Synthesis of Cobalt-Based Carbide Particles by Polyol Process and Their Magnetic Properties

Abstract

The metastable cobalt carbide with the Fe 3 C-type structure (Pnma) has attached much attention as a promising candidate for application to rare-earth free permanent magnets, because large magnetoanisotropy of Co3c has been predicted by the first-principal calculation [1]. The particles of Fe 3 C-type cobalt carbide have been synthesized by reduction of cobalt ions in polyol solution [2]. In this study, particles were synthesized at about 563 K for 30 min in tetraethyleneglycol (TEG) using chemicals such as Co acetate tetrahydrate, Ni acetate tetrahydrate and polyvinylpyrrolidone (PVP). The influence of both PVP and coexisting Ni ions on the precipitation of Fe 3 C-type cobalt carbide particles in polyol solution was examined. Figure 1 shows X-ray diffraction patterns of particles synthesized with and without PVP. When Co ions are reduced in solution without PVP, particles composed of fcc and hcp structures are precipitated. On the other hand, the diffraction peaks of particles synthesized in solution containing PVP are assigned primarily as the Fe 3 C-type structure. Therefore, the presence of PVP in TEG solution accelerates the formation of Fe 3 C-type cobalt carbide. The carbide particles exhibited a coercivity of about 1.7 kOe at room temperature. As shown in Fig. 1, particles synthesized in solution with the initial molar ratio of Co:Ni = 9:1 are also identified as the Fe 3 C-type structure. Their local structure around Ni atoms was investigated by a extended X-ray absorption fine structure (EXAFS) spectrum measurement. The result is shown in Fig. 2(a). The shape of EXAFS spectrum of particles synthesized in solution containing Ni ions is similar to that of Co K edge of Fe 3 C-type cobalt carbide particles as indicated in Fig. 2(b). Consequently, particles of Ni-substituted cobalt carbide with the Fe 3 C-type structure are obtained by controlling reduction conditions.