Magnetism of coexisting rhombohedral and orthorhombic Hf2Co11 phases in rapidly quenched Hf2Co11B

Evolution of crystalline structure caused by heat treatment and its influence on magnetic properties of two melt-spun Hf2Co11B alloys in fully amorphous and partially crystalline states have been investigated. The as-cast ribbons were annealed in order to improve the hard magnetic properties by inducing the formation of nanocrystalline phases with high anisotropy. The X-ray diffraction and magnetic measurements confirmed the coexistence of two Hf2Co11 phases, crystallizing in different structures, with the rhombohedral one showing hard magnetic properties. Annealing of fully amorphous alloy resulted in the lower coercive field in comparison to that of annealed partially crystalline ribbon. From the point of view of exchange coupling between magnetic phases and magnetic hardening, the optimum annealing temperature and time were determined to be 570 °C and 60 min, for partially crystalline sample. Annealing at 650 °C for 1 h leads to excess of the HfCo3B2 phase above the critical value and results in a decrease in the coercive field. Isothermal annealing increases the magnetocrystalline anisotropy constant K1 from 7.52 for the as-quenched sample to 12.11 Merg/cm3 for the partially crystalline ribbon. The existence of quenched-in nanocrystals resulted in higher coercive fields, up to 3.23 kOe, after annealing.