High-temperature series analysis of spin-one uniaxial ferromagnets:NiMF6·6H2O compounds

For the first time high-temperature series for the susceptibility and heat capacity have been employed to analyze data on ferromagnetic systems with single-ion anisotropy comparable with or greater than the pair interactions between ions. The salts $\mathrm{Ni}M{\mathrm{F}}_6$·6${\mathrm{H}}_2$O with $M=\mathrm{S}\mathrm{i},\mathrm{T}\mathrm{i},\mathrm{S}\mathrm{n},\mathrm{Z}\mathrm{r}$ are spin-one ferromagnetic systems. Each has an easy-axis single-ion anisotropy and long-range dipole-dipole interaction in addition to the more commonly discussed short-range exchange interactions. We show that the same set of interaction parameters can fit simultaneously and quantitatively both susceptibility and heat-capacity data. We also discuss the unusually small deviations of the data from the mean-field predictions for the three large single-ion anisotropy systems ($M=\mathrm{Z}\mathrm{r},\mathrm{T}\mathrm{i},\mathrm{S}\mathrm{n}$).