Mechanochemical Synthesis of (Sm,Pr)2(Co,Fe) $_{17}$ Anisotropic Hard Magnetic Powders

Anisotropic Sm_0.85Pr_0.15(Co_1-xFe_x)_z alloy powders with x = 0.2-0.5, consisting mostly of the rhombohedral (Sm,Pr)₂(Co,Fe)₁₇ compound, were prepared from a mixture of cobalt, ferric oxide and rare earth oxides subjected to high-energy ball-milling in the presence of calcium reducing agent and calcium oxide dispersant. The milling was followed by a short-time annealing and multi-step washing. The resulting powders were polydispersed, but their average particle size could be tuned from 80 to 300 nm by controlling the excess Ca metal and the annealing temperature. A nanometer-thin oxide shell was observed on a surface of the particles, and it may be, at least in part, responsible for their excellent stability to air. The significant effect of the excess Ca on the particle size, anisotropy and chemical/structural homogeneity is discussed based on the supposed solution-precipitation mechanism of particle growth. Intrinsic coercivity H_c up to 14 kOe was obtained; powders with x = 0.4 and an average particle size of 121 nm exhibited a remanence of 106 emu/g and H_c of 8.3 kOe.