Giant X-ray circular dichroism in a time-reversal invariant antiferromagnet

X-ray circular dichroism, which arises from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool for measuring the magnetization of ferromagnetic materials and identifying 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 on the observation of giant natural circular dichroism in the Ni L-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 the giant X-ray natural circular dichroism, reflecting the magnetic susceptibility of Ni₃TeO₆. Our findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. hl{Additionally, we establish the first example of a new class of magnetic materials exhibiting circular dichroism with time-reversal symmetry.