Impure Exchange-Coupled Paramagnets; Electron-Paramagnetic-Resonance Studies

The effects of substituting ${\mathrm{Ni}}^{++}$, ${\mathrm{Co}}^{++}$, and ${\mathrm{Fe}}^{++}$ for ${\mathrm{Mn}}^{++}$ in RbMn${\mathrm{F}}_3$ on the EPR linewidth and $g$ values have been studied as a function of impurity concentration and temperature above $T_N$. Severe line broadening for relatively small impurity concentrations is observed with ${\mathrm{Fe}}^{++}$ and ${\mathrm{Co}}^{++}$ but not with ${\mathrm{Ni}}^{++}$. The results are interpreted using both a high-temperature-moment method and a coupled-equation-of-motion approach. Both have as a central feature the importance of the interplay of host-impurity exchange rate (${\omega }_{\mathrm{ex}}^{\prime }$) and impurity-single-ion spinlattice damping rate (${\delta }_{\mathrm{SL}}$) in determining the effects that the impurity has on the resonance linewidth. Quantitative agreement is found for the magnitude and temperature dependence of the linewidths for resonable values of ${\omega }_{\mathrm{ex}}^{\prime }$ and ${\delta }_{\mathrm{SL}}$. The interesting correspondence between these effects and the impurity-conduction-electron spin-resonance problem in metals is noted. An analysis of the sign and temperature dependence of the $g$ shift in the RbMn${\mathrm{F}}_3$: Ni system indicates that the local susceptibility of the ${\mathrm{Ni}}^{++}$ ion is negative throughout most of the temperature region studied.