Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T14: Focus Session: Magnetic Vortices |
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Sponsoring Units: GMAG DMP Chair: Kristen Buchanan, Colorado State University Room: 316 |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T14.00001: Reproducible control of the magnetic vortex chirality on a nanosecond timescale Invited Speaker: Vojt\v{e}ch Uhl\'I\v{r} Magnetic vortices are curling magnetization structures which represent the lowest energy state in sub-micron size magnetic disks or polygons. The vortex core, a singularity at the vortex center, features magnetization pointing either up or down perpendicular to the disk plane. The binary character of the chirality of the curl and the polarity of the vortex core leads to four possible stable magnetization configurations that can be utilized in a multi-bit memory cell. Both the vortex polarity and chirality are stable against static magnetic fields. It has been shown that when excited with ultrafast magnetic field or current stimuli, the core polarity can be reversed on a 100 ps timescale. We demonstrate ultrafast switching of vortex chirality using nanosecond magnetic field pulses by imaging the process with full-field x-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core and the reversal is achieved at significantly reduced field amplitudes when compared to quasi-static switching. Controlled switching of the chirality requires removing the vortex core out of the disk and then reforming the vortex with opposite chirality. This can be achieved by using a static magnetic field and exploiting a geometric asymmetry in the object. However, scaling this process down in time, using nanosecond and shorter magnetic field pulses, necessarily introduces complex magnetization dynamics that might prevent efficient and reproducible switching of the vortex chirality. We show that these issues can be overcome by selecting magnetic disks of an appropriate geometry along with the field pulse parameters. Finally we discuss that faster switching rates can be achieved by scaling down the disk size. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T14.00002: Interacting magnetic nanodisks pairs Joao Paulo Sinnecker, Helmunt Eduardo Vigo Cotrina, Erico Novais, Fl\'avio Garcia, Alberto Passos Guimaraes Nanodots with magnetic vortex configuration are considered as promising elements of recording media [1]. When vortices are excited from their equilibrium position and allowed to relax, they perform a motion called gyrotropic, with a characteristic frequency. When two magnetic disks are close to one another there arises a frequency splitting due to the dynamic interaction [1]. Expressions for the magnetic vortex excitation frequencies and coupling constants in a pair of coupled identical circular disks were obtained previously by Shibata et al. [2]. The goal of this work is to calculate analytically the frequency of the dynamic excitation of coupled vortices in a pair of disks with different radii, with the same thickness. We considered a magnetostatic interdot interaction using the linearized Thiele's equations of motion of the vortex core, neglecting the damping term. Through micromagnetic simulation, we have investigated the interaction of these pairs of nanodisks using a recently developed tool, the magnetic vortex echoes (MVE) [3]. An analytical model of the MVE is presented. \\[4pt] [1] H. Jung, et al. Sci. Rep. 59, 1-6 (2011).\\[0pt] [2] J. Shibata et al. Phys. Rev. B67, 224404 (2003).\\[0pt] [3] F.Garcia et al., Journal of Applied Physics (in press). [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T14.00003: Element Specific Observation of Ferromagnetic Interlayer Exchange Coupled Dual Vortex Core Nano Systems Javier Pulecio, Dario Arena, Peter Warnicke, Mi-Young Im, Shawn Pollard, Peter Fischer, Yimei Zhu We report on the magnetic evolution of magnetic vortices in nanoscale and multilayer disk structures. ~The tri-layer structure consists of Co and Permalloy (Py) layers, coupled across a thin (1nm) Cu spacer that provides strong coupling between the Co and Py layers. ~Element-resolved full-field XMCD microscopy is combined with ultra-high resolution Lorentz transmission electron microscopy, permitting measurement of both layer-resolved domain patterns and the vortex structure averaged across the tri-layer. ~We examine the evolution of the vortex structure while the nanostructure is cycled through the M-H hysteresis loop. ~In particular we will discuss the effects of strong interlayer exchanged coupling on a dual vortex core system, including analysis of the layer-resolved coercivity, and the evolution, deformation, annihilation, and nucleation of the vortices. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T14.00004: Configurational Anisotropy and Single Domain Behavior in Sub-Micron Square Nickel Dots Daniel Endean, E. Dan Dahlberg Magnetic thin films patterned as regular polygons discourage the formation of a vortex magnetic state in favor of single domain behavior due to the presence of sharp corners. We report on measurements of the magnetic properties of Nickel films patterned as isolated square dots with side lengths varying from 1 micron down to 100 nm and thicknesses of 10 nm. The magnetic field dependence of the dot magnetization is probed using a 4-terminal resistance measurement through the anisotropic magnetoresistance (AMR) effect. By measuring the resistance analog of a hysteresis loop, we observe single domain behavior consistent with the presence of 4-fold configurational anisotropy energy. Using a Stoner-Wohlfarth model, we quantify the magnitude of the anisotropy through the easy axis coercivity and the rotational hysteresis and compare to micromagnetic simulations. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:48AM |
T14.00005: Resonant-spin-ordering of vortex cores in interacting mesomagnets Invited Speaker: Shikha Jain The magnetic system of interacting vortex-state elements have a dynamically reconfigurable ground state characterized by different relative polarities and chiralities of the individual disks; and have a corresponding dynamically controlled spectrum of collective excitation modes that determine the microwave absorption of the crystal. The development of effective methods for dynamic control of the ground state in this vortex-type magnonic crystal is of interest both from fundamental and technological viewpoints. Control of vortex chirality has been demonstrated previously using various techniques; however, control and manipulation of vortex polarities remain challenging. In this work, we present a robust and efficient way of selecting the ground state configuration of interacting magnetic elements using resonant-spin-ordering approach. This is achieved by driving the system from the linear regime of constant vortex gyrations to the non-linear regime of vortex-core reversals at a fixed excitation frequency of one of the coupled modes. Subsequently reducing the excitation field to the linear regime stabilizes the system to a polarity combination whose resonant frequency is decoupled from the initialization frequency. We have utilized the resonant approach to transition between the two polarity combinations (parallel or antiparallel) in a model system of connected dot-pairs which may form the building blocks of vortex-based magnonic crystals. Taking a step further, we have extended the technique by studying many-particle system for its potential as spin-torque oscillators or logic devices. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T14.00006: Magnetization manipulation in ferromagnetic nanoscale disks Wenming Ju, Madeline Shortt, Mina Khan, Jessica Bickel, Kathy Aidala, Mark Tuominen A ferromagnetic nanodisk, several hundred nanometers in radius and several tens of nanometers in thickness, has in-plane curling magnetization distribution around the center and out-of-plane magnetization vortex core at the center. Here, permalloy disks were patterned by electron-beam lithography. We investigated the in-plane curling magnetization direction (i.e., clockwise or counter-clockwise) by applying a uniform external magnetic field and observing the motion of vortex core via Magnetic Force Microscopy (MFM). We conducted experiments to reverse the in-plane curling direction for the magnetization by applying a circular magnetic field around the disk center with a conducting AFM tip [1]. Micro-magnetic simulations were performed to give a comparison and better understanding of the experimental work.\\[4pt] [1] T. Yang, et al. ``Manipulation of magnetization states of ferromagnetic nanorings by an applied azimuthal Oersted field,'' Applied Physics Letters 98, 242505 (2011). [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T14.00007: Calculation of energy barriers for magnetic vortices in sub-100 nm dots Pavel Lapa, Andrew King, Igor V. Roshchin Interest in switching of magnetic vortices in nanodots is stimulated by their potential application for magnetic memories and nano-oscillators. By combining analytical and micromagnetic techniques, we calculated energy barriers for vortex switching in 20 nm-thick iron dots as a function of applied in-plane field and dot diameter. Using analytical formula for magnetization distribution in the vortex\footnote{ N. A. Usov and S. E. Peschany, J. Magn. Magn. Mater. \textbf{118}, 290 (1992).}, we performed micromagnetic calculations of the dot energy for different vortex core positions. In contrast to the ``rigid body approximation,'' the core size and core shape in our calculations were varied to achieve the energy minimum for every core displacement. The energy barriers required for vortex nucleation and annihilation were calculated as a function of magnetic field. By comparing these barriers to the thermal energy k$_{\mathrm{B}}$T we obtained the temperature dependences of the vortex nucleation and annihilation fields in good agreement with the experiment.\footnote{R. K. Dumas \textit{et al}., Appl. Phys. Lett. \textbf{91}, 202501 (2007).} Work is supported by Texas A{\&}M University, TAMU-CONACyT Collaborative Research Program. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T14.00008: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T14.00009: Nanomechanical Detection of Magnetic Hysteresis of a Single-crystal Yttrium Iron Garnet Micromagnetic Disk Joseph Losby, Zhu Diao, Jacob Burgess, Shawn Compton, Fatemeh Fani Sani, Tayyaba Firdous, Douglas Vick, Miro Belov, Wayne Hiebert, Mark Freeman A micromagnetic disk was milled from a monocrystalline yttrium iron garnet film using a focused ion beam and micromanipulated onto a nanoscale torsional resonator. Nanomechanical torque magnetometry results show a unipolar magnetic hysteresis characteristic of a magnetic vortex state. Landau-Lifshitz-Gilbert-based micromagnetic simulations of the disk show a rich, flux-enclosed, three-dimensional domain structure. On the top and bottom faces of the disk, a skewed vortex state exists with a very small core. The core region extends through the thickness of the disk with a smooth variation in core diameter reaching a maximum along the midplane of the disk. The single crystalline nature of the disk lends to an observed absence of Barkhausen-like steps in the magnetization-versus-field curves, qualitatively different in comparison to the magnetometry results of an individual polycrystalline permalloy microdisk. Prospects for the mechanical detection of spin dynamical modes in these structures will also be discussed. [Preview Abstract] |
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