  MnBi2Te4 is a van der Waals topological insulator with intrinsic intralayer
ferromagnetic exchange and A-type antiferromagnetic interlayer coupling.
Theoretically, it belongs to a class of structurally centrosymmetric crystals
whose layered antiferromagnetic order breaks inversion symmetry for even layer
numbers, making optical second harmonic generation (SHG) an ideal probe of the
coupling between the crystal and magnetic structures. Here, we perform magnetic
field and temperature-dependent SHG measurements on MnBi2Te4 flakes ranging
from bulk to monolayer thickness. We find that the dominant SHG signal from
MnBi2Te4 is unexpectedly unrelated to both magnetic state and layer number. We
suggest that surface SHG is the likely source of the observed strong SHG, whose
symmetry matches that of the MnBi2Te4-vacuum interface. Our results highlight
the importance of considering the surface contribution to inversion
symmetry-breaking in van der Waals centrosymmetric magnets.

Materials Science;Mesoscale and Nanoscale Physics