Giant Moments in Ni–Cu Alloys Near the Critical Composition

The diffuse scattering of neutrons from Ni–Cu alloys near the critical composition (∼44 at.% Ni) was measured at 4.2°K. For the ferromagnetic alloys of 50−46 at.% Ni (with Curie temperatures of 40°−9°K and spontaneous moments μ0 of 0.06−0.02 μB/at.) the results give a magnetic-scattering cross section (dσ/dΩ) that is unexpectedly large at low scattering vectors (Κ) and decreases rapidly with increasing Κ. If the ferromagnetic moment were distributed equally among all the Ni atoms, the predicted dσ/dΩ would be much smaller and essentially constant over the same range of Κ (∼0.2–0.9 Å−1). Instead, our Ni–Cu results closely resemble those previously reported1 for the ferromagnetic polarization about an Fe impurity atom in Pd and can be interpreted analogously in terms of polarization clouds extending over a few hundred atoms. From the measured values of dσ/dΩ and μ0 it is deduced that the average moment per polarization cloud is ∼8 μB while the concentration of these clouds varies from ∼0.7% (for the 50% Ni alloy) to ∼0.2% (for 46% Ni). Some chemical clustering is also detected in these alloys but of much shorter range. Significant neutron scattering by critical fluctuations is observed for the paramagnetic alloys closest to the critical composition. A fuller report on this work will appear elsewhere.