillatory dependence of current driven domain wall motion on current pulse length Invited Speaker: The motion of domain walls (DW) in magnetic nanowires driven by spin torque from spin-polarized current is of considerable interest. Most previous work has considered the effect of dc or $\sim $microsecond long current pulses. Here, we show that the dynamics of DWs driven by nanosecond-long current pulses is unexpectedly complex. In particular, we show that the current driven motion of a DW, confined to a pinning site in a permalloy nanowire, exhibits an oscillatory dependence on the current pulse length with a period of just a few nanoseconds [1]. This behavior can be understood within a surprisingly straightforward one dimensional analytical model of the DW's motion. When a current pulse is applied, the DW's position oscillates within the pinning potential out of phase with the DW's out-of-plane magnetization, where the latter acts like the DW's momentum. Thus, the current driven motion of the DW is akin to a harmonic oscillator, whose frequency is determined by the ``mass'' of the DW and where the restoring force is related to the slope of the pinning potential. Remarkably, when the current pulse is turned off during phases of the DW motion when it has enough momentum, the amplitude of the oscillations can be amplified such that the DW exits the pinning potential well \textit{after} the pulse is turned off. This oscillatory depinning occurs for currents smaller than the dc threshold current, and, moreover, the DW moves against the electron flow, opposite to the propagation direction above the dc threshold. These effects can be further amplified by using trains of current pulses whose lengths and separations are matched to the DW's oscillation period. In this way, we have demonstrated a five fold reduction in the threshold current required to move a DW out of a pinning site, making this effect potentially important for technological applications. [1] L. Thomas, M. Hayashi, X. Jiang, R. Moriya, C. Rettner and S.S.P. Parkin, Nature 443, 197 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J14.00005: Spin-Transfer-Torque-Driven Domain-Wall Dynamics in Permalloy Nanowires Shuqiang Yang, James Erskine Pulse-current-driven domain-wall dynamics in Permalloy nanowires are studied using high-temporal-resolution magneto-optical techniques. The time-resolved measurements elucidate mechanisms responsible for stochastic variation in pulse-current-stimulated wall displacements, and resolve factor-of-10 disagreements between prior experimental $^{[1,2]}$ and theoretical determinations $^{[3]}$ of domain-wall velocity and spin-flip efficiency in magnetic nanowire structures. Current pulses with different width and amplitude are used to probe the domain-wall motion. By reducing the pulse width, higher current densities can be achieved, leading to more complex domain structures (probed by MFM) in the final state. [1] A. Yamaguchi et al. PRL 92, 077205-1, 2004 [2] M. Klaui et al. PRL 95, 026601-1, 2005 [3] Z. Li and S. Zhang, PRL 92, 207203-1, 2004 [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J14.00006: Coherent precession of propagating domain walls in permalloy nanowires Masamitsu Hayashi, Luc Thomas, Charles Rettner, Rai Moriya, Stuart Parkin We report on domain wall (DW) dynamics in permalloy nanowires. We demonstrate the precessional nature of the DW propagation above the Walker breakdown field. Time resolved resistance measurements were performed on 200 nm wide 10 nm thick permalloy nanowires. Oscillations in resistance are observed when the DW propagates along the nanowire. The frequency of this oscillation varies linearly with magnetic field, according to the Larmor precession frequency. By contrast, current passing through the nanowire has relatively little effect on the oscillation frequency even though it is large enough to influence the DW velocity. To explore the origin of this resistance oscillation, dc resistance measurements were performed on permalloy nanowires with a pinning center located along the nanowire. The state of the DW pinned at the pinning center can be inferred from the nanowire's resistance. By using a combination of current and magnetic field, the time at which the DW arrives at the pining center can be tuned, allowing us to show that the chirality of the domain walls reverses periodically as the wall propagates along the nanowire. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J14.00007: Damping in Ferromagnets: Landau-Lifshitz versus Gilbert Wayne M. Saslow, Mark D. Stiles, Andrew Zangwill We first note a number of qualitative and quantitative arguments favoring Landau-Lifshitz over Gilbert damping in ferromagnets. We then explicitly demonstrate a classical Fokker-Planck-like derivation of the macroscopic damping rate in terms of thermodynamic fluctuations (fluctuation-dissipation). Because out-of-equilibrium fluctuations are driven toward equilibrium by their excess thermodynamic energy, the damping term is proportional to the transverse component of the effective field, thus yielding Landau-Lifshitz damping with an explicit expression for the damping coefficient. This damping is unaffected to lowest order in systems with spin transfer torque (STT). Recent experiments on current-driven domain wall motion have a natural interpretation in terms of the so-called adiabatic STT. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J14.00008: Interactions between spin polarized current and local spin systems: A Quantum Mechanical Approach Fatih Dogan, Lucian Covaci, Wonkee Kim, Frank Marsiglio In this talk, we present the first fully quantum mechanical calculation of the spin flip interaction between spin polarized current and local spin systems. Dynamics of local spins will be illustrated as many electrons pass through the chain. The talk will focus on the description of the approach, density matrix formalism used in the calculations and the behavior of the system for different configurations. The interplay between electron-spin and spin-spin interaction, effect of domain walls, limiting cases for interaction strengths, spin degree of freedom, and comparison to LLG model will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J14.00009: Giant voltage response to magnetic field of model granular magnetic films and spin mixing effects Jean-Philippe Ansermet, M. Hillenkamp, G. Didomenicantonio, C. Felix, L. Gravier, S. Serrano-Guisan, M. Abid Magneto-thermogalvanic voltage (MTGV) is the magnetic field dependence of the AC voltage measured across a sample subjected to a DC current and to an oscillation of its temperature. Large field sensitivity was found when the measurement was applied to a thin film made by the technique of cluster-assembled materials. [1] Clusters of Co with an average size of 15 atoms per cluster in a copper matrix gave this result. Similar large changes were observed using silver as a matrix. The size of this effect compared to GMR, its temperature and field dependence, in these and other nanostructures, demonstrate that a different process than that responsible for GMR is the determining mechanism. Argument in favour of an asymmetry in the spin mixing process is given. \newline \newline [1] S. Serrano-Guisan et al., \underline {Nature Materials} \textbf{5,} 730 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J14.00010: Dynamics of spin flipping Lucian Covaci, Wonkee Kim, Fatih Dogan, Frank Marsiglio Interactions between a spin polarized current and a ferromagnetic spin chain will lead to the eventual flipping of the spins. We study the dynamics of spin flipping due to Kondo- like interactions between an electron and a spin chain. Interactions within the chain are taken to be of Heisenberg type. The full time dependent quantum mechanical problem is solved within a density matrix formulation. We present the time evolution of the electron wave packet and of the spin expectation values as the electron passes through the chain. The electron transmission probability is calculated as a function of electron momentum and interaction coupling strength. We observe excitations induced by spin transfer and resonant transmission regimes. Deviations from quasi-classical treatments of magnetic moments are discussed. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J14.00011: Non-Equilibrium Spin Dynamics in the Subpicosecond Regime Adnan Rebei Femto-second laser pulses are becoming an important tool that allows us to explore non-equilibrium spin dynamics at short time (high frequency) scales [1]. It has therefore become apparent [2] that more rigorous treatments are needed to correctly address spin relaxation at these energies. I will show how functional-methods of calculations of correlation energies in electron gas [3] can be successfully adapted to the problem of relaxation in magnetic systems [4]. The study of short time response entails a careful treatment of initial conditions. Our formalism naturally takes care of this and avoids the assumption that the system has been in equilibrium in the infinite past, an assumption common in Boltzmann-type treatments. As an example, we discuss possible non-equilibrium effects due to ultrasonic attenuation on spin relaxation when the magnon sub-system is initially near the Curie point. \newline [1] A. V. Kimmel et al, Nature 435, 655 (2005); L. Guidoni et al., Phys. Rev. Lett. 89, 017401 (2002). \newline [2] A. Rebei and J. Hohlfeld, Phys. Rev. 97, 117601 (2006); A. Rebei and M. Simionato, Phys. Rev. B 71, 174415 (2005). \newline [3] A. Rebei and W. N. G. Hitchon, Int. J. Mod. Phys. B 17, 973 (2003). \newline [4] A. Rebei, W.N.G. Hitchon, and G. J. Parker, Phys. Rev. B 72, 064408 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J14.00012: Thermal hysteresis in transport properties of chromium films due to Spin Density Wave (SDW) quantization and Domain-wall scattering Ravi K. Kummamuru, Yeong-Ah Soh Magnetotransport measurements were made on four Cr films of thicknesses 3500$^{o}$A, 560$^{o}$A, 430$^{o}$A and 175$^{o}$A sputtered on MgO substrates. We observe thermal hysteresis in the resistivity and Hall coefficient. Two types of hysteresis are observed, one in a temperature range of tens of Kelvin above 200 K and the other from the Neel temperature down to about 50 K. The first type is seen in two of the films, 175$^{o}$A and 560$^{o}$A. By looking at the Hall conductance in the vicinity of this hysteresis, we show that it arises directly from the SDW. The second type of hysteresis is absent in the thinnest 175$^{o}$A film, but increases in magnitude with film thickness, and resistivity is always higher during cooling than warming. We conclude that the first type of hysteresis is caused due to discrete changes in the number of incommensurate SDW nodes due to confinement in the thickness dimension, and the second type is caused due to changes in the domain wall configuration with temperature, leading to a reduction in anti-ferromagnetic (AFM) SDW domain wall scattering. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J14.00013: Direct measurement of antiferromagnetic domain fluctuations Oleg Shpyrko, Eric Isaacs, Jonathan Logan, Hyekyung Kim, Yejun Feng, Rafael Jaramillo, Thomas Rosenbaum, Gabriel Aeppli, Paul Zschack, Michael Sprung, Suresh Narayanan, Alec Sandy We present coherent x-ray speckle measurements of slow nanoscale dynamics of domain walls separating microscopic regions with different orientations of the spin- (charge-) density waves in Cr bulk samples. Between 150K and 30K domain wall fluctuations slow down as the sample temperature is lowered, consistent with classical thermal activation model. Below 30K, however, the characteristic domain fluctuation timescale remains constant, possibly due to the cross-over between thermally activated and quantum tunneling mechanisms of domain wall relaxation. [Preview Abstract] |
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