Abstract
We have observed Raman scattering of 5145-Å argon-laser radiation by two-magnon excitations in the antiferromagnetic nickel fluorides KNi and Ni and ferrimagnetic RbNi. For both the cubic perovskite structure of KNi and the tetragonal structure of Ni, which behaves like a two-dimensional antiferromagnet, the experimental results are in excellent agreement with the theoretical spectra computed by a Green's-function technique. This method is reviewed for the general case of a two-sublattice Néel ferrimagnet with antiparallel spins. From a comparison with this theory, we find values of , respectively, for the Heisenberg exchange constants in these two materials. The experimentally observed temperature dependence of the two-magnon Raman spectra, which persists above the Néel temperature in both cases, is described. For hexagonal RbNi, with the aid of magnon-assisted optical-absorption and magnetic-susceptibility data, we have calculated the magnon dispersion relations and estimated the dominant exchange constants. These values are 48 for the antiferromagnetic 180° - - superexchange interaction and 113 for the 90° ferromagnetic interaction, with a small second-nearest interaction constant approximately 4 . The identification of the low-temperature Raman line at 510 as arising from two different Brillouin-zone-edge magnons is consistent with our exchange constants when magnon-magnon binding effects are taken into account. A Bethe-Peierls-Weiss analysis using the present exchange parameters gives a Curie temperature close to the experimental value, 139 °K, and predicts high-temperature sublattice magnetizations in good agreement with the results of nuclear-magnetic-resonance measurements. Finally, comparisons are made among these nickel fluorides, and with results found from experiments in other magnetic insulators.
- Received 15 June 1970
DOI:https://doi.org/10.1103/PhysRevB.3.1709
©1971 American Physical Society