Surface crystallization and magnetic properties of iron-based metallic glasses

We have studied surface crystallization of amorphous Fe₇₈Si₉B₁₃ and its consequences on the magnetic properties. Thin crystalline surface layers on both ribbon sides were induced in various thicknesses by suitably annealing at elevated temperatures. As a consequence an out-of-plane anisotropy K developed in the amorphous bulk; the magnitude of K being proportional to the thickness of the crystallized surface layer. The temperature variation of K was studied from room temperature up to Curie temperature T_c and was predominantly determined by the one of saturation magnetostriction; a further contribution was due to the different thermal expansion of crystallized surface and amorphous bulk. The magnetization in the surface layer, finally, could be separated from that of the bulk by recording magnetization loops around T_c. The experimental results are in good agreement with a recently proposed theory which is based on the calculation of the internal stress distribution generated by the crystallized surface.