Controllable vortex chirality switching on spherical shells

Abstract

A simple mechanism of controllable switching of magnetic vortex chirality is proposed. We consider curvilinear magnetic nanoshells of spherical geometry whose ground state is a vortex magnetization distribution. Chirality of this magnetic vortex can be switched in controllable way by applying a Gaussian pulse of spatially uniform magnetic field along the symmetry axis of the shell. The chirality switching process is explored in detail numerically for various parameters of magnetic pulse: the corresponding switching diagram is build. The role of the curvature is ascertained by studying the switching diagram evolution under the continuous transition from hemispherical shell to the disk shaped sample with the volume and thickness kept constant. Magnetic vortex has two binary characteristics: chirality C = ±1, the counterclockwise (C = +1) or clockwise (C = −1) direction of magnetization circulation; and polarity p = ±1, the up (p = +1) or down (p = −1) direction of the vortex core magnetization. Each of these quantities can be potentially used for storing of a bit of information in a high-speed magnetic random access memory. [5] In this respect the possibility of controllable manipulating of the chirality and polarity values is crucial one. Though a number of mechanisms of controllable vortex polarity switching are already proposed, [6][7][8][9] the controllable vortex chirality switching is still a challenging problem because it requires a fine asymmetrical tuning the nanoscale system: the asymmetry must be introduced into geometry of the nanomagnet [10] or into the spatial distribution of the applied magnetic fields. [11] In symmetrical planar systems e.g. magnetic nanodisks, the vortex chirality control requires highly accurate setting of parameters of the of magnetic field pulse[12] or spin-current pulse.[13]Here we propose a simple mechanism of vortex chirality control for a symmetrical system by using a simple spatially uniform pulse of magnetic field. The idea is to proceed from planar to curvilinear magnetic shells with spherical geometry, see Fig. 1. We choose the magnetic pulse with the Gaussian temporal profile B = −e z B 0 exp[−(t − 3τ ) 2 /τ 2 ], where B 0 and τ are ampli- tude and width of the field pulse respectively. The pulse is applied along the symmetry axis of the system, B||e z . Note, that recently we reported on the vortex polarity switching for spherical shells under action of the same Gaussian field pulse applied within the xy-plane [14].To study the magnetization dynamics induced by the field pulse we perform numerical simulation of the Landau-Lifshitz equation using the NMAG code. [15] We start from the hemispherical shell, see Fig. 1 with inner radius R 0 = 100 nm, radial thickness h = 10 nm and material parameters of Permalloy (Ni 80 Fe 20 ), namely the saturation magnetization M s = 7.96×10 5 A/m, exchange constant A = 1.3 × 10 −11 J/m, anisotropy is neglected, and damping coefficient α = 0.01. The chosen material parameters result in characteristic length scale of the system, an excha...

Multi-segmented nanowires for vortex magnetic domain wall racetrack memory

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