Effects of Nanocrystallization on the Magnetostriction of Co-based Amorphous Alloys

The influence of thermal treatments on the magnetostriction constant of VitrovacR 6025 amorphous alloys is reported. Annealing conditions (673 and 523 K at several annealing times) were chosen to result in nanocrystallization. The presence of nanocrystals led to a significant magnetic softening. No changes were observed on the saturation magnetization value at room temperature on the treated samples, as compared with the as-quenched sample. The magnetostriction constant, λs, shifts from -0.14 x 10-6 for the as-quenched sample to positive values for the treated alloys. Samples annealed at 673K exhibited two components of λs which depended on the tensile stress, σ one for low stress, and the other for high stress values, with the change at σ = 400 MPa. This behavior is interpreted in terms of a two-phase model, where one phase is the Fe-enriched residual amorphous fraction, and the other one is formed by Co-rich nanocrystals. For annealing at 723K, a well-known relationship, λs(σ) = λs(0) - aσ, was observed, with no resolution of components. This difference is interpreted on the basis of the nanocrystals size: for 673K, the nanocrystals are smaller than the exchange length, while they are larger for 723K.