Dynamics of the Bloch point in an asymmetric permalloy disk

Mi-Young Im,Min-Seung Jung,Young-Sang Yu,Weilun Chao,Peter Fischer,Sooseok Lee,Jung-Il Hong,Hee-Sung Han,Ki-Suk Lee,Seongsoo Yoon

doi:10.1038/s41467-019-08327-6

Abstract

A Bloch point (BP) is a topological defect in a ferromagnet at which the local magnetization vanishes. With the difficulty of generating a stable BP in magnetic nanostructures, the intrinsic nature of a BP and its dynamic behaviour has not been verified experimentally. We report a realization of steady-state BPs embedded in deformed magnetic vortex cores in asymmetrically shaped Ni80Fe20 nanodisks. Time-resolved nanoscale magnetic X-ray imaging combined with micromagnetic simulation shows detailed dynamic character of BPs, revealing rigid and limited lateral movements under magnetic field pulses as well as its crucial role in vortex-core dynamics. Direct visualizations of magnetic structures disclose the unique dynamical feature of a BP as an atomic scale discrete spin texture and allude its influence on the neighbouring spin structures such as magnetic vortices.

Highlights

A Bloch point (BP) is a topological defect in a ferromagnet at which the local magnetization vanishes
One example in magnetism is the point-like topological defect referred as the Bloch point (BP), which was originally proposed by Feldtkeller[6] and Döring[7]
Due to the limitations in achieving both temporal and spatial resolutions simultaneously to capture the dynamic motions of BPs, experimental attempts have been focused on observing the static state of a BP and its quasi-static motions[13,16,32]

Summary

Introduction

A Bloch point (BP) is a topological defect in a ferromagnet at which the local magnetization vanishes. We generate steady-state BPs in association with magnetic vortex cores of asymmetrically shaped 100 nm thick permalloy (Ni80Fe20, Py) disks and their dynamic behaviours are directly observed along with their quasi-static motions using the magnetic transmission soft X-ray microscopy (MTXM) at the Advanced Light Source with a spatiotemporal resolution of 25 nm and 70 ps[33].

Results

Conclusion