Structural Stability and Magnetic Properties of the TbCu$_{7}$ -Type SmCo$_{x - 0.4}$Ti$_{0.4} (x = 5.0\hbox{–}8.5)$ Alloys

We have studied the composition dependence, thermal stability, long-term stability at 500°C, and magnetic properties of the nanostructural TbCu
₇
-type (1:7) Sm-Co-Ti alloys. We prepared the SmCo
x
-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 CaCu
₅
-type (1:5) structure. At an annealing temperature higher than 800°C, a minor Th
₂
Zn
₁₇
-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.