On the N\'eel Vector Dependence of X-ray Magnetic Circular Dichroism in Altermagnets

TL;DR

This paper derives a general theoretical relationship between the Ne9el vector orientation and x-ray magnetic circular dichroism (XMCD) signals in altermagnets, expanding understanding of symmetry and geometry effects in magnetic materials.

Contribution

The authors develop a group-theoretic framework linking the Ne9el vector to XMCD signals in altermagnets within the free valence spin approximation, generalizing previous specific cases.

Findings

XMCD dependence on Ne9el vector orientation is system-specific.

The derived relationship applies to arbitrary spin point groups.

XMCD signals can be present, emergent, or forbidden depending on symmetry.

Abstract

Dependence of x-ray magnetic circular dichroism on the experimental geometry is described by a frequency-dependent Hall vector. Using group theory, we derive a general relationship between the Hall vector and the orientation of the N\'eel vector $\bL$ in altermagnets within the free valence spin (FVS) approximation, where the spin-orbit coupling of the valence electrons and their exchange interaction with the core electrons are neglected. For a given spin point group, the full $\bL$-dependence of the Hall vector can be expressed in terms of several irreducible spectral functions. This derivation generalizes earlier results for the special cases of MnTe and MnF$_2$. Depending on the system symmetry, XMCD in the FVS approximation may be present, emerge only when the neglected terms are included, or be completely forbidden.