High-temperature EPR in solid and molten paramagnets

The electron-paramagnetic-resonance (EPR) linewidth ΔH of the strongly exchange coupled paramagnets, MnF₂, KMnF₃, RbMnF₃, and CsMnF₃, was studied from 300 to 1450 K. Relative to the N\'eel temperature TN this regin encompasses temperatures of the order of 3<TTN<15 in the solid and some 200-350^∘{}K into the molten state. The XMnF₃ (X=K, Rb and Cs) salts all show a ΔH which monotonically increases with temperature up to the melting point Tₘ. At Tₘ an abrupt doubling of ΔH occurs, followed by a decreasing linewidth with increasing temperature. The linewidth of MnF₂ exhibits a most striking behavior; beginning some 200^∘{}C below Tₘ the linewidths ΔH^∥ and ΔH^⊥ (parallel or perpendicular to c axis) decrease rapidly, with the anisotropy (1-ΔH^∥ΔH^⊥ in ΔH changing sign before melting. No abrupt change in ΔH is found upon melting. EPR in magnetically diluted but structurally isomorphic salts (e.g., KMn_1-xMgₓF₃) was investigated in both solid and molten states. Resolved ⁵⁵Mn and ¹⁹F 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 ΔH 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 KMnF₃ and RbMnF₃ compounds, the temperature dependence of the exchange interaction is deduced in the region T=4TN to T=Tₘ. The anomalous behavior of the MnF₂ linewidth and its anisotropy belowTₘ is interpreted as resulting from the activated interchange of nearest-neighbor Mn²⁺ ions along the c axis, with E₀≈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) τₘ10^-12 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.