Preparation and magnetic properties of anisotropic MnBi powders

MnBi low-temperature phase (LTP) powders with composition of Mn50Bi50 were prepared using traditional casting followed by annealing and subsequent low energy ball milling with the assistance of heptane. The microstructure and magnetic performances were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and magnetometer measurements. The content of LTP MnBi increases with the extension of annealing time, and reaches 89.1 wt. % after 25 h annealing. The particle size decreases with increased milling time and powders with an average size of less than 1 μm can be effectively obtained by extending milling time to 5 h. The coercivity increases with milling time, but long-time ball milling greatly induces Ms. The magnetically aligned MnBi powders ball milled for 2 h exhibits a ratio Mr/Ms value of 96.2%, Ms of 55.1 emu/g and Hc of 14.1 kOe, respectively. The nanocrystal can be obtained during early brief time milling process, and more than 2 h ball milling has negligible effect on grain size.