Creation and Annihilation of Skyrmions for Neuromorphic Computing Applications

Abstract

Magnetic Skyrmions are topologically protected swirling structures induced by chiral interactions in non-centrosymmetric magnetic compounds or thin films with broken inversion symmetry [1]. The spintronic devices based on these textures promise increased density and energy efficient data storage due to small nanometric size and topological protection [2][3]. In this work we present the effect of DMI, surface anisotropy, magnetic field and spin orbit torque (SOT) on the creation and annihilation of the skyrmions. Fig.1 (a) shows that the skyrmion density is increased as we increase the magnitude of DMI coefficient from 1mJ/m2 to 3mJ/m2. But with increasing the surface anisotropy the density is reduced and skyrmions are annihilated at some threshold anisotropy Kth which is proportional to the magnitude of the DMI coefficient. Furthermore, the size of skyrmion decreases with increasing anisotropy as shown in Fig. 1(b). In Fig. 1 (c-d) we study the effect of external magnetic field on the skyrmions. Fig. 1 (c) shows the MOKE images of the fabricated sample depicting the transition of magnetic texture from Labriynth domains to skyrmions around 8 Oe. The skyrmion density increases proportionally with field till 20 Oe and begins to roll off after 20 Oe. These results are supported by MuMax simulations as shown in Fig. 1(d). These results suggest that by proper optimization of material parameters and stack geometry we can tweak the skyrmion density for its application in memory and logic. In Fig. 2(a) we show MTJ with skyrmions in the free layer, using SOT control we propose this device for its application in neuromorphic computing. The SOT manipulation of skyrmion size, density and motion is shown in Fig. 2 (b). Increasing current Ic via the heavy metal increases the skyrmion density from 21µm-2 (at 2×1011 A/m2) to 40 21µm-2 (at 8×1011 A/m2). Further increase in Ic results in decrease of the skyrmion density. Finally, we implement a simple neuromorphic circuit using the proposed device.  Fig. 1: (a) Skyrmion density vs anisotropy and DMI coefficient. (b) Density and size of skyrmion vs anisotropy. (c) MOKE images (d) MuMax simulations.  (a) Skyrmion-MTJ neuromorphic device. (b) Skyrmion density vs current density