Circular Dichroism in Resonant Inelastic X-Ray Scattering: Probing Altermagnetic Domains in MnTe.

X-ray magnetic circular dichroism provides a means to identify ferromagnetic, chiral, and altermagnetic orders via their time-reversal-symmetry (T) breaking. However, the symmetry properties that govern circular dichroism (CD) in resonant inelastic x-ray scattering (RIXS) remain poorly understood. We show that, due to the inherent irreversibility of the RIXS process, RIXS-CD does not require T breaking to be present, but reflects the change in unitary symmetries associated with magnetic ordering. Using the altermagnetic MnTe as a model system, we observe an azimuthal-angle dependent RIXS-CD signal in the magnon excitations. Our findings highlight the sensitivity of RIXS-CD to the relativistic symmetry in magnetic systems and its potential application as a probe of magnetic domains.