Microstructure, improved magnetic properties, and recoil loops characteristics for MnBi alloys

MnBi is a potential candidate for permanent magnets that can be used in high-temperature applications due to the large magnetocrystalline anisotropy and unique positive temperature coefficient of coercivity. In this work, MnBi magnets with a high content of low temperature phase (LTP) were prepared by arc melting followed by heat treatment. The high coercivity of 7.11 kOe at 300 K and 21.97 kOe at 500 K can be obtained for MnBi ingots, respectively. Two-step thermal annealing can promote the transformation from the Mn and Bi phases to form LTP MnBi, allowing a maximum remanence of 38.3 emu/g to be acquired. Open recoil loops for MnBi ingots have never been reported and are worthy of investigation. It was found that the non-uniform distribution of the Mn phase, Bi phase, LTP MnBi, and even the amorphous phase, as well as their large variation in size, could produce a non-negligible local demagnetization field with non-uniform distribution and resulted in open recoil loops. Nearly closed recoil loops can be achieved by improving the non-uniform distribution of the microstructure. The dependence of the magnetic properties and the recoil loops on the microstructure was investigated in detail.