Pinning and gyration dynamics of magnetic vortices revealed by correlative Lorentz and bright-field imaging

TL;DR

This study uses advanced Lorentz microscopy techniques to map and analyze how grain boundaries influence the pinning and dynamic behavior of magnetic vortex cores in polycrystalline permalloy films, revealing key effects on vortex gyration.

Contribution

It introduces a quantitative method to map pinning potentials and demonstrates the impact of grain size on vortex dynamics using time-resolved imaging.

Findings

Pinning sites are primarily governed by grain boundaries.

Larger grains increase vortex dissipation.

Grain size affects vortex bistability.

Abstract

Topological magnetic textures are of great interest in various scientific and technological fields. To allow for precise control of nanoscale magnetism, it is of great importance to understand the role of intrinsic defects in the host material. Here, we use conventional and time-resolved Lorentz microscopy to study the effect of grain size in polycrystalline permalloy films on the pinning and gyration orbits of vortex cores inside magnetic nanoislands. To assess static pinning, we use in-plane magnetic fields to shift the core across the island while recording its position. This enables us to produce highly accurate two-dimensional maps of pinning sites. Based on this technique, we can generate a quantitative map of the pinning potential for the core, which we identify as being governed by grain boundaries. Furthermore, we investigate the effects of pinning on the dynamic behavior of the vortex core using stroboscopic Lorentz microscopy, harnessing a new photoemission source that accelerates image acquisition by about two orders of magnitude. We find characteristic changes to the vortex gyration in the form of increased dissipation and enhanced bistability in samples with larger grains.