Relaxation phenomena and host exchange parameters in Tm van Vleck compounds

The ESR linewidth of Gd in TmP (Sugawara et al. 1975) TmSb and TmBi (measured in the present work and by Davidov and Baberschke 1975) exhibits an appreciable temperature dependence. This behaviour is attributed to the fluctuation spectra of the host Tm ions. A previous theory (Davidov et al. 1977) for impurity relaxation in weakly coupled van Vleck paramagnets based on the Bloch-Redfield kinetic equation is extended and applied to the interpretation of the ESR linewidth in the Tm pnictides. In particular the second moment calculation of the host fluctuation spectra has been extended to include both pair correlation and autocorrelation contributions. Explicit expressions are given for Tm and Pr cubic van Vleck compounds. The Tm-Tm host exchange has been estimated by fitting theory to the experimental results. The host exchange parameter in TmSb is very small confirming previous studies on this compound.