High-temperature EPR in solid and molten paramagnets

The electron-paramagnetic-resonance (EPR) linewidth $\Delta H$ of the strongly exchange coupled paramagnets, Mn${\mathrm{F}}_2$, KMn${\mathrm{F}}_3$, RbMn${\mathrm{F}}_3$, and CsMn${\mathrm{F}}_3$, was studied from 300 to 1450 K. Relative to the Néel temperature $T_N$ this regin encompasses temperatures of the order of $3<\mathrm{}\frac{T}{T_N}<15\mathrm{}$ in the solid and some 200-350°K into the molten state. The $X\mathrm{Mn}{\mathrm{F}}_3$ ($X=K, \mathrm{Rb} \mathrm{and} \mathrm{Cs}$) salts all show a $\Delta H$ which monotonically increases with temperature up to the melting point $T_m$. At $T_m$ an abrupt doubling of $\Delta H$ occurs, followed by a decreasing linewidth with increasing temperature. The linewidth of Mn${\mathrm{F}}_2$ exhibits a most striking behavior; beginning some 200°C below $T_m$ the linewidths $\Delta H^{\parallel }$ and $\Delta H^{\perp }$ (parallel or perpendicular to $c$ axis) decrease rapidly, with the anisotropy ($1-\frac{\Delta H^{\parallel }}{\Delta H^{\perp }}$ in $\Delta H$ changing sign before melting. No abrupt change in $\Delta H$ is found upon melting. EPR in magnetically diluted but structurally isomorphic salts (e.g., $K{\mathrm{Mn}}_{1-x}{\mathrm{Mg}}_x{\mathrm{F}}_3$) was investigated in both solid and molten states. Resolved ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ hfs were found in the very dilute solids but not in the liquids. Linewidths as functions of concentration were obtained in both cases. A theoretical interpretation of the temperature dependence of $\Delta H$ in the dense magnetic salts is given. Rigid-lattice spin dynamical contributions are examined and are found to be small. Lattice vibrational effects (harmonic and anharmonic) on the temperature dependence of the dipolar and exchange interactions, as they affect the linewidth, were studied. From the combined theoretical and experimental studies, for the simple cubic KMn${\mathrm{F}}_3$ and RbMn${\mathrm{F}}_3$ compounds, the temperature dependence of the exchange interaction is deduced in the region $T=4\mathrm{}{T}_N$ to $T=T_m$. The anomalous behavior of the Mn${\mathrm{F}}_2$ linewidth and its anisotropy below$T_m$ is interpreted as resulting from the activated interchange of nearest-neighbor ${\mathrm{Mn}}^{2+}$ ions along the $c$ axis, with $E_0\approx 0.3$ eV. The magnitudes of the linewidth in the molten states of all of the dense paramagnets are shown to correspond to a correlation time for spin exchange or motion (and/or chemical exchange) ${\tau }_m\simeq {10}^{-12\mathrm{}}$ sec. The dilution experiments in the molten state suggest that spin exchange continues to be important in the liquid until very low magnetic ion concentrations.