Crystal structure, magnetic properties and electronic structure of the MnBi intermetallic compound

The low-temperature phase of the MnBi alloy has a coercivity μ 0 H c of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product ( BH ) max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6 μ B , and that the Bi atom has a magnetic moment of −0.15 μ B which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6 μ B at a volume expansion rate of ΔV / V ≈ 100 % .