Magneto-Optical Imaging of Vortex Domain Deformation in Pinning Sites

TL;DR

This study employs magneto-optical microscopy to visualize and quantify how vortex cores in ferromagnetic disks deform and move under varying magnetic fields, revealing detailed pinning and unpinning behaviors.

Contribution

It introduces a sensitive differential magneto-optical imaging technique to observe vortex deformation and pinning dynamics with sub-micron resolution.

Findings

Pinned vortex cores tilt in the direction of applied field.

Magnetization away from the core reorients to align with the field.

Unpinning causes the vortex core to translate across the disk.

Abstract

We use a sensitive magneto-optical microscopy technique to image the magnetization response of micron-scale ferromagnetic disks to changes in applied magnetic field. This differential technique relies on a modulated applied magnetic field which allows us to measure changes in magnetization <1% with sub-micron resolution. The disks are magnetized in single vortex domains, with defects in the material serving to pin the vortex core at particular positions. By applying a small AC magnetic field, we measure the deformation of the magnetization while the core remains pinned. We can also characterize the strength of the pinning site by increasing the AC magnetic field to unpin the vortex core. While pinned, we find that the magnetization away from the core reorients slightly to better align with an applied field. Additionally, an applied field causes the pinned core itself to tilt in the direction of the field. Once the field is large enough to unpin the core, this tilt disappears, and the core instead translates across the disk.