Magneto-optic response of the metallic antiferromagnet Fe2As to ultrafast temperature excursions

The linear magneto-optic Kerr effect (MOKE) is often used to probe magnetism of ferromagnetic materials, but MOKE cannot be applied to collinear antiferromagnets due to the cancellation of sublattice magnetization. Magneto-optic constants that are quadratic in magnetization, however, provide an approach for studying antiferromagnets on picosecond timescales. Here, we combine transient measurements of linear birefringence and optical reflectivity to study the optical response of Fe2As to small ultrafast temperature excursions. We performed temperature-dependent pump-probe measurements on crystallographically isotropic (001) and anisotropic (010) faces of Fe2As bulk crystals. We find that the largest optical signals arise from changes in the index of refraction along the z axis, perpendicular to the Néel vector. Both real and imaginary parts of the transient optical birefringence signal approximately follow the temperature dependence of the magnetic heat capacity, as expected if the changes in dielectric function are dominated by contributions of exchange interactions to the dielectric function.