Structural and Magnetic Properties of ${\hbox {TbCu}}_{7}$ -type ${\hbox {SmCo}}_{7-x}{\hbox {Ni}}_{x}$

We have prepared nanograin magnetic powders of SmCo_7-xNi_x (x =0, 0.2, 0.3, 0.4, 0.5, 0.6, 1.0, 1.5, 2.0) alloys by ball milling followed by annealing at 700degC for 2 h. Samples with x=0 -1.0 show a single TbCu₇-type phase (1:7) structure, and the lattice parameters a and c of this structure decrease as the Ni content increases. The Curie temperature (T_c) of the 1 :7 phase also decreases with increasing x, from 897degC at x= 0 to 771degC at x =1.5 in these samples, but it is still found that the T_c of these SmCo_7-xNi_x samples is much higher than that of the equivalent SmCo_7-xM_x compounds, where M represents substituting atoms such as Ti, Zr, Cu, and Hf. Good square hysteresis loops are observed in the fine-grained samples with single 1:7 phase structure, and high room-temperature intrinsic coercivities (_iH_c) of larger than 1.1 T are obtained in the samples with x = 0-1.5. The temperature coefficient (beta) of the _iH_c was about -0.2%/degC in the temperature range of 25degC-400degC.