Spin polarized transport of iron nitride Fe₄N: Analysis using a combination of first principles calculation and model calculation

To propose a ferromagnet exhibiting a highly spin-polarized transport, we analyze the spin polarization ratio of the bulk Fe₄N (Curie temperature: 761 K), which is focused on by taking into account the fact that a ferromagnetic electrode containing Fe and a light element enhances the tunnel magnetoresistance effect at room temperature. The spin polarization ratio is here defined by P = (σ↑-σ↓)/(σ↑+σ↓), with σ↑(σ↓) being the conductivity at zero temperature of up spin (down spin). The conductivity is obtained using the Kubo formula and the Slater-Koster tight binding model, where parameters are determined by fitting the band structure of the model to that of the first principles calculation. In the vicinity of the Fermi energy, |P| takes almost 1.0, which is 5.0 times as large as that of bcc-Fe. It may suggest that Fe₄N is very effective as a spin injection electrode. Also, by comparing Fe₄N to fcc-Fe in the ferromagnetic state with the equilibrium lattice constant of Fe₄N, it is shown that N plays an important role in increasing |P|.