Suppressing antiferromagnetic coupling in rare-earth free ferromagnetic MnBi-Cu permanent magnet

Rare-earth free, ferromagnetic MnBi shows a positive temperature coefficient of coercivity from room temperature to 400K and energy product (BH)(max) of 17.7MGOe at 300K. However, MnBi undergoes a first-order structural phase transformation from a ferromagnetic low-temperature phase (LTP) to a paramagnetic high-temperature phase at 613K below the Curie temperature (T-c) of 716K. The transformation is attributed to Mn diffusion into the interstitial site of LTP MnBi unit cell. Interstitial Mn antiferromagnetically couples with the Mn at lattice 2a site, lowering the magnetization. Cu-occupied bipyramidal sites are investigated as a possible means to suppress Mn diffusion into the bipyramidal sites using first-principles calculations based on the density functional theory. Saturation magnetization, magnetocrystalline anisotropy constant (K), and T-c of (Mn0.5Bi0.5)(100-x)Cu-x (x=0-33) are reported. The magnetocrystalline anisotropy changes to the out-of-plane direction (x=13) from the in-plane direction (x=0.0). T-c decreases gradually to 578K at x=33 from 716K at x=0.0. The calculations show a slightly lower (BH)(max) of 15.6MGOe while it is expected that Cu-occupied interstitial sites will significantly suppress Mn diffusion and raise the temperature of the phase transformation.