Abstract

Repeated heat treatment on manganese-bismuth (MnBi) in a tube furnace increased the homogeneity of rare-earth-free magnets. Ferromagnetic low temperature phase (LTP) MnBi, was formed after heating Mn and Bi in a 2:1 ratio at 1000°C for 1 h and then at 400°C for 1 h. Areas with comparable amounts of Mn and Bi were detected, but some Mn and Bi elements remained segregated after using the above heat treatment schedule for 3 times. The subsequent annealing at 340°C gave rise to higher magnetizations and coercivity compared to using 410°C as an annealing temperature. Increasing the starting Mn:Bi ratio to 4:1 reduced the coercivity and remanent magnetization as a result of an increase in the Mn oxidation.