Electronic structure of lithium nitridoferrate: Effects of correlation and spin-orbit coupling

Lithium nitride Li₃N, in which part of Li is substituted by Fe, orders ferromagnetically with a very large magnetic moment on the iron ions. On specific iron sites a huge hyperfine field was also observed. Using the supercell approach we calculate the electronic structure of the Li₂[(Li_1-xFeₓ)N] system. For the iron ions which have only Li as the nearest cation neighbors, the dxy and d_x²-y² bands are intersected by the Fermi level and they are very narrow. Splitting of these bands by the spin-orbit coupling gives rise to a large orbital momentum, which is further enhanced by the correlation effects. To describe the strong correlation of iron $3d$ electrons the LDA+U method is used. The spin and orbital contributions to Fe magnetic momentum are then found to be comparable, while the orbital part of the hyperfine field in fact dominates. Huge, easy-axis-type magnetocrystalline anisotropy is predicted.