Revealing the origin of XMCD in an altermagnet via three-dimensional control of spins

TL;DR

This study demonstrates that XMCD signals in altermagnets are governed by spin-direction symmetry breaking and can be used to map nanoscale spin textures, advancing understanding of altermagnetic phenomena.

Contribution

It reveals that XMCD in altermagnets is driven by spin-direction-induced symmetry breaking, decoupled from magnetic canting, and introduces a method to map nanoscale spin textures.

Findings

XMCD is governed by spin-direction symmetry breaking in altermagnets.

XMCD signals are highly anisotropic and decoupled from magnetic canting.

Complete vectorial maps of spin textures in $ ext{Fe}_2 ext{O}_3$ thin films were reconstructed.

Abstract

Altermagnets are an emerging class of collinear antiferromagnets that exhibit unconventional spin-polarised electronic bands, potentially unlocking new functionalities that do not rely on spin-orbit coupling (SOC). Experimental signatures traditionally associated with spin polarisation, like X-ray magnetic circular dichroism (XMCD), are thus being used as a validation of altermagnetism. However, unlike altermagnetic spin-splitting, these responses require SOC and are not invariant under spin-space rotations. This brings into question the extent to which they can be considered direct signatures of altermagnetism. Here, we exploit the g-wave altermagnet $\alpha$-Fe$_{2}$O$_{3}$ to demonstrate that XMCD is governed precisely by the spin-direction-induced symmetry breaking that altermagnetic spin groups are designed to ignore. Strikingly, the XMCD is highly anisotropic and is decoupled from the weak magnetic canting. We show that this anomalous XMCD can be described by on-site Faraday tensors capturing the locally uncompensated spin-orbital anisotropies - a scenario that can be applied to other altermagnets. Leveraging this, we reconstruct complete vectorial maps of nanoscale textures in $\alpha$-Fe$_{2}$O$_{3}$ thin films, including domain walls and topological solitons, which are promising for building future spintronics and magnonics devices.