Nonlinear and nonlocal moment disturbance effects in Ni-Cr alloys

Abstract

We have made polarized-neutron diffuse-scattering measurements on Ni-rich Ni-Cr alloys to determine the magnetic moment distribution on an atomic scale. In contrast to previous unpolarized-neutron measurements, which contain all of the static moment fluctuations from the average, these measurements select out only those fluctuations at one site correlated with the presence of an impurity at another site. The polarized measurement is therefore intrinsically linear in the impurity site occupation. Comparison of the polarized and unpolarized results show that nonlinear effects are important for this system and therefore that both measurements are required to describe the spatial moment distribution away from the dilute impurity region. The total moment disturbance per impurity, obtained by fitting the polarized-neutron data to the localenvironment model, decreases rapidly over the concentration region for which $\frac{d\overline{\ensuremath{\mu}}}{\mathrm{dc}}$ from magnetization data remains constant. We conclude that the impurity-induced moment disturbance has two components: a local-environment effect which we detect with neutrons and a nonlocal effect with range \ensuremath{\ge} 12.5 \AA{} that remains undetected in the neutron experiment. We propose a charge-transfer model that contains both the nonlinear and nonlocal effects.