Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V32: Focus Session: Vortex and Domain Wall Dynamics |
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Sponsoring Units: GMAG DMP FIAP Chair: Ilya Krivorotov, Univerisity of California Irvine Room: 336 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V32.00001: Dynamics of Exchange-Biased Magnetic Vortices T. Y. Chen, M. K. Chan, P. A. Crowell We have studied magnetization dynamics in micron-sized circular disks composed of ferromagnetic (FM)-antiferromagnetic (AFM) bilayers. The patterned samples of FeMn/NiFe are field-cooled (FC) or zero-field cooled (ZFC) from above the blocking temperature to room temperature. Time-resolved Kerr microscopy measurements show that the vortex gyrotropic mode fluctuates in frequency as the vortex core is displaced by a static in-plane magnetic field. The average gyrotropic frequency and the magnitude of its fluctuations, which are due to pinning of the vortex core, are larger than in single layer FM films. The enhancement of the gyrotropic frequency is largest in the ZFC samples, in which the effective field due to exchange coupling is expected to enhance pinning of the vortex core at the center of the disk. We find, however, that micromagnetic simulations incorporating uniform or vortex-like exchange-bias fields do not explain our results quantitatively. We interpret this discrepancy as a consequence of randomly orientated AFM domains, which are comparable in size to the vortex core. This work was supported by NSF and the Univ. of Minnesota Graduate School. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V32.00002: Ferromagnetic resonance force spectroscopy of a magnetic vortex G. de Loubens, O. Klein, A. Riegler, F. Lochner, G. Schmidt, L.W. Molenkamp, H. Hurdequint, F. Boust, N. Vukadinovic, A.N. Slavin Due to its nanometer size (of the order the exchange length), probing the high frequency dynamics of a magnetic vortex core is an experimental challenge. Precessional dynamics of the magnetization of individual nano-disks of NiMnSb perpendicularly magnetized is measured in a wide range of bias magnetic fields using a magnetic resonance force microscope (MRFM). A full dynamic phase diagram, demonstrating excitation of a Kittel-type dipolar mode in the saturated disks and the gyrotropic mode of vortex core rotation in the vortex-state unsaturated disks, is established. Switching of the vortex core polarity in a negative (anti-parallel to core) bias magnetic field is registered dynamically. Analytic theory and micromagnetic simulations provide a quantitative description of the experimental results. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V32.00003: Spin motive electric field driven by magnetic vortex motion Jun-ichiro Ohe, Stewart E. Barnes, Sadamichi Maekawa The current-induced magnetization dynamics realized in spintronics devices involve both of charge and spin degrees of freedom. Recently, it has been pointed out that the magnetization dynamics induces an effective electric field acting on the conduction electrons through the spin Berry phase. The effective electric field, or a ``spin motive electric field,'' was investigated for a simple one-dimensional domain wall. It is difficult to estimate analytically this effective electric field in actual systems, because the magnetization dynamics obeys the non-linear Landau-Lifshitz equation. In this report, we describe numerical studies of the spin motive electric field induced by the dynamics of a vortex core. The vortex structure can be realized in a Permalloy disc. It is known that the magnetic vortex core shows a resonant motion when the oscillating magnetic field is applied. The direction of the core is switched rapidly by applying a pulsed such magnetic field. During the core motion, we obtain an electric field near the core. The direction of the electric field is perpendicular to the direction of the core motion. We also obtain the electric field driven by spin waves which are excited by the core switching. We propose an experimental setup for measuring the electric field. The calculated voltage is large enough to measure. We show that the voltage induced by core switching is quite large. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V32.00004: Thickness and field dependence of the driven dynamic mode-splitting of magnetic vortices Kristen Buchanan We have explored the effects of increased driving field amplitude on the dynamics of magnetic vortices using a microwave reflection technique and found that the vortex translational eigenmode profile first takes on a distorted shape and then splits into two well-defined peaks as the field is increased [1]. Here we examine the thickness and field dependence of this mode-splitting phenomenon via measurements of lithographically patterned micron-sized Permalloy ellipses with thicknesses of 20, 40, and 60 nm. The experimental results will be compared to numerical calculations that incorporate a critical velocity parameter and provide new insight into the origin of the observed vortex dynamic mode splitting. Acknowledgments: Thank you to Marcos Grimsditch, Frank Fradin, Sam Bader, and Val Novosad for stimulating discussions. [1] Buchanan et al. PRL 99,267201 (2007). [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V32.00005: Time-resolved X-ray Microscopy of Magnetic Antivortex Dynamics Markus Bolte, Thomas Kamionka, Michael Martens, Bernd Guede, Guido Meier, Kang Wei Chou, Tolek Tyliszczak, Michael Curcic, Bartel Van Waeyenberge, Hermann Stoll The study of magnetic singularities, vortices and antivortices, has recently intensified as they have been suggested as non-volatile data storage elements or for spin-wave logic applications. Magnetic antivortices occur during the switching process of their topological counterparts, the vortices [1], as well as in crosstie-domain walls and special geometries [2]. Understanding the dynamics of antivortices [3] is therefore fundamental for gaining a detailed knowledge necessary to design new spintronic applications. Here we show by time-resolved X-ray microscopy experiments that magnetic antivortices indeed gyrate when excited by alternating currents, in agreement with theoretical models and micromagnetic simulations [3], albeit with much lower efficiency than their topological counterparts, the vortices. [1] B. Van Waeyenberge et al., Nature \textbf{444}, 461 (2006). [2] K.Shigeto et al., APL \textbf{80}, 4190, (2002). [3] A. Drews et al., PRB \textbf{77}, 094413 (2008); B. Krueger et al., JAP \textbf{103}, 07A501 (2008). [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V32.00006: High frequency spin dynamics in soft magnetic dots in biased vortex state: precise probing and nature of the eigenmodes Farkhad Aliev, Juan Francisco Sierra, Ahmad Awad, Gleb Kakazei, Dong-Soo Han, Sang-Koong Kim, Konstantin Guslienko, Bojan Ilic, Vitali Metlushko Regular arrays of soft magnetic dots in the vortex state are being considered as a potentially new high-density nonvolatile recording media characterized by two binary properties: chirality and polarity of magnetic vortex core. Here we unambiguously demonstrate the existence of two distinct dynamic vortex (stable and metastable) regimes with qualitatively different spin wave eigenmodes. We find that dynamic response in the metastable vortex state qualitatively changes with relative orientation of the driving rf and bias magnetic fields. These findings, supported by numerical simulations, open new possibilities for development of magnetic devices with precise control over the magnetization switching process. They also underscore importance of understanding of dynamic response in different nanostructured materials with vortices in confined and stratified conditions. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V32.00007: Spin-Torque Ferromagnetic Resonance Spectroscopy of Permalloy Nanowires Carl Boone, Jordan Katine, Jeff Childress, Jian Zhu, Xiao Cheng, Ilya Krivorotov We develop a technique for studies of spectral properties of spin waves excited by spin transfer torque in metallic ferromagnetic nanowires, and apply this technique to measure frequencies and damping constants of several low-energy quantized spin wave modes in permalloy nanowires of rectangular cross section. Our measurements demonstrate that the spin wave spectrum of nanowires as narrow as 100 nm is well described by an analytic theory of dipole-exchange spin waves in thin ferromagnetic strips. Geometric quantization of the spin wave spectrum in nanowires significantly reduces the phase space for magnon-magnon scattering leading to opening and closing of discrete scattering channels as a function of magnetic field. These scattering channels manifest themselves as peaks in plots of spin wave damping versus magnetic field. In particular, we observe damping enhancement of the lowest energy spin wave mode at the values of magnetic field corresponding to three-magnon confluence processes in which two lowest energy magnons merge into a single higher-energy mode magnon. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V32.00008: Effects of disorder and temperature on vortex domain wall dynamics Hongki Min, Michael Donahue, Mark Stiles Domain wall motion, whether driven by applied magnetic fields or electrical current, can be strongly affected by sample irregularities. Using micromagnetic simulations and a collective coordinate approach, we study the dynamics of domain wall motion driven by a spin-polarized current or an external magnetic field in the presence of extrinsic random potential at finite temperatures. We compare these calculations and discuss the region of validity of the approximations in the simple model. Information about the strength of the random potential is taken from recent magnetic resonance experiments. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V32.00009: Electron Drift Speed And Current-Induced Drive Torques On A Domain Wall Luc Berger It has become fashionable to describe [1] current-induced torques on a DW in terms of an electron drift speed u = - P*j*mu$_B$/e*M where mu$_B$ is the Bohr magneton and M the saturation magnetization. While appropriate for adiabatic torques, this quantity u is misleading and not the best choice in the case of non-adiabatic torques. For example, it leads [2] to beta not equal to alpha, where beta represents the intensity of the non-adiabatic torque, and alpha is the damping parameter. By writing equations of motion for conduction- electron spins in a moving frame where the electron gas is at rest, we find [3] a direct relation between damping and non- adiabatic torques. The correct electron drift speed turns out to be the speed of the frame, and is v = P*j/(n*q) where n and q are the carrier density and charge. It is related to the ordinary Hall constant R$_0$ by v ~ P*R$_0$*j. After substituting v for u in the expression of the non-adiabatic torque, we find that beta = alpha holds now. Because v is larger than u in Permalloy, it can explain better the large current-induced DW speeds found [4] experimentally. In materials where R$_0 > 0$ and the carriers are dominantly hole-like, v and u have opposite signs, leading to different predictions for the sense of DW motion. We discuss examples of such materials. 1. G. Tatara and H. Kohno, Phys. Rev. Lett. 92, 086601 (2004). 2. H. Kohno et al., J. Phys. Soc. Japan, 75, 113706 (2006). 3. L. Berger, Phys. Rev. B 75, 174401 (2007). 4. M. Hayashi et al., Phys. Rev. Lett. 98, 037204 (2007). [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V32.00010: Universal Electromotive Force Induced by Domain Wall Motion Qian Niu, Shengyuan Yang, Geoffrey Beach, Carl Knutson, Di Xiao, Maxim Tsoi, James Erskine The electromotive force induced by a moving magnetic domain wall in a nanostrip has been calculated theoretically and detected experimentally. It is found that the emf depends only on the domain wall transformation frequency through a universal Josephson type relation, which is closely related to the topological nature of the domain wall. Our experimental measurements confirm the theoretical prediction. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V32.00011: Motion of a vortex domain wall in a rough nanowire Paula Mellado, David Clarke, Oleg Tchernyshyov The motion of a vortex domain wall in a ferromagnetic nanowire under the influence of an applied magnetic field has been recently cast in the language of collective coordinates [1]. The theory, taking into account the two softest modes of the domain wall, works well below and immediately above Walker's breakdown [2] and can be extended to include the influence of spin current. Here we examine the motion of a vortex domain wall in a wire with rough edges. Integrating out the transverse coordinate yields an effective one-dimensional problem of a massive particle moving in a viscous medium. The edge roughness translates into a combination of a random pinning potential and a random Zeeman force. We calculate the average velocity of the domain wall and the probability of passing a wire of specified length as a function of the applied magnetic field. [1] O. A. Tretiakov \textit{et al.,} Phys. Rev. Lett. \textbf {100,} 127204 (2008). [2] D. J. Clarke et al., Phys. Rev. B \textbf{78,} 134412 (2008). The work was supported in part by the NSF Grant DMR-05204291. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V32.00012: Domain pinning and disorder in Fe/Gd magnetic multilayers. Jyoti Mohanty, Ashish Tripathi, Erik Shipton, Keith Chan, SangSoo Kim, Ian McNulty, Eric Fullerton, Oleg Shpyrko We study the evolution of magnetic domains and effect of pinning centers in thin film magnetic systems as a function of magnetic field, temperature, and dopants to identify the role the disorder in formation and stability of the domains in these systems. We have studied Fe/Gd multilayer exhibiting ordered stripes due to perpendicular magnetic anisotropy (PMA). Samples are well characterized using Polar Kerr effect and Vibrating sample magnetometry. Magnetic Force Microscopy (MFM) measurements show out-of-plane magnetized stripe domains. We study the effects of field pinning of the local magnetic structure of these systems through their magnetization hysteresis loops and their temperature driven dynamics. Using element sensitivity and depth resolution of resonant magnetic x-ray coherent scattering technique we investigate the magnetic domain structure and intermittent switching dynamics. Comparison of the magnetic speckles (in momentum space) provides information on correlation between the magnetic structures (in real space). We will present the X-ray Coherent Speckle Metrology approach to study of Barkhausen noise spectrum as a function of the applied magnetic field, and will discuss extension of this study to Tb-doped Fe/Gd magnetic films, which would induce strong PMA. [Preview Abstract] |
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