Anomalous concentration dependence of the hyperfine field in orderedDy1−xGdxAl2alloys

We have studied the hyperfine field at the ${}_{}{}^{163}{}_{}{}^{}\mathrm{Dy}$ nucleus across the whole range of Gd concentrations in ferromagnetic ${\mathrm{Dy}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Al}}_2$ alloys by nuclear magnetic resonance. Contrary to expectation, the hyperfine field for each observed resonance is found to decrease with increasing Gd content or equivalent increasing molecular field. The reduction in hyperfine field appears to be correlated with a reduction expected in the transferred hyperfine field suggesting that fields due to $4f$ and self-polarization effects remain essentially constant. The results for hyperfine field and quadrupolar splitting at the ${}_{}{}^{163}{}_{}{}^{}\mathrm{Dy}$ nucleus in Dy-rich and Gd alloys are also consistent and suggest the $\mathrm{Dy} 4f$ electron $〈J_z〉$ value remains constant across the series. Enhancement-factor measurements are consistent with Al nuclear-resonance and torque measurements and suggest the Dy anisotropy dominates in these compounds even for very small concentrations of Dy.