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

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 (Cₘ) is shown to derive from dilute concentrations of magnetic clusters with giant moments and low effective spins. From the field and temperature dependences of Cₘ, 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.