Directly observed dynamics of distorted vortex cores including asymmetric Bloch walls utilizing soft X-ray microscopy

Spin structures including domain walls and magnetic vortices have attracted enormous interests not only due to their fascinating topological textures but also their potentials in a wealth of technological applications such as high efficient storage and memory devices. In the research of those spin structures, synchrotron-based microscopes have been playing key roles by direct imaging of static and dynamic behaviors of spin structures and therefore providing a powerful insight into the underlying physics of nanospin phenomena and an essential knowledge for their applications in advanced nanotechnologies [1, 2]. In our work, we employed a full-field soft X-ray microscope (XM-1) at Advanced Light Source (ALS) to directly observe non-trivially distorted vortex cores consisting of asymmetric Bloch walls and their dynamics. Fig. 1shows the deformed vortex core observed in an asymmetric permalloy (Py, Ni
80
Fe
20
) disk with a height of h = 100 nm, a diameter of D = 500 nm, and an asymmetric ratio of r = 0.3D (a) together with simulated vortex core and the out-of-plane (OOP) magnetic component (m
z
) larger than 0.7 (b). The distorted vortex core was found to be vortex cores placing non-coaxially on top and bottom surface of the disk, which are connected by an asymmetric Bloch wall creating flux closer domain. Such core structure is significantly distinguished from common circular vortex cores characterized by a single vortex core (polarity, p) aligned on both surfaces of a magnetic element pointing either up or down and a circular in-plane domain (circularity, c) rotating either clockwise or counter-clockwise [3, 4]. Interestingly, the nontrivially shaped vortex core shows an abnormal dynamic behavior. Unlike the traditional gyrotropic motions of circular vortex cores, sloshing motion was observed in the distorted core although micromagnetic simulations demonstrated that vortex cores on top and bottom surfaces still have gyrotropic motions. The unique dynamic motion of the deformed vortex core is likely due to the asymmetric Bloch wall restricting the motions of vortex cores on surfaces [5]. This research was also supported by Leading Foreign Research Institute Recruitment Program through NRF (2012K1A4A3053565) and by the DGIST R&D program of the Ministry of Science, ICT and future Planning (17-BT-02. Work at the ALS was supported by the U.S. Department of Energy (DE-AC02-05CH11231).