Effect of Annealing Temperature and Composition on Magnetic Properties of Manganese-Bismuth Homogenized in Tube Furnace

Repeated heat treatment on manganese-bismuth (MnBi) in a tube furnace increases the homogeneity of rare-earth-free magnets. Ferromagnetic low temperature phase (LTP) MnBi is formed after heating Mn and Bi in a ratio of ۲:۱ at ۱۰۰۰ °C for ۱ h and then ۴۰۰ °C for ۱ h. Areas with comparable compositions of Mn and Bi are detected, but some Mn and Bi remains segregate after using this stepped heating ۳ times. The subsequent annealing at ۳۴۰ °C gives rise to higher magnetizations and coercivity than those using ۴۱۰ °C annealing. Increasing the starting Mn:Bi ratio to ۴:۱ reduces the coercivity and remanent magnetization due to the increase in Mn oxidation at the expense of ferromagnetic phase.

Keywords:

MnBi , Rare-earth-free magnet , Coercivity , Remanent magnetization , Tube furnace