Comment on the Gd crystalline field and the Gd-Pr exchange in the Van Vleck monopnictides

We report on the observation of resolved fine structure in the ESR of Gd in PrSb, TmSb, and PrBi single crystals. The fourth-order crystalline-field parameter b4 was found to increase with the lattice constant upon going from the antimonides to the bismuthides, consistent with the variation of b4 in the analogous nonmagnetic pnictides. A correlation between b4 and the Korringa rate in the analogous nonmagnetic compounds suggests that the variation of b4 is associated with conduction-electron effects. Also the variation of the Gd-Pr exchange parameter, JGd-Pr across the pnictides is attributed to variation of the conduction-electron d-d overlap. The data enable us to estimate the pressure derivative of b4 and JGd-Pr.