Three-dimensional Magnetic Vortex Structure Transformation

Abstract

A flux-closure domain structure such as a magnetic vortex structure is formed from the competition between demagnetization and exchange energies [1]. Such magnetic configuration has been studied in a two-dimensional (2D) structure. However, as increasing the thickness of the sample, a magnetic vortex structure becomes a complicated three dimensional (3D) structure [2, 3] to reduce demagnetization energy along the thickness direction. For example, with increasing aspect ratio, asymmetric Bloch wall (ABW) with Néel cap appears between two shifted vortex cores as shown in Fig. 1. In this system, two types of ABWs can be generated. Depending on the positions of the top and bottom vortex core, the ABW CW and CCW are determined: they are exactly same energy states (degenerate states). In this study, we found that 3D magnetic vortex structure can be formed even in a square disk sample under an external in-plane magnetic field from magnetic transmission soft X-ray microscopy (MTXM) measurements and micromagnetic simulations. A symmetric vortex core structure at the center of disk pattern is shifted along +y direction by applying magnetic field and it abruptly transformed to the ABW structure; top and bottom vortex core structures are separated opposite direction, +x and -x for top and bottom vortex core, respectively. And this separation direction determines the rotation sense of ABW, CCW. In this presentation, we will discuss about the detailed mechanism of 3D vortex structure transition and its related physical significance.