Giant X‐Ray Circular Dichroism in a Time‐Reversal Invariant Antiferromagnet

Abstract

X‐ray circular dichroism, arising from the contrast in X‐ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X‐ray magnetic circular dichroism because of time‐reversal symmetry, yet exhibit weak X‐ray natural circular dichroism. Here, we report the observation of giant natural circular dichroism in the Ni L3‐edge X‐ray absorption of Ni3TeO6, a polar and chiral antiferromagnet with effective time‐reversal symmetry. To unravel this intriguing phenomenon, we propose a phenomenological model that classifies the movement of photons in a chiral crystal within the same symmetry class as that of a magnetic field. The coupling of X‐ray polarization with the induced magnetization yields giant X‐ray natural circular dichroism, revealing typical ferromagnetic behaviors allowed by the symmetry in an antiferromagnet, i.e., the altermagnetism of Ni3TeO6. Our findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, we establish the first example of a new class of magnetic materials exhibiting circular dichroism with time‐reversal symmetry.This article is protected by copyright. All rights reserved