DD-04 - Spin-Polarised Current Bloch Point Manipulation

Abstract

Recently, it was demonstrated that confined helimagnetic nanostructures, composed of grains of different chirality (and consequently, a different sign of Dzyaloshinskii-Moriya energy constant), can host stable Bloch points [1]. In addition, it was shown that Bloch points undergo hysteretic behaviour and that their type can be changed using an external magnetic field. In this work, using Ubermag [2] micromagnetic simulations with OOMMF [3] as a computational backend, we explore if Bloch point magnetisation states can be manipulated using spin-polarized currents in nano-disk and nano-strip FeGe samples.We start by showing that a zero-field stable Bloch point emerges not only in thin-film disk samples [1] but also in an extensive range of planar geometries, including nanostrips. We then explore whether Bloch point's type can be switched using spin-polarized current pulses. We simulate a FeGe [4] nanodisk with 150 nm diameter, 20 nm thickness of the bottom layer with D<0 and 10 nm thickness of the top layer with D>0. We begin by relaxing a uniform +z magnetisation state, which results in a head-to-head Bloch point. To a relaxed head-to-head Bloch point, we apply a spin-polarised current pulse with current density J and duration T, polarised in the negative z-direction. We vary current density J and pulse duration T and observe whether the type of Bloch point changes. In Fig. 1 we show a diagram depicting J-T regions for which Bloch point type switching occurs from head-to-head to tail-to-tail.In the second part of our work, we simulate nanostrips with 450 nm length and 90 nm width. Similar to nanodisks previously simulated, the thickness of the bottom layer with negative Dzyaloshinskii-Moriya constant is 20 nm and the thickness of the top layer with positive DM constant is 10 nm. We apply current polarised in -y-direction and track both x and y positions of the Bloch point. In Fig. 2, we show how Bloch point moves in the x-direction for different current densities J.Our findings demonstrate that Bloch points can be manipulated using spin-polarized currents. Using current pulses polarised in -z-direction, we were able to switch the Bloch point's type, whereas, with currents polarised in the negative y-direction, we managed to move Bloch point in the x-direction. Our results, in addition to being of interest to fundamental physics, bring Bloch points one step closer to being potential candidates for future data storage and information processing devices. **