Study of Confinement Effects on Skyrmions in Amorphous Multilayer Nanodots

Abstract

Magnetic skyrmions are nanoscale spin structures which can be stabilized at room temperature in multilayer films [1, 2]. In particular, the amorphous multilayer film presents a pinning-free material platform, enabling efficient spin-orbit-torque manipulation of skyrmion dynamics. The device-relevant configuration of dots is of immense interest for realizing skyrmionic tunnel junctions for next-generation memory and computing applications [3]. Here, we first establish an amorphous multilayer composite stack comprising [Pt/CoB(x)/Ir]3/[Pt/CoB(y)/MgO], where x: 1.7-2.2 nm and y: 1-1.4 nm, with demonstrable skyrmions of sizes ~100 nm stabilized at zero field (ZF). By varying both the bottom (x) and top (y) CoB thicknesses, the magnetic anisotropy, Keff, can be tuned by an order from -0.03 to 0.126 MJ/m3. Next, we present ZF stabilization of skyrmions confined in such multilayer nanodots over a range of Keff and geometric sizes (w: 300 – 1000nm). Kerr magnetometry characterization of these nanodot arrays shows a monotonic increase in squareness with decreasing w. Expectedly, at ZF, we observe a gradual evolution in magnetic textures from the labyrinthine stripe to skyrmion phase, and eventually to the uniform magnetisation phase with w reduction (Figure 1). Notably, the stabilization of ZF skyrmions is observed over a wide w range from 400 to 700 nm across samples and skyrmion multiplets are stabilized in dots with low Keff. Finally, we demonstrate that at intermediate OP applied fields, skyrmion multiplets can be stabilized across varying w. Micromagnetic simulations are performed to model the geometric confinement effect of the nanodots on the skyrmion nucleation probability and size. These results provide a platform for harnessing the properties of nanoscale skyrmions in confined geometries for device applications.