Infrared Absorption and Raman Scattering by Two-Magnon Processes in NiO

Temperature-dependent Raman scattering by two magnons has been observed in both green and black (from excess oxygen) NiO single crystals, peaking at 1555 cm−1 at 1.4°K. The shape of the band is reasonably well described by a dominant intersublattice exchange of J2=148 cm−1, in excellent agreement with the value J2=153 cm−1, obtained from neutron scattering.1 The high-frequency cutoff of the Raman scattering lies near 1900 cm−1 in agreement with the J1=0 two-magnon joint density-of-states cutoff at 176 cm−1. The absorption line2 peaking at 1940 cm−1 has been reexamined in light of this data, and is found to exhibit a cutoff on the high-frequency side at 2460 cm−1, and a threshold near the peak of the Raman scattering. These data support a model for the absorption band in which two magnons are created simultaneously with a photon, with the cutoff corresponding to the creation of two zone boundary magnons and an L0 phonon of 580 cm−1 at k=0. Such a phonon sideband of a two-magnon transition probably derives its intensity from the phonon modulation of exchange as suggested by Koide et al.3