Abstract
In this and the following two papers, low-temperature spin-wave properties of quadratic-layer antiferromagnets having the Ni structure are reported and analyzed in detail. Here we present the results of a least-squares adjustment of spin-wave theory to the temperature variation of the sublattice magnetization in the compounds Ni, Mn, and Mn, as reflected by NMR frequency measurements in zero field. Lowest-order temperature-dependent and temperature-independent corrections to simple spin-wave theory, as formulated by Oguchi, are included in the analysis. The free parameters of the fits are taken to be the exchange coupling, the zero-temperature spin-wave gap energy, and the zero-temperature NMR frequency. Our conclusions are as follows. Spin-wave theory accounts for the sublattice magnetization of these compounds up to somewhat less than one-half the Néel temperature, with the temperature-dependent corrections yielding less than 20% improvement in the range of fit for the compounds and a negligible improvement for Ni. The breakdown of spin-wave theory is clearly not ascribable to spin-wave interaction effects and is apparently caused by excitations of a fundamentally different nature. Exchange values obtained are in excellent agreement with data from neutron and susceptibility measurements. The "effective" spin-wave-energy-gap values obtained give some evidence for interplanar exchange coupling between second-neighbor planes, yielding upper limits for such coupling of a few parts in 1 of the primary exchange. Earlier conclusions regarding the large zero-point spin reduction in Ni are refined here, giving a result slightly larger than but within error limits of the spin-wave-theory value (17.7%).
- Received 7 November 1972
DOI:https://doi.org/10.1103/PhysRevB.8.285
©1973 American Physical Society