Vortex Core Switching by Coherent Excitation with Single In-Plane Magnetic Field Pulses

Abstract

The response of magnetic vortex cores to subnanosecond in-plane magnetic field pulses was studied by time-resolved x-ray microscopy. Vortex core reversal was observed and the switching events were located in space and time. This revealed a mechanism of coherent excitation by the leading and trailing edges of the pulse, lowering the field amplitude required for switching. The mechanism was confirmed by micromagnetic simulations and can be understood in terms of gyration around the vortex equilibrium positions, displaced by the applied field. DOI: 10.1103/PhysRevLett.102.077201 PACS numbers: 75.40.Gb, 75.60.Jk, 75.75.+a The magnetic vortex is a typical ground state configuration of micron and submicron sized ferromagnetic thin film structures [1]. It minimizes the stray field energy by forming an in-plane curling magnetization. In order to avoid a singularity in the center of the structure, the magnetization turns out of plane, forming the vortex core which can point either up (vortex polarization p ¼ þ1) or down (p ¼ À1). This configuration is very stable; static out-ofplane magnetic fields of about 0.5 T are required to switch the core polarization [2]. The vortex also has a specific excitation mode, the so-called gyrotropic mode, which can be excited by an oscillating in-plane magnetic field [3,4]. It corresponds to a circular motion of the vortex around its equilibrium position.Recently, it was discovered that switching the polarization of the vortex core is not only possible by static fields, but also by excitation of the gyration mode [5]. In this case, only field strengths of a few millitesla are needed. Micromagnetic modeling of these experiments revealed that the vortex switching occurs by the creation and subsequent annihilation of a vortex-antivortex pair [5]. This discovery has triggered a variety of studies on vortex core switching by various excitation methods. In particular, it was found by micromagnetic simulations that the polarization may also be switched by very short in-plane magnetic field pulses. It was shown that switching times as short as 40 ps are possible [6]. Except for a brief report on switching events observed with 2 ns long spin-polarized current pulses [7], the possibilities of such excitations have not been explored experimentally so far.In this work, we have experimentally investigated vortex core switching by in-plane magnetic field pulses, using time-resolved magnetic x-ray microscopy. By taking advantage of the pulsed nature of synchrotron light, stroboscopic imaging was set up at scanning transmission x-ray microscopes [8] at beam line 11.0.2 of the Advanced Light Source and 10ID-1 of the Canadian Light Source. Using the x-ray magnetic circular dichroism [9] as a contrast mechanism, magnetization dynamics could be studied with 25 nm spatial and 70 ps temporal resolution.The samples studied in this work are 500 nm  500 nm and 1 m  1 m square-shaped magnetic Permalloy (Ni 80 Fe 20 ) elements with a thickness of 50 nm. These structures are defined on top of a 2:5 m wide, 1...

Spin wave mediated unidirectional vortex core reversal by two orthogonal monopolar field pulses: The essential role of three-dimensional magnetization dynamics

“…Since the vortex polarity is very stable and well controllable, such structures may be used in the future for information storage and data processing devices. It has been demonstrated that the vortex core polarity can be dynamically reversed by exciting one of the eigenmodes of the vortex structure, i.e., the gyrotropic mode [1][2][3][4] or dipolar spin-wave modes. [5][6][7] For the vortex gyrotropic mode G 0 , the vortex core performs a translational motion with frequencies in the range of 100 MHz to 1 GHz (depending on the disk dimensions and the material of the disk).…”

Optimal control of vortex-core polarity by resonant microwave pulses

“…In a vortex-state magnetic nanodisc [1][2][3] , the static magnetization curls in the plane, except in the core region, where it points out of plane 4,5 , either up or down, leading to two possible stable states of opposite core polarity p. Dynamical reversal of p by large-amplitude motion of the vortex core [6][7][8][9] has recently been demonstrated experimentally [10][11][12][13][14] , raising the prospect of practical applications, in particular in magnetic-storage devices…”

“…Still, selective core-polarity reversal is possible using a circularly polarized microwave magnetic field because the sense of the core rotation is linked by a right-hand rule to its polarity 12 . Control of polarity switching can also be achieved by precise timing of non-resonant magnetic-field pulses 13,19 , in a similar fashion as domain-wall propagation in magnetic nanowires 20 . Resonant amplification 21 of the vortex gyrotropic motion enables us to reverse the core polarity with minimum excitation power 12,14,15 , as it enables us to concentrate the energy in a narrow frequency band.…”

Electrical determination of vortex state in submicron magnetic elements

“…This threshold will differ from the type of in-plane excitation e.g. pulse [67,68], burst [63], or continuous excitation that is applied to the structure, and at least is still a factor of about 100 lower than the switching threshold with a static perpendicular field.…”

“…Scanning Transmission X-ray Microsopy (STXM) [63] techniques that utilize Xray Magnetic Circular Dichroism (XMCD) [95] to obtain the magnetic contrast are emerging as one of the premier probes. The vortex core dynamics induced by a pulsed magnetic field has also recently been detected in STXM [68,91]. We have utilized this technique in our experimental work, the detailed description of which will be presented in following chapters.…”