Changes in Curie temperature, physical dimensions, and magnetic anisotropy during annealing of amorphous magnetic alloys

We have measured the effects of annealing in air on various properties of several amorphous alloys. Reported here are results on the changes in Curie temperature, in the physical dimensions of lengths of amorphous ribbon, and in the magnetic anisotropy. Increases in Curie temperature up to 35°C have been measured. All the alloys examined show a steady increase in Tcon annealing at low temperatures, but some compositions show a smaller increase in Tcon annealing near the crystallization temperature than on annealing at lower temperatures. There appear to be two competing mechanisms influencing Tc. All the alloys examined show a clearly measurable decrease in length on annealing; we interpret this as an increase in bulk density. The kinetics of the annealing are similar to those of the stress relaxation. Finally, annealing experiments on the shape of 60 Hz hysteresis loops show a decrease in the anisotropy associated with non-uniform internal stresses, but in some cases also show the slow development of a fairly strong uniaxial anisotropy with its easy axis perpendicular to the ribbon axis. This uniaxial anisotropy is tentatively ascribed to the development of an oxide layer during annealing, which in turn produces a uniform compressive stress due to differential thermal contraction and therefore a stress-magnetostriction anisotropy. The changes in Curie temperature and in sample dimensions cannot be ascribed to oxidation. All the results described above are for annealing treatments that do not cause crystallization. The time for crystallization at various temperatures has been measured, and activation energies for crystallization derived.