Macroscopic theory of reflection from antiferromagnetic Cr2O3

Reflection matrices with the correct space-time symmetry properties may be obtained from the standard Maxwell boundary conditions only when the constitutive relations for the D and H fields satisfy covariance requirements. In this paper we present the first application of covariant multipole forms for D and H, to the order of electric quadrupole and magnetic dipole, to a magnetic crystal in order to compare the predictions obtained from this theory with recent experimental results. In particular, the theory is used to determine the Fresnel reflection amplitudes for antiferromagnetic Cr2O3 when a monochromatic plane light wave is incident normally from a vacuum on a crystal face perpendicular first to the C-3 axis and then to the C-2 axis. The theory agrees well with experiment. In addition, it reveals a novel relationship between certain components of the frequency-dependent magnetoelectric tensor, which, with the aid of transmission and reflection data, would enable the surface and bulk contributions in reflection to be distinguished.