Spin-glass-like distribution of interaction fields in Ni-Cu alloys

Abstract

Low-temperature heat-capacity measurements were made in magnetic fields up to 42 kOe on several Ni-Cu alloys close to (on both sides of) the critical composition for ferromagnetism. The magnetic component of the specific heat ($C_m$) is shown to derive from dilute concentrations of magnetic clusters with giant moments and low effective spins. From the field and temperature dependences of $C_m$, it is also deduced that the interaction (exchange and anisotropy) fields seen by the clusters extend from large positive to large negative values, relative to the applied field. This spin-glass-like distribution of interaction fields, as it pertains to the weakly ferromagnetic alloys, suggests a coexistence of spin-glass and ferromagnetic order. This interaction-field distribution can also account for the anomalously slow approach to saturation of the low-temperature magnetization of these alloys in high fields.