Paramagnetic Resonance Linewidths in Some Rare-Earth Double Nitrates

We have measured the paramagnetic resonance linewidth $\delta H$ at microwave frequencies for the three trivalent rare-earth ions Ce, Nd, and Sm present in small concentrations in ${\mathrm{La}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{H}}_2$O. The linewidths appear to be strongly dependent upon the angle $\theta$ between the $c$ axis of the crystal and the magnetic field, and in most cases the observed linewidths seem to be reasonably well explained by a simple theory which assumes a spatial distribution of paramagnetic resonance $g$ values throughout the crystal.