Symmetry Breaking in 3D Magnetic Vortex Core Structure

In nanopatterned thin film of soft-ferromagnetic materials, a flux-closure domain structure such as a Landau domain 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 fluxclosure domain structure turn to a complex 3D structure [2, 3] to reduce demagnetization energy along the thickness of the sample. For example, in a rectangular disk, asymmetric Bloch wall (ABW) with Néel cap is positioned between two shifted vortex cores as shown in Fig. 1(a). In this system, a rotating sense of an ABW, clockwise (CW) and counter-clockwise (CCW) is an additional degree of freedom [2]: they are exactly same energy states (degenerate states). In this study, we show that 3D flux-closure magnetic domain structure can be formed even in a square disk sample under an external magnetic field by utilizing magnetic transmission soft X-ray microscopy (MTXM) and micromagnetic simulations. As shown in Fig. 1(b), symmetric vortex core structure at the center of Landau domain 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. Interestingly, we found symmetry breaking in this transformation. The separation direction, i.e., the rotation sense of ABW occurred always one direction for +x magnetic field as shown in Fig. 1(c). In contrast, the opposite sense of ABW was formed when the magnetic field applied –x direction. Which reveals that the energy states of ABW with opposite rotating sense are not the degenerate states anymore in this transformation from the symmetric vortex core to ABW. In this presentation, we will discuss about the detailed energy states on this ABW structure and its transformation mechanism