Microstructure and coercivity of Al-diffused Sm(Fe,Co,B)12 film

Fe-rich SmFe12-based compounds are considered as potential next generation permanent magnets due to their excellent intrinsic magnetic properties. However, their reported coercivities remain low, only -12 % of the magnetic anisotropy field of the SmFe12-based compound. In this work, we successfully increased the coercivity of B-doped Sm(Fe0.8Co0.2)12-based thin films from 1.1 T to 1.8 T by diffusion of Al into the grain boundaries. Detailed microstructural characterization using scanning transmission electron microscopy (STEM) and atom probe tomography (APT) revealed that the high coercivity is due to the segregation of Al into -4 nm thin B-rich amorphous intergranular phase, resulting in a reduction of its magnetization. This compositional modification of the intergranular phase can prevent the nucleation of reversed magnetic domains in the triple junctions and increase the pinning force of the grain boundary region against the magnetic domain wall motion. Based on detailed microstructural observations, the mechanism of coercivity enhancement is discussed.