Boundary spin disorder in nanocrystalline FeRh alloys

The magnetic properties of the ball-milled nanocrystalline fcc phase of FeRh alloys are reported and discussed. The observed features (maximum of ac and zero-field cooling susceptibility at a temperature Tₘₐₓ weakly dependent on the frequency and on the applied field) suggest a progressive blocking of moments of nanograins with decreasing temperature, followed by their collective freezing in random directions at Tₘₐₓ. The shifted hysteresis loop observed after field cooling as well as the remarkable irreversibility in high fields indicates that Tₘₐₓ corresponds to the freezing temperature of the strongly deformed grain boundaries, which behave as a spin-glass-like phase.