Magnetic behaviour during the first crystallisation stages in Co-B amorphous alloys: a test of the exchange penetration through interfaces

Two amorphous compositions of the Co-B system, which present quite different magnetic behaviour, have been studied in the as-cast state and after low temperature annealings. The Co₈₀B₂₀ amorphous alloy submitted to thermal treatments undergoes a primary crystallisation which transforms the material into a two phase system composed of nanocrystals of cobalt embedded in a Co-B amorphous matrix. The role of the cobalt grains on the magnetisation processes and the coercive field is analysed for temperature ranging from 77 to 300 K. It is found that the hardening effect of the grains diminishes on increasing the temperature. The large crystalline anisotropy of the Co grains as well as the magnetic state of the grain boundary which transmits the exchange interaction, are mainly responsible for the observed behaviour.