Exploring secondary phases in the Sm–Fe–V system beneficial for coercivity

Magnetic isolation of hard magnetic grains with an intergranular phase is a key factor in obtaining high coercivity, which has been established in the microstructure of Nd-Fe-B magnets owing to the phase equilibrium of hard magnetic Nd2Fe14B with a low melting point phase. In this work, we investigated the phase equilibrium in the Sm-Fe-V system to establish an optimum composition range to develop high coercivity SmFe12-based permanent magnets. In addition to the solid 1:12/liquid Sm-rich phase equilibrium, an additional V-rich phase is found for the first time in the Sm-Fe-V phase diagram for V contents between 19 to 35 at.% at 1500 K. The equilibrium between the 1:12 phase and the two phases with low melting points is computationally modeled for Sm-Fe-V metastable phase diagram. This is experimentally confirmed by analyzing the microstructure of the Sm8Fe72V20 thin film, where anisotropic columnar 1:12 grains are enveloped with predominantly V-rich phases along with some Sm-rich intergranular phases resulting in a coercivity (µ0Hc) of 0.9 T. Although this large V content in Sm(Fe,V)12-based alloy is not beneficial for achieving a large magnetization, the finding of a new V-rich intergranular phase can benefit the development of anisotropic sintered magnets via two alloy powder methods; one alloy powder with large V-content for forming intergranular phase and the other with V-lean content to maintain a large magnetizations. To further improve the coercivity to its highest limit μ0Hc ≈0.2-0.3Ha ≈2-3 T, decreasing the ferromagnetic elements in the intergranular phase is essential.