Magnetic Dichroism in Rutile NiF$_2$: Separating Altermagnetic and Ferromagnetic Effects

TL;DR

This paper uses numerical simulations to analyze x-ray magnetic circular dichroism in NiF$_2$, demonstrating how to distinguish altermagnetic and ferromagnetic effects and control magnetic orientations via applied magnetic fields.

Contribution

It introduces a method to separate altermagnetic and ferromagnetic contributions in XMCD signals and shows how to control magnetic orientations systematically.

Findings

XMCD signal is significant for light perpendicular to magnetic moments.

Altermagnetic and ferromagnetic contributions can be distinguished by field dependence.

XMCD can be modeled as a linear combination of spectral components.

Abstract

We present numerical simulations of x-ray magnetic circular dichroism (XMCD) at the L$_{2,3}$ edge of Ni in the weakly ferromagnetic altermagnet NiF$_2$. Our results predict a significant XMCD signal for light propagating perpendicular to the magnetic moments, which are approximately aligned along the [100] easy-axis direction. The analysis shows that the altermagnetic and ferromagnetic contributions to the XMCD signal can be uniquely distinguished by their dependence on an applied magnetic field. By varying the angle of the field relative to the easy axis, the in-plane orientation of both the N\'eel vector and the net magnetization can be systematically controlled. We further demonstrate that the XMCD signal, even under fields as strong as 40 T and for any in-plane orientation, can be accurately described as a linear combination of two spectral components, with geometrical prefactors determined by the field magnitude and direction. This insight enables experimental validation of the distinctive relationship between the N\'eel vector orientation and the x-ray Hall vector in the rutile structure. Quantitative simulations supporting these findings are provided.