Structural Stability and Magnetic Properties of the TbCu $_{7}$ -Type SmCo $_{x - 0.4}$ Ti

We have studied the composition dependence, thermal stability, long-term stability at 500°C, and magnetic properties of the nanostructural TbCu₇-type (1:7) Sm-Co-Ti alloys. We prepared the SmCox-0.4Ti_0.4 alloys with a wide composition range from x = 5.0 to x = 8.5 by high-energy ball-milling, followed by annealing at 700-1100°C for 2 h. After annealing at 700°C, the powders with x = 7.0-8.5 showed a single 1:7 structure, while the powders with x = 5.0-6.5 presented the 1:7 plus CaCu₅-type (1:5) structure. At an annealing temperature higher than 800°C, a minor Th₂Zn₁₇-type (2:17) phase precipitated in the matrix of the 1:7 phase. Intrinsic coercivity iHc exhibits a maximum of 2.3 T at room temperature and 0.4 T at 500°C in the x = 7.0 samples annealed at 700°C. The temperature coefficient of iHc seems stable as the Sm/Co ratio changes from 1/6.5 to 1/7.5. The coercivity decreased with increasing annealing temperature Ta, from 2.3 T at Ta = 700°C to 1.3 T at Ta = 1100°C, which is mainly attributed to the grain growth from 35 nm for Ta = 700°C to 1 ¿m for Ta = 1100°C. After holding at 500°C for up to 360 h, the microstructure and magnetic properties of the 1:7-type nanograin alloys remained almost unchanged, indicating a structurally and magnetically long-term stabilization at the potential high-temperature application environment.