Magnetic-field-enhanced reactive synthesis of MnBi from Mn nanoparticles

The isotropic nanoparticles (NPs) and anisotropic bulk samples of MnBi were prepared from Mn NPs by using zero-field and field-assisted reactive sintering, respectively. The MnBi NPs are irregular in shape with size d < 600 nm, which is slightly larger than the precursor Mn NPs. The fraction of MnBi in the zero-field sintered NPs is higher than 68.6 wt%. The large surface area and small size of the Mn NPs enhanced the formation of ferromagnetic phase during sintering and the magnetic performance of the MnBi NPs, which show a coercivity (H-c) up to 0.8 T and a remanent magnetization (M-r) up to 44.5 Am-2/kg at room temperature. The magnetic fields enhanced the formation of MnBi phase significantly and oriented the c-axis of the MnBi grains grown along the external fields during reactive sintering. A fraction of 63 wt% MnBi was formed in field-sintering within 2 h, whereas only 58.5 wt% MnBi was formed in zero-field-sintering for 48 h. The room-temperature H-c and M-r of the field-sintered bulk anisotropic MnBi reached up to 0.12-0.17 T and 37 Am-2/kg, respectively.