Second harmonic generation of magnetic-dipole type in the centrosymmetric antiferromagnets NiO and KNiF3

Abstract Optical second harmonic generation (SHG) spectra are studied in two model antiferromagnets, NiO and KNiF3, in which the leading-order electric-dipole type SH process is forbidden by the centrosymmetry of the compounds. The observed SHG is attributed to an intrinsic two-photon excitation involving a resonance enhanced magnetic-dipole transition in the spectral region of the lowest d–d transition ( 3 Γ 2 + → 3 Γ 5 + ). SHG in a magnetic field up to 7 T reveals the excitonic eigenstates of the first d band in NiO.