Magnetic and structural properties of SmCo_7-xTi_x magnets

The main phase in as-cast SmCo_7-xTi_x (x = 0.3, 0.4, 0.5, 0.6) magnets is found to have the TbCu₇-type structure. The c/a ratio decreases with increasing Ti concentration x, but the unit cell volume increase with x. A minor amount of a rhombohedral Th₂Zn₁₇-type phase was detected at x = 0.3, 0.4, and an another Ce₂Ni₇-type phase with low Curie temperature appeared when x exceeds 0.5. No more than 0.1 T of intrinsic coercivity was obtained in the as-cast samples. After annealing, the TbCu₇-type SmCo_7-xTi_x magnets decomposed into 1:5 and 2:17 phases, with a slight increase in coercivity compared with the corresponding as-cast samples up to a maximum of 0.24 T. The as-milled amorphous SmCo_7-xTi_x magnets recrystallised with TbCu₇-type structure after annealing at 750°C for 2 h. High intrinsic coercivities of more than 2.0T were obtained for almost all compositions, and maximum coercivity of 2.5T was found for the ball-milled SmCo_6.6Ti_0.4 magnet. The high coercivity of the ball-milled SmCo_7-xTi_x magnets is attributed to the strong pinning of the walls of the 'interaction domains' by the network of grain boundaries between the nanocrystalline grains.