Temperature dependence of structural and magnetic relaxation in amorphous and nanocrystalline Co-based alloys

Structural and magnetic relaxations in amorphous alloys containing a nanostructured phase are compared with those of the amorphous precursors Metglas 2714A and Vitrovac 6025, in the temperature range 20K to the Curie temperature. The analysis is performed by considering the magnetic after-effect (MAE) of the reluctivity which is represented by its relative change Deltar/r between two fixed times after demagnetization. The nanostructured samples, obtained by suitable thermal treatments of the amorphous alloys, have superior soft magnetic properties (much higher initial permeability and lower coercive field) but are characterized by a lower structural and magnetic stability in wide ranges of temperature. The characteristics of the MAE in nanostructured alloys seem to be connected with the mean radius of the grains. The activation energy spectra are evaluated on the basis of the logarithmic kinetics observed for MAE, It results that the formation of nanocrystallites makes more uniform the distribution of these energies without modifing significantly the range of their values. (C) 2002 Elsevier Science B.V. All rights reserved.