Giant-moment clusters in paramagnetic and weakly ferromagnetic Pd-Ni alloys

A series of Pd-Ni alloys, of Ni concentrations up through the critical value (∼ 2.5 at.%) for ferromagnetism, was studied by detailed magnetization measurements from 2.4 to 50 K in fields up to 56 kOe. The paramagnetic susceptibilities and high-field magnetizations are both found to consist of two components arising from stable magnetic clusters and locally enhanced Pauli paramagnetism. The magnetic clusters are shown to be nucleated by groups of three or more nearest-neighbor Ni atoms, as reported earlier, and also by nearest-neighbor Ni pairs that have other Ni pairs (and/or Ni singles) in close proximity. Both types of clusters have giant moments (∼ 17 and ∼ 12${\mathrm{\mu }}_{\mathit{B}}$, respectively) and anomalously low spins (2 and 1, respectively), suggesting that the large induced polarization of the Pd atoms around the cluster nuclei has no independent degrees of freedom. Since the fraction of Ni pairs that nucleates magnetic clusters increases rapidly above ∼ 2 at.% Ni and does not contribute significantly to the locally enhanced susceptibility, this susceptibility component has a broad finite peak near the critical concentration. Thus, the onset of ferromagnetism does not derive from any critical exchange enhancement but from a percolation among the interacting clusters.