Pair spectra and the magnetic properties ofMn2+in double nitrate crystals

Experimental electron-paramagnetic-resonance spectra for ${\mathrm{Mn}}^{2+}$ pairs in ${\mathrm{La}}_2$ ${\mathrm{Zn}}_3$ ${\left({\mathrm{NO}}_3\right)}_{12}$·${24\mathrm{H}}_2$O host crystals are interpreted to obtain the exchange interaction, $J{\overrightarrow{\mathrm{S}}}_1·{\overrightarrow{\mathrm{S}}}_2$, for pairs of ions in nearest-neighbor $X$ and $Y$ sites, $J(X,Y)=0.0193\mathrm{}\pm 0.001$ ${\mathrm{ç}\mathrm{m}}^{-1\mathrm{}}$, and for pairs of ions in nearest-neighbor $X$ sites, $J(X,X)=0.0309\mathrm{}\pm 0.0015$ ${\mathrm{cm}}^{-1\mathrm{}}$. The zero-field splitting parameter $D$ has also been determined for the members of these pairs. For ions in the $X$ site, these values are much different than for the isolated ions. The information obtained for isolated pairs is used as an aid in the interpretation of measurements on the concentrated crystal ${\mathrm{La}}_2$ ${\mathrm{Mn}}_3$ ${\left({\mathrm{NO}}_3\right)}_{12}$·${24\mathrm{H}}_2$O reported by other authors. From this analysis, a plausible magnetic ordering arrangement is found for the concentrated material below its magnetic critical point. It is concluded from a comparison of calculated and measured magnetothermal properties that the magnetic interactions are the same in the concentrated material as for isolated pairs within the quoted accuracy of the latter. From the trends of the zero-field splitting for ions in the $Y$ site as the diamagnetic constituents are varied, it is deduced that $D=-0.0209\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ions in the $Y$ site of the concentrated material. The anisotropy of the ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ $\gamma$ rays from magnetically ordered ${\mathrm{La}}_2$ ${\mathrm{Mn}}_3$ ${\left({\mathrm{NO}}_3\right)}_{12}$·