Magnetic-Moment Distributions in Ferromagnetic Ni-Cu Alloys

Abstract

The elastic diffuse scattering of neutrons from ferromagnetic Ni-Cu alloys of 2-40 at.% Cu has been measured at 4.2\ifmmode^\circ\else\textdegree\fi{}K. Analysis of the results within the formalism of Marshall yields a description of the magnetic-moment distribution among the various atoms. The disturbance in moment produced by a copper atom at dilute concentrations appears to be confined almost completely to its nickel near neighbors, which is consistent with the short-range chemical screening effects predicted from coherent-potential theory. At higher copper concentrations, the moment disturbance extends over several neighbor shells, and it is argued that this longer-range effect is predominantly magnetic in origin. Inasmuch as the bulk moment per atom also contains a contribution from a uniform conduction-electron polarization ${\ensuremath{\mu}}_{\mathrm{cond}}$, the average nickel and copper $3d$ moments ${\ensuremath{\mu}}_{\mathrm{Ni}}$ and ${\ensuremath{\mu}}_{\mathrm{Cu}}$ can only be determined in terms of the combinations ${\ensuremath{\mu}}_{\mathrm{Ni}}+{\ensuremath{\mu}}_{\mathrm{cond}}$ and ${\ensuremath{\mu}}_{\mathrm{Cu}}+{\ensuremath{\mu}}_{\mathrm{cond}}$. The quantity ${\ensuremath{\mu}}_{\mathrm{Cu}}+{\ensuremath{\mu}}_{\mathrm{cond}}$, whose major component is probably ${\ensuremath{\mu}}_{\mathrm{cond}}$, has a constant value of about $\ensuremath{-}0.1{\ensuremath{\mu}}_{B}$ over the entire composition range studied.