Ni61Mössbauer Effect in Ni-Pd Alloys

Abstract

We have obtained nuclear $\gamma$-resonance (NGR) absorption spectra with the 67.4-keV transition from ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ in Ni-Pd alloy absorbers throughout the concentration range 0-99.5 at.% Pd. The source contained the parent isotope ${}_{}{}^{61}{}_{}{}^{}\mathrm{Co}$, produced by the reaction ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}(p, \alpha ){}_{}{}^{61}{}_{}{}^{}\mathrm{Co}$, in a nonmagnetic ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$-14 at.% V foil. Both source and absorber were immersed in liquid helium. Values of the absorber recoilless fraction, energy shift, and absolute value of the ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ hyperfine (hf) field were obtained for each concentration studied. In the ferromagnetic region 0-98 at.% Pd, the spectra showed a partly resolved magnetic hyperfine splitting with a distribution of magnetic hyperfine fields. The average hyperfine field is negative in pure Ni (-76 kOe), changes sign near 50 at.% Pd, and rises to a large positive value (+173 kOe) at 90 at.% Pd. Qualitative agreement with these results is obtained with a model based on the assumption that $〈H_{\mathrm{hf}}〉$ in Ni-Pd has the same contributions from core polarization and bulk conduction-electron polarization as in other Ni-based alloys, plus a large positive contribution from Pd atoms on neighboring lattice sites. From measurements with an external magnetic field in alloys containing 50-99.5 at.% Pd, we find that the calculated average hyperfine fields ${〈H_{\mathrm{hf}}〉}_{\mathrm{calc}}$ are in agreement with the values expected for no distribution of fields. The distribution width of hyperfine fields is $\Gamma =80\mathrm{}\pm 9$ kOe in the mid-concentration range. In an alloy containing 0.5 at.% Ni, the most dilute alloy studied, the moment $\mu =\left[\frac{J}{\mathrm{}(J+1\mathrm{})\mathrm{}}\right](0.73\mathrm{}\pm 0.05){\mu }_B$ was obtained. From a temperature-dependence study of the second-order Doppler shift, we have deduced the relative isomer shift between ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ in Pd and ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ in Ni to be ${\delta }_{\mathrm{IS}}^{\mathrm{Pd}}-{\delta }_{\mathrm{IS}}^{\mathrm{Ni}}=-23\mathrm{}\pm 15$ μ/sec.