Instability toward the Formation of Local Moments in Ni-Cu Alloys

An examination of the nonlocal exchange-enhanced susceptibility of disordered alloys such as Cu-Ni leads to the development of criteria for the formation of localized extended magnetic moments. A simplified model is developed for Cu-Ni, in which the nonlocal susceptibility without exchange enhancement Γᵢⱼ is assumed to be finite only if i and j are nickel sites. Using the random-phase approximation for the exchange enhancement, and assuming that Γᵢⱼ extends only to nearest neighbors, the instability criterion is examined by a variational method, and conditions for formation of both localized and extended moments are obtained. The results are consistent with the neutron diffraction findings that, near the critical concentration for ferromagnetism, giant moments or polarization clouds occur. These are nucleated by small nickel clusters which are due to statistical fluctuations in the nickel concentration. The number of these centers is enhanced by short-range-order effects. The disappearance of all the effects above the nickel Curie temperature is discussed.