Investigation of microstructural and magnetic property evolution in mechanically milled Co–Zr metallic ribbon

Zr2Co11 has emerged as a promising candidate for rare-earth-free magnetic materials with the exceptional energy products in thin-film nanocomposites. Despite this, its potential in bulk magnet form remains largely unexplored. Powder metallurgy offers a promising route to fabricate these alloys in bulk magnet form with controlled microstructure and soft and hard magnetic phase distribution essential for optimizing magnetic properties. This study investigates the effects of mechanical milling on the microstructure, and magnetic properties of a metallic ribbon having composition of Co85Zr15, aiming toward development of novel permanent magnet material. The microstructure and phase composition of both the as-cast ribbon and the milled powder samples were characterized using transmission electron microscopy (TEM) and synchrotron X-ray diffraction (XRD). The as-cast ribbon predominantly consists of the hard magnetic Zr2Co11 phase along with a soft magnetic Zr6Co23 phase. TEM analysis confirmed that mechanical milling reduced the ribbon to fine particles with an average size of 11 nm after 14 h of milling. With the increase in milling duration, the saturation magnetization was found to increase whereas the coercivity was found to decreases. This study shows that milling has a significant impact on the exchange coupling between soft and hard magnetic phases at nanoscale. To rationalize the experimental observations, first-principles calculations were performed considering rhombohedral structure of Zr2Co11 to estimate the change in the total magnetic moments in the unit cell as a function of isotropic compression/expansion, as well as to examine the effect of off-stoichiometry and defect formation. The ball milling process parameters were optimized to achieve particle sizes close to the critical size of the soft and hard magnetic phases (~ 11 nm) required for effective magnetic coupling between the two so as to manifest the exchange spring behavior in nanocomposite magnets.