Superparamagnetism, Nonrandomness, and Irradiation Effects in Cu-Ni Alloys

Abstract

The effect of neutron irradiation on copper-nickel alloys has been investigated by means of magnetic susceptibility measurements. The susceptibilities were measured by the Gouy method between 300°K and 2.0°K for a series of alloys ranging from 17.22 to 46.5 atomic percent nickel. It is shown that without superparamagnetism an unreasonable magnetic moment per nickel atom has to be assumed. This model is confirmed by irradiation studies in which samples were exposed to neutron fluxes at the Brookhaven reactor, ranging up to 2.2×${10}^{19}$ neutrons/${\mathrm{cm}}^2$ while at 80°C, and the magnetic susceptibilities were found to increase following the irradiation. The increase was easily observable due to its strong temperature dependence, and was greatest for the samples with the highest nickel content and for samples exposed to the highest neutron fluxes. The susceptibilities of the alloys returned to their original values following an anneal in or above the temperature range where self-diffusion becomes important, while no changes in the susceptibilities were observed following anneals at lower temperatures. It is suggested that the copper-nickel system is not a perfect random solid solution but tends toward segregation, and that the neutron irradiation enhances diffusion toward a true equilibrium at room temperature. This is in agreement with several other observations.