Dynamics of ferromagnetic nanomagnets with vortex or single-domain configuration

Abstract

We study the dynamics of flat circular permalloy nanomagnets for 1.) magnetic vortex and 2.) single-domain configurations using micromagnetic simulation. Dynamical studies for isolated vortex structures show that both the vorticity and the polarity of the out-of-plane component can be switched fast (50-100 ps). Micromagnetic simulations of the switching process in thin cylindrical Permalloy(Py) nanoparticles with an initial stable singledomain state show nearly homogeneous single-domain behavior followed by excitation of spin waves. The magnetism of small ferromagnetic structures has become increasingly important e.g. for nonvolatile random access memories (MRAMs) [1]. Nanostructured Permalloy is a possible candidate for such devices. As it is an essential feature in the technology of magnetic recording to read and write magnetic states as fast as possible we study the dynamics of flat circular permalloy nanomagnets with magnetic vortex or singledomain configuration using micromagnetic simulations.There are four equivalent vortex states since the vorticity (clockwise/counterclockwise) and the central polarization (up or down) are independent. The possibility of storing and switching two bits of information instead of only one makes these vortex structures quite interesting.It is known from [3] that the vorticity of a magnetic dot can be switched in ≈ 40 ps with strong enough and short enough perpendicular out-ofplane field pulses.In the present work, we consider 1.) the question, whether also the polarization can be switched reproducibly and as fast as the vorticity.Therefore the results of a micromagnetic simulation performed with OOMMF, [4], on a cylindrical Py dot with diameter 300 nm and 10 nm thickness are shown. We have used Py param- * Tel.: +49-941/9432605; Fax: +49-941/9434544; E-mail address: andreas.killinger@physik.uni-regensburg.de eters (M s = 800000 A m for the saturation magnetisation, A = 1.3 * 10 −11 J m for the exchange stiffness and α = 0.008 for the Gilbert damping). The initial magnetisation configuration is a (relaxed) vortex state. We apply a typical out-ofplane field pulse (100 ps duration) of B z = 0.2 T, plus B x = 0.002 T to induce torque at x = y ≈ 0.

Edge-Soliton-Mediated Vortex-Core Reversal Dynamics

“…For example, domain wall motion can be described in terms of the dynamics of various types of magnetic topological solitons, i.e., transverse, vortex, and antivortex walls, via the sequential processes of the creation, propagation, and annihilation of them [3,4] in the Walker breakdown regime in magnetic nanostrips or simple motions of single solitons without any transformation of their internal structures [5]. Also, a single magnetic vortex in nanodots can be transformed into a new vortex with its core orientation opposite to the original core orientation via the serial processes of the creation and subsequent annihilation of vortexantivortex (VAV) pairs, with the help of vortex-core (VC) gyration motions [11][12][13][14][15][16][17].Furthermore, it has been reported that VC reversals can take place, possibly avoiding VC motions, under a specific condition of an intended immobile VC [18], by application of strong pulse fields perpendicular to the sample plane [19], or through a thermal excitation [20]. Regardless of the particular conditions and types of driving forces, the common reversal mechanism found thus far is the dynamic process of VAV-pair creation followed immediately by the annihilation of the newly created antivortex and the original vortex inside the nanoelements.…”

“…Furthermore, it has been reported that VC reversals can take place, possibly avoiding VC motions, under a specific condition of an intended immobile VC [18], by application of strong pulse fields perpendicular to the sample plane [19], or through a thermal excitation [20]. Regardless of the particular conditions and types of driving forces, the common reversal mechanism found thus far is the dynamic process of VAV-pair creation followed immediately by the annihilation of the newly created antivortex and the original vortex inside the nanoelements.…”