Mapping different skyrmion phases in double wedges of Ta/FeCoB/TaOx trilayers

Abstract

Skyrmions are chiral magnetic textures that have immense potential for applications in spintronic devices. However, their formation is quite challenging and necessitates a subtle balance of the magnetic interactions at play. Here, we study Ta/FeCoB/TaOx trilayer using crossed double wedges, i.e., thickness gradients of FeCoB and of top Ta, which is subsequently oxidized, leading to an oxidation gradient. This enabled us to observe micron-sized skyrmions in the vicinity of different transition regions of the sample: from perpendicular magnetic anisotropy to paramagnetic phase and also from perpendicular to in-plane magnetic anisotropy. These observations can be explained by the isolated skyrmion model taking into account the different energy contributions at play, namely, anisotropy, exchange, Dzyaloshinskii-Moriya, dipolar, and Zeeman. We also qualitatively compare the current-induced motion of skyrmions obtained in different transition regions. Our study not only provides an effective means to form skyrmions by tuning the interfacial magnetic properties but also highlights the differences pertaining to the skyrmions observed in different transition zones, which is extremely crucial for any envisaged application.