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 MnF2. 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.
@article{arxiv.2508.07234,
title = {On the N\'eel Vector Dependence of X-ray Magnetic Circular Dichroism in Altermagnets},
author = {Jan Kuneš},
journal= {arXiv preprint arXiv:2508.07234},
year = {2025}
}