EFFECTS OF NANOCRYSTALLIZATION ON THE MAGNETOSTRICTION OF CO-BASED AMORPHOUS-ALLOYS

The influence of thermal treatments on the magnetostriction constant o f Vitrovac(R) 6025 amorphous alloys is reported. Annealing conditions (673 and 523 K at several annealing times) were chosen to result in na nocrystallizatiion. The presence of nanocrystals led to a significant magnetic softening. No changes were observed on the saturation magneti zation value at room temperature on the treated samples, as compared w ith the as-quenched sample. The magnetostriction constant, lambda(s), shifts from -0.14 x 10(-6) for the as-quenched sample to positive valu es for the treated alloys. Samples annealed at 673K exhibited two comp onents of lambda(s) which depended on the tensile stress, sigma; one f or low stress, and the other for high stress values, with the change a t sigma = 400 MPa. This behavior is interpreted in terms of a two-phas e model, where one phase is the Fe-enriched residual amorphous fractio n, and the other one is formed by Co-rich nanocrystals. For annealing at 723K, a well-known relationship, lambda(s)(sigma) = lambda(s)(0) - A sigma, was observed, with no resolution of components. This differen ce is interpreted on the basis of the nanocrystals size: for 673K, the nanocrystals are smaller than the exchange length, while they are lar ger for 723K.