Abstract
The variation with temperature of the sublattice magnetization of the quadratic double-layer antiferromagnet has been determined by measuring the NMR frequency of the nuclei adjacent to the Mn sites in the double layer. The data been analyzed in terms of a two-dimensional four sublattice spin-wave theory. Temperature-dependent and temperature-independent renormalization as formalized by Oguchi, as well as temperature variation of hte energy gap have been included, while the integrations over the Brillouin zone have been carried out exactly. THe dispersion is 2 × 2 fold degenerate, where teh lower branches represent in-phase and the upper branches, with energy larger than out-of-phase precession of the spins in the paired layers, respectively. In a least-squares adjustment, spin-wave theory then appears to account for the sublattice magnetization up to 28 K, to be compared with K. The least-squares adjustment yielded for the exchange constant K and for the zero-temperature energy gap K (including effects of residual -axis dispersion), which corresponds to an anisotropy field kG. In contrast to the signle-layer structure Mn, Oguchi renormalization resulted in a marked improvement, by 9 K, of the range of concurrence over the unrenormalized theory. The functional dependence of the sublattice magnetization of on temperatuer is found to be close to that in the single-layer Mn, at least at such low temperatures, that the upper magnon branches are not yet excited. Finally, evidence is found for zero-point spin reduction in good accord with the spin-wave value .
- Received 15 November 1976
DOI:https://doi.org/10.1103/PhysRevB.15.4348
©1977 American Physical Society