The effect of heat treatment on the Curie temperature of a metallic glass

Metallic glasses undergo structural relaxation at elevated temperatures and their mechanical properties are often dramatically altered as a consequence. We have discovered that the magnetization behavior of a (Ni.5Co.5).75P.16B.06Al.03 glass is similarly sensitive to heat treatment. The ferromagnetic transition in this material is not sharp, and heat treatment serves to broaden the transition further. Tc cannot be defined by conventional methods; e.g., (M)1/β vs (H/M)1/γ plots or a.c. permeability measurements. Both the initial permeability and the high field magnetization increase upon annealing, at rates that are indentical to those of structural relaxation processes. While transmission electron microscopy examination of the glass failed to reveal either crystallization or phase separation, the magnetic properties suggest that a microscopically inhomogeneous distribution of the ferromagnetic species is developed during quenching and heat treatment.