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, the observation of giant natural circular dichroism in the Ni L₃‐edge X‐ray absorption of Ni₃TeO₆ is reported, a polar and chiral antiferromagnet with effective time‐reversal symmetry. To unravel this intriguing phenomenon, a phenomenological model is proposed 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. The findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, the first example of a new class of magnetic materials exhibiting circular dichroism is established with time‐reversal symmetry.