Enhancement of hard magnetic properties in rapidly quenched Zr–Co–Fe–B ribbons through vacuum annealing

Abstract

In the present work, the importance of vacuum annealing on the nanostructured Zr2Co9.5Fe1.5B ribbons fabricated through rapid solidification method was investigated for their hard magnetic properties. Optimized vacuum annealing promotes the formation of rhombohedral Co5.1Zr hard magnetic phase, in addition to the orthorhombic Co5.1Zr, fcc-Co, Co23Zr6 and Co3ZrB2 phases, which were formed via melt spinning. Remanent magnetization (Mr) and coercivity (HC) was found to enhance post-annealing from 22.8 emu/g and 1.24 kOe in as melt-spun (MS) ribbons to 50.3 emu/g and 1.64 kOe for the ribbons annealed at 550 °C/1 h. Furthermore, the optimally annealed ribbons contribute towards an increase in magnetic energy product (BH)max ~ 2 MGOe in comparison to as-spun ribbons having 0.7 MGOe. Annealing process leads to enhancement of hard magnetic properties through the crystallization of rhombohedral Co5.1Zr hard magnetic phase. To account for an increase in coercivity, the coercivity mechanism of the annealed optimized ribbon was also studied and is attributed to the nucleation mechanism as deduced from the measurement of minor loops. The effect of annealing conditions on the phase evolution, microstructure and magnetic behaviour of Zr2Co9.5Fe1.5B MS ribbons, indicates that appropriate control of the thermal annealing schedule is a key to achieve high remanence, coercivity as well as magnetic energy product. The nanostructured Zr2Co9.5Fe1.5B MS ribbons could serve as a potential candidate for rare-earth free permanent magnet materials.