Exchange coupling and microstructure in annealed cobalt-rich amorphous alloys

Low-field magnetic properties have been measured in annealed ribbons of amorphous Co_95-xFe₅(BSi)_x. The M-H loops show coercivities of order 100 mOe and weak but consistent exchange anisotropy of order 10 to 100 mOe. The direction of the field offset can be reversed by reversing the ribbons in the pickup coil or by applying a negative field of order 30 to 40 Oe. We have made an extensive microstructural investigation of these alloys to understand the origin and nature of the exchange-coupling component responsible for the offset. Annealing in air first causes surface oxidation to B₂ O₃ and SiO₂ which reduces the glass-former content in the underlying amorphous material. Continued heating well below the bulk crystallization temperature results in premature crystallization of a layer of order 10 to 100 nm in thickness. The crystallization product is either fcc or hcp Co (plus solutes). The fcc cobalt is highly-faulted which seems to be necessary for the exchange anisotropy