# Defects in magnetic domain walls after single-shot all-optical switching

## TL;DR

This paper investigates defects in magnetic domain walls formed after ultrafast all-optical switching and finds that defect density is influenced by material properties and domain wall motion speed.

## Contribution

The study reveals that domain wall defects during all-optical switching are comparable to those from field cycling and are caused by high-speed propagation over pinning sites.

## Key findings

- A high density of vertical Bloch line defects is observed in domain walls after all-optical switching.
- Defect density is independent of optical pulse parameters but varies significantly between materials.
- Micromagnetic simulations suggest that domain walls moving above the Walker breakdown accumulate defects over pinning sites.

## Abstract

Helicity-independent all-optical switching (HI-AOS) is the fastest known way to switch the magnetic order parameter. While the switching process of extended areas is well understood, the formation of domain walls enclosing switched areas remains less explored. Here, we study domain walls around all-optically nucleated magnetic domains using x-ray vector spin imaging and observe a high density of vertical Bloch line defects. Surprisingly, the defect density appears to be independent of optical pulse parameters, significantly varies between materials, and is only slightly higher than in domain walls generated by field cycling. A possible explanation is given by time-resolved Kerr microscopy, which reveals that magnetic domains considerably expand after the initial AOS process. During this expansion, and likewise during field cycling, domain walls propagate at speeds above the Walker breakdown. Micromagnetic simulations suggest that at such speeds, domain walls accumulate defects when moving over magnetic pinning sites, explaining similar defect densities after two very different switching processes. The slightly larger defect density after AOS compared to field-induced switching indicates that some defects are created already when the domain wall comes into existence. Our work shows that engineered low-pinning materials are a key ingredient to uncover the intrinsic dynamics of domain wall formation during ultrafast all-optical switching.