Neutron study of local-environment effects in ferromagnetic Ni-Rh alloys

Neutron diffuse-scattering measurements were made on Ni-Rh alloys containing 2-, 5-, 10-, 15-, 25-, 30-, and 35-at.% Rh to determine the spatial distribution of the magnetic moments. We find that ${\overline{\mu }}_{\mathrm{Ni}}$ increases slightly out to about 5-at.% Rh, then decreases slowly to about 15-at.% Rh and beyond that decreases rapidly toward zero at the critical concentration of 37-at.% Rh. The concentration dependence of ${\overline{\mu }}_{\mathrm{Rh}}$ is approximately described by a $P_{12}$ dependence and is consistent with moments of $2{\mu }_B$ for those Rh atoms surrounded by 12 Ni nearest neighbors with a small residual moment of about $0.1{\mu }_B$ for Rh atoms with other environments. The diffuse cross sections exhibit small-$K$ peaks which become sharper and more pronounced with increasing Rh content. These peaks show the presence of magnetic-moment fluctuations about the average moments and, from a comparison of the $K=0\mathrm{}$ intercepts with $\frac{d\overline{\mu }}{\mathrm{dc}}$, we find that these fluctuations are due to local-environment effects. Because the range of these fluctuations increases with increasing Rh, we conclude that magnetic environment is important in determining the moment distribution of these alloys. The data are fitted to a local-environment model that includes the unusually large chemical-environment effect at Rh sites and both chemical- and magnetic-environment effects at Ni sites. The resulting parameters are physically reasonable and approach the correct limits at the extremes of the ferromagnetic region. One of the more interesting results of this fitting is that the chemical effect at Ni sites is positive, i.e., a Rh atom causes an increase in the moment of nearest-neighbor Ni atoms.