Preparation of YCo5, PrCo5 and SmCo5 anisotropic high-coercivity powders via mechanochemistry

Polydispersed RCo5 powders consisting mostly of single crystal particles with an average size ranging from 280 nm to 400 nm have been prepared mechanochemically for R=Y, Pr, Pr-Sm and Sm. The synthesis included (i) mechanical activation of a mixture of rare earth oxides, Co, Ca and CaO, (ii) short annealing and (iii) separation of the RCo5 particles through a multi-step washing process. The highest room-temperature coercivities of the oriented YCo5, PrCo5 and SmCo5 powders were 20.6, 19.1 and 41.5 kOe, respectively. This improvement compared to traditionally ground powders is attributed to a lower density of crystalline defects. An investigation of a broad, 1050-1320 K, range of the synthesis temperatures revealed nearly universal evolution of crystalline anisotropy of the particles and their remanent magnetization, significant variations in the particle growth rate-with growth of the YCo5 particles being the most sluggish-and, surprisingly, no correlation between the average size of the particles and their coercivity. These findings extend the practical limits associated with hard magnetic materials and demonstrate that the hard magnetic properties of the mechanochemically synthesized YCo5 and PrCo5 powders are similar to those of traditionally prepared SmCo5.