Coercivity of titanium-substituted high-temperature permanent magnets

The temperature dependence of the coercivity of Sm-Co based magnets is investigated by magnetization measurements and model calculations. The Zr-free titanium-substituted Sm-Co material exhibits a positive temperature coefficient dH_c/dT of the coercivity (TCC) above room temperature, a reasonable hysteresis-loop shape, and an appreciable coercivity of 12.3 kOe at 500°C for the nominal composition Sm(Co_6.2Cu_0.8Ti_0.3). The samples were produced by heat-treating the disordered 1:5 alloy commonly referred to as the TbCu₇ (or 1:7) phase. X-ray diffraction analysis shows that, upon annealing at 1165°C, the starting material segregates into more-or-less stoichiometric 1:5 and 2:17 phases. The TCC is explained by taking into account that two-phase Sm-Co magnets are of the pinning type, that is the coercivity is realized by capturing (or repelling) domain walls at 1:5/2:17 phase boundaries. Starting from a planar-defect approach, the TCC is modeled as a function of the anisotropy constants of the involved phases. The present approach yields a fair agreement between theory and experiment, and explains the existence of a coercivity maximum in terms of the Cu concentration