Matrix and Impurity-Cluster Polarization in Ni-Pt and Ni-Pd Alloys

A study of hyperfine magnetic fields at nuclei of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ diffused as impurities in the transition-metal alloys Ni-Pt and Ni-Pd and at nuclei of ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ in the Ni-Pt system has been carried out using the Mössbauer-effect technique. The fields were measured as a function of composition and temperature on the atomically disordered alloys. The experimentally determined Curie temperatures $T_C$ and saturation hyperfine fields are presented for the entire composition range of each system. Analysis of the Mössbauer spectra of specific samples for various diffusion times of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ traces gives clear indication of iron impurity clustering within the Ni-Pd and Ni-Pt alloy matrices. A simple phenomenological theory is presented in an attempt to explain the $T_C$ vs concentration dependence in these alloys.