Giant topological magneto-optical effect in noncoplanar antiferromagnet

Geometrical frustration on triangular lattice is expected to exhibit diverse quantum spin and electronic states endowed with emergent electromagnetic phenomena. The all-in-all-out (AIAO)-type antiferromagnetic spin structure is one such example, possessing the scalar spin chirality that generates giant emergent magnetic field with vanishingly small magnetization. Here, we report on the large spontaneous magneto-optical Kerr effect (MOKE) caused by the AIAO/AOAI state in quasi-two-dimensional triangular-lattice compound CoNb₃S₆. Over the entire measured energy region from 55 to 2000 meV, the MOKE is found to be dominated only by the spin chirality. Essential role of momentum-space Berry curvature for both MOKE and dc Hall effect is demonstrated by the spectral analysis of optical Hall conductivity derived from MOKE. The figure of merit of observed topological MOKE, light-polarization rotation angle divided by magnetization, largely exceeds other magnets including time-reversal-symmetry broken antiferromagnet Mn₃Sn. Our findings demonstrate the strong light-spin coupling through the spin chirality, paving the way for antiferromagnetic spintronics and future optospintronic devices.