Temperature dependence of structure and magnetic properties in MnBi-based alloy films

MnBi alloy films, which have large magnetocrystalline anisotropy and high remanence, are synthesized with small dopants of rare-earth elements (RE=Dy, Sm, or Tb) by vacuum evaporation method. In contrast with most ferromagnetic materials, the low temperature phase (LTP) of MnBi film shows a large coercivity, Hc, over an extended temperature range. The hexagonal crystal structure of MnBi LTP thus has good thermal stability. The experiments found that doping further extends the temperature range through which the LTP is stable. Moreover, MnBi-doped films still exhibit square hysteresis loops at high temperatures. We attribute the unusual temperature dependence of the coercivity to an increased perpendicular anisotropy field, Ha, with temperature. For this reason, MnBi-based alloy films have great potential as permanent magnets at high temperatures.