Correlation between stacking fault formation, allotropic phase transformations and magnetic properties of ball-milled cobalt

A correlation between microstructure, allotropic phase transformations and magnetic properties of cobalt subjected to ball milling is presented. The starting cobalt, which is a mixture of hexagonal close-packed (hcp) and face-centered cubic (fcc) phases, is found to develop into an almost pure hcp phase after short-term milling. At the same time, the coercivity, HC, and the uniaxial magnetic anisotropy, Ku, increases. For longer milling times, plastic deformation mechanisms introduce large amounts of stacking faults in the hcp structure and, consequently, fcc-Co is in part recovered, even though this phase is usually metastable at room temperature. After long-term milling, the high degree of structural disorder and the partial transformation from hcp to fcc-Co significantly reduce both HC and Ku. The effect of the milling intensity, i.e. increase of the ball-to-powder weight ratio, is mainly to accelerate the structural and magnetic changes induced in Co.