Hyperfine Fields at Solutes in Ferromagnets: Cadmium and Ruthenium in Nickel

Perturbed-angular-correlation studies of temperature-dependent hyperfine fields at ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ and ${}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}$ solutes in nickel above and below the Curie point are presented. Trends among observed hyperfine fields at solutes in Fe, Co, and Ni are discussed within the framework of a formal spin Hamiltonian. Origins of the hyperfine fields on $4f$ and $5p$ group solutes are discussed. Conduction-electron and core-polarization contributions to $H_{\mathrm{hf}}$ in transition-series solutes are analyzed, and localized moments are derived for the solutes Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt, and are compared with neutron-scattering results. The observed lack of agreement may be associated with the fact that neutron scattering and hyperfine fields measure different quantities. It is suggested that a molecular-field model that accounts for conduction electrons as well as core polarization might also be applicable above $T_C$. This model fits $H_{\mathrm{hf}}\mathrm{}\left(T\right)\mathrm{}$ in ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}\mathrm{Fe}$ for all spins between ½ and $\frac{5}{2}$, suggesting that $j$ cannot be determined from $H_{\mathrm{hf}}\mathrm{}\left(T\right)\mathrm{}$ alone. This analysis yields a local moment of $(1.5\mathrm{}\pm 0.5){\mu }_B$ for ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ in iron, just in agreement with the results of Jaccarino et al., but higher than allowed by neutron data. Angular-correlation apparatus capable of 0.1% accuracy is described. A localized moment of $\cong 0.5{\mu }_B$ is determined from $H_{\mathrm{hf}}$ for ${}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}$ in Ni. For $T>\mathrm{}{T}_C$, paramagnetic Knight shifts up to -80% are observed. A model due to Lovesey also fits the data below $T_C$ for $j=\frac{1}{2}$. For ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}\mathrm{Ni}$, no local moment is indicated below $T_C$, and comparison with a model due to Lovesey and Marshall suggests that Cd atoms have the same temperature-dependent magnetization as nearest-neighbor nickel atoms. Above $T_C$, paramagnetic Knight shifts up to -60% are in excellent agreement with predictions of the conduction-electron polarization model.