NUCLEAR SPIN-LATTICE RELAXATION IN ANTIFERROMAGNETIC FEF2

The nuclear spin-lattice relaxation (NSLR) of F19 nuclei in antiferromagnetic FeF2 has been studied experimentally and theoretically, and is shown to be due to a two-magnon Raman scattering process. The assumption of isotropic magnon dispersion allows simplification of the calculations while still including zone-boundary effects. This model adequately describes the total density of states for generally cubic crystals whose exchange interaction may be expressed by a single parameter. However, processes which depend on magnon scattering, such as NSLR, can be calculated with reasonable accuracy only for relatively dispersion-less spin-wave spectra. This is due to replacing the true cubic Brillouin zone, in which different directions in k space do not contribute equally to the density of states, with the model's spherical Brillouin zone in which all directions contribute equally. Comparison of the FeF2 data with those for the isostructural antiferromagnet MnF2 indicates that the temperature above which NSLR is insensitive to changes in anisotropy is much lower than the corresponding temperature for magnetization data. © 1969 The American Physical Society.