Magneto-optic Kerr effect (MOKE) is a powerful probe of broken time-reversal
symmetry ($\mathcal{T}$), typically used to study ferromagnets. While MOKE has
been observed in some antiferromagnets (AFMs) with vanishing magnetization, it
is often associated with structures whose symmetry is lower than basic
collinear, bipartite order. In contrast, theory predicts a mechanism for MOKE
intrinsic to all AFMs of A-type, i.e. layered AFMs in which ferromagnetic
layers are antiferromagnetically aligned. Here we report the first experimental
confirmation of this mechanism. We achieve this by measuring the imaginary
component of MOKE as a function of photon energy in MnBi$_2$Te$_4$, an A-type
AFM where $\mathcal{T}$ is preserved in combination with a translation. By
comparing the experimental results with model calculations, we demonstrate that
observable MOKE should be expected in all collinear A-type AFMs with
out-of-plane spin order, thus enabling optical detection of AFM domains and
expanding the scope of MOKE to few-layer AFMs.

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