Magnetic anisotropy of C and N doped bulk FeCo alloy: A first principles study

•Investigation of the magnetic properties of C and N doped FeCo alloy.•The tetragonal lattice distortion due to C or N impurity induced a large magnetic anisotropy.•The hybridization with impurity atom was not a major factor for the large anisotropy.•In addition, we found a large coercive field in C or N doped system.•Overall, we propose that the C or N doped FeCo can be utilized as potential rare earth free permanent magnet.

Using the full potential linearized augmented plane wave (FLAPW) method, we investigated the magnetocrystalline anisotropy of carbon and nitrogen doped FeCo in the interstitial site. Here, we have considered 3.125% impurity doping concentration. The impurity doping induces a tetragonal distortion in the FeCo lattice, and both carbon and nitrogen impurities play a similar role for lattice distortion. The local magnetic moment of Fe atom around the impurity site was greatly reduced, whereas the Co had rather robust magnetic moment. We found a uniaxial magnetocrystalline anisotropy constant of 0.65 and 0.58 MJ/m3 for C and N doped bulk FeCo, and this was mainly due to the tetragonal distortion induced by C and N impurity, not from the hybridization effect with Fe or Co. Additionally, the estimated maximum energy product and coercive field were 81.4, 72.5 MGOe and 600, 530 kA m−1 for C and N doped bulk FeCo, respectively. These results may imply that the interstitial C or N doped FeCo can be used for potential rare earth free permanent magnet although those values are likely to be suppressed in real samples due to micromagnetic factors such as structural defects, geometrical effect, or grain boundary effect.