Hyperfine field in ordered ${\mathrm{Dy}}_{1-x}{M}_x{\mathrm{Al}}_2$ ($M=\mathrm{Y},\mathrm{Gd}$) compounds

We report the results of nuclear-magnetic-resonance experiments on the ${}_{}{}^{163}{}_{}{}^{}\mathrm{Dy}$ nucleus in ferromagnetic alloys of ${\mathrm{Dy}}_{1-x}{\mathrm{Y}}_x{\mathrm{Al}}_2$ and ${\mathrm{Dy}}_{0.975}$ ${\mathrm{Gd}}_{0.025}$ ${\mathrm{Al}}_2$. The substitution of Dy by Y or Gd produces "satellite" lines in the ${}_{}{}^{163}{}_{}{}^{}\mathrm{Dy}$ NMR spectrum which we show are due to modification of the $4f$ hyperfine field. This modification is produced by a variation in the molecular field which modifies the angular momentum of the ground-state $4f$ wave function and hence the $4f$ moment. From the Gd substitution experiment we are able to estimate the molecular field appropriate to Gd${\mathrm{Al}}_2$, which agrees with other experiments.