Temperature-Dependent Magnon-Energy Theory of FeF2and MnF2

The temperature dependence of the antiferromagnetic-resonance frequency, sublattice magnetization, and magnetic specific heat of Fe${\mathrm{F}}_2$ and Mn${\mathrm{F}}_2$ is calculated by use of the temperature-dependent magnon-energy theory, which was recently presented by the authors. The calculations were done in both the conventional random-phase approximation (RPA) and the magnon-renormalization approximation (MRA). The experimental data are mostly found between these two approximate calculations. Thus, it seems that the magnon interaction effects are overestimated in the MRA. The calculation of the magnon energy at the first Brillouin-zone edge in the case of Mn${\mathrm{F}}_2$ also supports this conclusion.