Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J19: Focus Session: Spin Transport & Magnetization Dynamics in Metals IV |
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Sponsoring Units: GMAG DMP FIAP Chair: Peter Fischer, Lawrence Berkeley National Laboratory Room: D170 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J19.00001: Vortex-Core Reversal Dynamics: Towards Vortex Random Access Memory Invited Speaker: An energy-efficient, ultrahigh-density, ultrafast, and nonvolatile solid-state universal memory is a long-held dream in the field of information-storage technology. The magnetic random access memory (MRAM) along with a spin-transfer-torque switching mechanism is a strong candidate-means of realizing that dream, given its nonvolatility, infinite endurance, and fast random access. Magnetic vortices in patterned soft magnetic dots promise ground-breaking applications in information-storage devices, owing to the very stable twofold ground states of either their upward or downward core magnetization orientation and plausible core switching by in-plane alternating magnetic fields or spin-polarized currents. However, two technologically most important but very challenging issues --- low-power recording and reliable selection of each memory cell with already existing cross-point architectures --- have not yet been resolved for the basic operations in information storage, that is, writing (recording) and readout. Here, we experimentally demonstrate a magnetic vortex random access memory (VRAM) in the basic cross-point architecture. This unique VRAM offers reliable cell selection and low-power-consumption control of switching of out-of-plane core magnetizations using specially designed rotating magnetic fields generated by two orthogonal and unipolar Gaussian-pulse currents along with optimized pulse width and time delay. Our achievement of a new device based on a new material, that is, a medium composed of patterned vortex-state disks, together with the new physics on ultrafast vortex-core switching dynamics, can stimulate further fruitful research on MRAMs that are based on vortex-state dot arrays. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J19.00002: Dynamics of coupled vortices in spin-valve nanostructures Paolo Bortolotti, N. Locatelli, V. Cros, J. Grollier, V.V. Naletov, G. de Loubens, C. Ulysse, G. Faini, O. Klein, A. Fert Recently, vortex dynamics driven by spin-transfer torque have been considered for new generation of nano-oscillators and memory devices. In this work we study the coupled vortex dynamics in FeNi(15nm)/Cu(10nm)/FeNi(4nm) samples where one single vortex state is favoured in both magnetic layers. Our experimental data are in good agreement with the corresponding simulations obtained through a 3D spin diffusion approach. Each vortex is characterized by a given chirality and polarity controllable separately by varying the external field (both in-plane and out-of-plane) and by applying a DC current perpendicular to the sample plane. The system modes are detected by static magneto-transport and microwave emissions analysis. In particular, it can be shown that vortex dynamics with large power appear only for configuration characterized by vortex cores pointing in opposite directions. The coupling of those two vortices allows to reach very narrow peak linewidth (down to 50 kHz), two order of magnitude smaller than in the uncoupled case. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J19.00003: Statistical Behavior of Formation Process of Magnetic Vortex State in Permalloy Nanodisks Mi-Young Im, Peter Fischer, Yamada Keisuke, Shinya Kasai Magnetic vortices in magnetic nanodots, which are characterized by an in-plane (chirality) and an out-plane (polarity) magnetizations, have been intensively attracted because of their high potential for technological application to data storage scheme and their scientific interest for an understanding of fundamental physics in magnetic nanostructures. Complete understanding of the formation process of vortex state in magnetic vortex systems is very important issue in both technical and scientific points of view. In our work, we have statistically investigated the formation process of vortex state in permalloy (Ni$_{80}$Fe$_{20})$ nanodisks through the direct observation of vortex structure utilizing a magnetic transmission soft X-ray microscopy (MTXM) with a high spatial resolution down to 20 nm. We found a particular selectivity between the circulation sense of chirality and orientation sense of polarity for each other in the formation process of vortex state. Dzyaloshinskii-Moriya interaction inevitably generated in magnetic nanodisks is mainly responsible for the experimentally witnessed selectivity between chirality and polarity. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J19.00004: Imaging Magnetic Normal Modes Driven by Spin Transfer Torque in Magnetic Nanopillars using Soft X-ray Microscopy Yong-Tao Cui, Lin Xue, Peter Fischer, Mi-Young Im, R.A. Buhrman, D.C. Ralph Motivated by the desire to understand the spatial structure of the high-frequency dynamical magnetic modes that can be excited by spin transfer from spin-polarized currents, we report measurements using X-ray microscopy to image magnetic normal modes in nanopillar devices resonantly excited by spin torque from a microwave frequency current. The frequency of the microwave current is phase locked to the incident X-ray pulses. We achieve 70 ps time resolution and 25 nm spatial resolution, enabling us to study the spatial configuration of the magnetization throughout the cycle of resonant magnetization dynamics. We will discuss the initial results of our measurements and comparisons with micromagnetic simulations. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J19.00005: Voltage-controlled Spin Wave Logic Device: Ring Interferometer Tianyu Liu, Giovanni Vignale Spin wave logic circuitry transmits information by propagating spin waves along magnetically insulating wave guides. This is less power-consuming than ordinary circuits and is expected to work at THz frequency and room temperature. Logical operations are performed by modulating the interference of spin waves through a phase shifter. A great deal of effort has been devoted to the problem of controlling the phase of spin waves by means of a spatially varying magnetic field, either extrinsic or intrinsic (i.e., by passing the spin wave through a non-uniform magnetic texture). Here we introduce a new approach, which exploits the response of spin waves to an external {\it electric field} via the spin-orbit coupling of this electric field to the electrons that mediate the magnetic interaction. Based on the Heisenberg Hamiltonian modified by spin-orbit coupling, we show how a ring interferometer made of a magnetic insulator (e.g, YIG) can be used to implement NOT logic (and other logic functions) under voltage control. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J19.00006: Phenomenology of Current-Induced Dynamics in Antiferromagnets Kjetil M.D. Hals, Yaroslav Tserkovnyak, Arne Brataas In antiferromagnets, an electric current can induce a torque on the staggered magnetization. We derive a novel phenomenology of current-induced dynamics in antiferromagnets. The theory includes effects of damping, external magnetic fields, and both adiabatic and non-adiabatic current-induced torques. We apply our theory to an antiferromagnetic domain wall system, and find an analytic solution for the domain wall motion in the low current density regime. In this regime, the domain wall velocity is proportional to the ratio between the non-adiabatic torque and the damping coefficient. In addition, the domain wall develops a net magnetic moment. This opens the route to an alternative way to observe current-induced effects in antiferromagnets. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J19.00007: Temperature dependence of domain wall dynamics in Permalloy nanowires Jusang Yang, James L. Erskine Current-driven [1] and current-assisted field-driven [2] domain wall dynamics in ferromagnetic nanowires have the thermal effects resulting from Joule heating, which make difficult to separate the spin-torque effects on domain wall displacements. To understand the thermal effects on domain wall dynamics, temperature dependence of field-driven domain wall velocity was studied using high-bandwidth scanning Kerr polarimetry. Domain wall velocity curves of 20 nm thick Permalloy nanowires with various widths (from 400 nm to 1000 nm) were measured with increasing temperature from 300 K to 400 K. Walker critical fields decreased with increasing temperature, which can be attributed to thermal excitations, and temperature-induced stochastic dynamics mode changes were observed. The results will be discussed in relation to internal domain wall structures.\\[4pt] [1] M. Klaui et al., Phys. Rev. Lett. 95, 026601 (2005).\\[0pt] [2] G.S.D. Beach et al., Phys. Rev. Lett. 97, 057203 (2006). [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J19.00008: Voltage induced by domain wall motion in a ferromagnetic nanowire Yang Liu, Oleg Tretiakov, Artem Abanov We study current-induced domain-wall motion in a narrow ferromagnetic wire. This motion is described by effective equations of motion which depend only on four parameters. These parameters are set by the magnetic Hamiltonian and the shape of the wire. We propose a new way to measure these parameters by measuring time dependent voltage generated by the domain wall motion. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J19.00009: Discrete positioning of domain walls due to localized pinning sites in current driven motion along nanowires Xin Jiang, Luc Thomas, Rai Moriya, Stuart Parkin Current driven domain wall motion is studied in spin-valve nanowires. The position of the domain wall after nanosecond long driving current pulses is determined with an accuracy of better than 50 nm by measuring the resistance of the nanowire. Although the domain wall displacement scales linearly with the current pulse length, its final position is discretized. This is attributed to relaxation of the domain wall into local pinning potential minima along the nanowire after the current pulse is turned off. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J19.00010: Direct imaging of domain wall pinning in artificially created asymmetric potentials D.E. Read, L. O'Brien, S. Ladak, K. Zeissler, T. Tyliszczak, A.-V. Jausovec, H.T. Zeng, E.R. Lewis, J. Sampaio, A. Fernandez-Pacheco, D. Petit, R.P. Cowburn, W.R. Branford Domain walls (DWs) in ferromagnetic nanowires are ideal candidates for a wide variety of technological applications including high density data storage devices. To realise functional DW devices the processes associated with DW pinning must be understood and controlled. Magnetostatic pinning of DWs has already been observed experimentally using spatially resolved MOKE measurements. Using high resolution scanning transmission x-ray microscopy (STXM) we were able to directly image DWs in NiFe nanowires pinned in asymmetric potentials which were created by nano-patterning additional ferromagnetic wires perpendicular to the DW conduit. Tailoring the pinning potential in this way allows us to probe the rigidity of the DW in the region of the pinning site, increasing our understanding of DW deformation in magnetostatic traps and paving the way for future commercial applications. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J19.00011: Spin-torque-driven ballistic switching with $<$ 50ps pulses OukJae Lee, Dan Ralph, Robert Buhrman Spin-torque-driven ballistic switching is a fast, energy-efficient, non-thermal operation in which the magnetization of a nanomagnet rotates from one equilibrium state to the other without any preceding small-angle precession. This reversal scheme can be implemented with a non-collinear structure in which the magnetic free layer is located between an out-of-plane spin polarizer and an in-plane polarizer. Both for achieving better fundamental understanding of magnetic dynamics and for realizing technological advances, it is desirable to demonstrate experimentally that the free layer can be reliably reversed with a current pulse as short as possible. Moreover it is necessary to achieve an asymmetrical response as the function of both the initial state and the pulse current polarity in order to obtain the desired final state with a simple unipolar pulse. We will discuss experimental results that show that the interval of pulse widths giving reliable switching is strongly dependent on the initial magnetic state and on the current. We will also discuss strategies to further improve ballistic switching operations. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J19.00012: First-principles calculation of the photon-shortage mystery in femtosecond magnetism Guoping Zhang, Mingsu Si, Yihua Bai, T.F. George Laser-induced femtosecond magnetism needs photons to influence the magnetization in a sample, but there is a debate on whether the photon-shortage really exists [1]. Here we directly compute the number of photons used in ferromagnetic nickel, and we find that for nearly all the experiments, there are enough photons. The key is that one has to compute this number correctly using the surface instead of volume as a parameter [1,2]. Then we use the first-principles method to compute the magnetization for a fixed number of photons. Our results show that the number of photons is not a decisive factor, since for a fixed number, the laser amplitude and pulse duration can be changed systematically. We suggest that it is more appropriate to use the laser amplitude and pulse duration as two decisive parameters to characterize the role of photons, instead of the photon number. [1] M. S. Si and G. P. Zhang, J. Phys.: Cond. Matt. {\bf 22}, 076005 (2010). [2] G. P. Zhang, W. H\"{u}bner, G. Lefkidis, Y. H. Bai, and T. F. George, Nature Phys. {\bf 5}, 499 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J19.00013: \emph{Ab initio} investigation of ultrafast spin-manipulation: $\Lambda$ processes in charged two-magnetic-center nanostructures with bridging atoms Chun Li, Wei Jin, Georgios Lefkidis, Wolfgang H\"ubner We present a fully {\it ab initio} investigation of ultrafast laser-induced magnetic switching mechanisms in charged two-magnetic-center nanostructures via $\Lambda$ processes [1,2]. In order to improve the spin transferability between the magnetic centers and fulfill the energy-difference requirements for the $\Lambda$ processes [3], a small number of nonmagnetic bridging atoms (O and Mg) is used to connect the magnetic centers. These bridging atoms influence the overlap between the magnetic centers. It is shown that both bridging atoms can redistribute the spin density on the structure by changing either the local spin density or even the total spin localization. Especially, the spin-transfer scenario achieved in [Fe-O(Mg)-Co]$^+$ confirms that using bridging atoms can significantly enhance the spin transferability between the magnetic centers. \\[0pt] [1] C. Li, T. Hartenstein, G. Lefkidis {\it et al}, PRB {\bf 79}, 180413(R) (2009).\\[0pt] [2] T. Hartenstein, C. Li, G. Lefkidis {\it et al}, JPD {\bf 41}, 164006 (2008).\\[0pt] [3] G. Lefkidis, G. P. Zhang, and W. H\"{u}bner, PRL {\bf 103}, 217401 (2009). [Preview Abstract] |
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