Effect of Co substitution for Ni on the microstructure and magnetic properties of (Fe, Ni)-based amorphous alloys produced by melt spinning

Transmission electron microscopy with selected area electron diffraction, and X-ray diffraction were applied to study the effect of Co substitution for Ni in (Fe, Ni)-based amorphous alloys. The investigation was performed to obtain information on correlations between microstructure and magnetic properties of the (Fe, Ni, Co)-based amorphous alloys. The examination of the microstructure reveals that there are small crystallized free surface regions because the actual quenching rate is distributed inhomogeneously over the cross-section of the ribbon. Since in the free surface region, the solidification rate is lower, a spontaneous annealing process occurs at the top surface of the ribbon. The crystallization degree of the free surface region is higher for alloy ribbons that contain Co up to a 15 at.% concentration. Magnetic domains pattern are sensitive to the surface crystallization and Co content of the (Fe, Ni)-based alloy ribbons. Fine-scaled stripe domains were evidenced on the free ribbon surface while on contact surface stress domain pattern appeared. With the increase of the Co content, the domain width became small and long stripes appeared. The striped domains are responsible for an increased coercivity of the ribbons. However, there is a critical Co content (xCo = 10) for which spontaneous narrow stripe domains are no longer more energetically favourable for ribbons with specific magnetic applications.