Magnetic hyperfine interaction studies of isolated Ni impurities in Pd and Pd-Pt alloys.

Abstract

The magnetic hyperfine fields at isolated Ni impurities in Pd and Pd-Pt alloys were studied with the perturbed-angular-distribution (PAD) method by measuring the temperature, magnetic field, and concentration dependence of the nuclear-spin Larmor precession of isomeric states in $^{63}\mathrm{Ni}$. The recoil-implanted Ni nuclei, as products of heavy-ion nuclear reactions, are present in extreme dilution (<1 ppm) in the hosts. The positive Larmor frequency shift (Knight shift) observed for Ni impurities in Pd follows a Curie-Weiss-like temperature dependence with a large Curie constant indicating a giant moment behavior. For Ni impurities in the Pd-Pt alloys a considerable positive shift remains even at 30 at. % Pt content. The variation of the shift with Pt concentration and temperature reflects the variation of the Pd-Pt alloy susceptibility. The different contributions to the hyperfine field could be differentiated by comparing the Knight shift for Ni in Pd with its susceptibility contribution obtained from extrapolated susceptibility measurements in dilute Pd-Ni alloys. The negative core-polarization field of the impurity spin moment is compensated for by a transferred hyperfine field correlated with the host polarization in the neighborhood of the impurity. The remaining positive hyperfine field is due to a weak orbital moment of 0.3${\ensuremath{\mu}}_{B}$ at the impurity site. The values obtained for the different contributions are compared with results of the Korringa-Kohn-Rostoker--coherent-potential-approximation calculations for concentrated Pd-Ni alloys extrapolated to the dilute limit.