Exchange-Induced Hyperfine Fields in the Samarium Monochalcogenides

We have observed large negative shifts of the ${\mathrm{Se}}^{77}$ nuclear magnetic resonance in SmSe and ${\mathrm{SmS}}_c{\mathrm{Se}}_{1-c}$ solid solutions. These shifts, which depend markedly on temperature and vary linearly with the sulfur concentration, are found to be correlated with the expectation value of the rare-earth spin. Simple models for the temperature and concentration dependence are used to obtain the hyperfine field at the selenium site and the approximate dependence of this quantity on the lattice spacing. For SmSe the hyperfine field per unit $4f$ spin is -29±4 kOe, where the error includes all probable temperature dependences of the relevant exchange integrals. In the pseudobinary compounds the dominant contribution to the concentration dependence of the shift is due to the dependence of the hyperfine field on the lattice spacing. The data suggest an inverse-seventh power relationship for this dependence.