Evidence of magnetic domains in the reentrant spin glasses ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ studied by neutron depolarization

Polarization analysis of the transmitted beam was performed using a pulsed neutron source in a reentrant-spin-glass (RSG) system ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$, for a concentration range (0.23≤x≤0.28) encompassing the critical concentration. The polarization P was measured as a function of temperature, neutron wavelength, and applied field. Unlike other RSG systems previously studied, P decreases at all temperatures below the Curie temperature even for the samples close to the critical concentration. This behavior is also observed in Fe-Zr alloys. In Ni-Mn the wavelength dependence of the polarization indicates the existence of magnetic domains, whose size δ decreases, with increasing x, from 10 to 50 μm at x=0.23 to a few thousands of angstroms at x=0.25. No depolarization occurs for x>0.25. As shown for x=0.23, the irreversibilities of the polarization, which depend on the cooling conditions, indicate that the domain arrangement freezes at low temperature. Clearly, in Ni-Mn, the drop of the susceptibility at low temperature does not result from a decrease of the size or magnetization of the domains but from a slowing down of their mobility.