Theoretical analysis of highly spin-polarized transport in the iron nitride ${\mathrm{Fe}}_4\mathrm{N}$

In order to propose a ferromagnet exhibiting highly spin-polarized transport, we theoretically analyzed the spin polarization ratio of the conductivity of the bulk ${\mathrm{Fe}}_4\mathrm{N}$ with a perovskite-type structure, in which N is located at the body center position of the fcc-Fe. The spin polarization ratio is defined by $P=({\sigma }_{\uparrow }-{\sigma }_{\downarrow })∕({\sigma }_{\uparrow }+{\sigma }_{\downarrow })$, with ${\sigma }_{\uparrow (\downarrow )}$ being the conductivity at zero temperature of the up spin (down spin). The conductivity is obtained by using the Kubo formula and the Slater-Koster tight binding model, where parameters are determined from the least-square fitting of the dispersion curves by the tight binding model to those by the first principles calculation. In the vicinity of the Fermi energy, $\mid P\mid$ takes almost 1.0, indicating a perfectly spin-polarized transport. In addition, by comparing ${\mathrm{Fe}}_4\mathrm{N}$ to fcc-Fe $\left({\mathrm{Fe}}_4{\mathrm{N}}_0\right)$ in the ferromagnetic state with the equilibrium lattice constant of ${\mathrm{Fe}}_4\mathrm{N}$, it is shown that the nonmagnetic atom N plays an important role in increasing $\mid P\mid$.

Figure 1

Density of states, conductivities, and spin polarization ratios of ${\mathrm{Fe}}_4\mathrm{N}$: (a) $D_s\left(E\right)$ calculated using the FP calculation. (b) Schematic illustration of partial DOSs. The $4s\text{−}4p$ components are partly covered by the $3d$ ones (see darkish parts in $3d$ components). (c) $D_s\left(E\right)$ and $P_{\mathrm{DOS}}\left(E\right)$ for $-1\mathrm{eV}⩽E-E_{\mathrm{F}}⩽1\mathrm{eV}$ of the TB model and the FP calculation. (d) ${\sigma }_{i,s}\left(E\right)$, ${\sigma }_s\left(E\right)$, and $P\left(E\right)$ for $-1\mathrm{eV}⩽E-E_{\mathrm{F}}⩽1\mathrm{eV}$. Here, ${\sigma }_{2\mathrm{s},s}\left(E\right)$ and ${\sigma }_{2\mathrm{p},s}\left(E\right)$ for N are not shown because of their small values.

Figure 2

The same as Fig. 1 for fcc-Fe $\left({\mathrm{Fe}}_4{\mathrm{N}}_0\right)$ in the ferromagnetic state with the equilibrium lattice constant of ${\mathrm{Fe}}_4\mathrm{N}$: (a) $D_s\left(E\right)$ calculated using the FP calculation. (b) Schematic illustration of partial DOSs. The $4s\text{−}4p$ components are partly covered by the $3d$ ones (see darkish parts in $3d$ components). (c) $D_s\left(E\right)$ and $P_{\mathrm{DOS}}\left(E\right)$ for $-1\mathrm{eV}⩽E-E_{\mathrm{F}}⩽1\mathrm{eV}$ of the TB model and the FP calculation. (d) ${\sigma }_{i,s}\left(E\right)$, ${\sigma }_s\left(E\right)$, and $P\left(E\right)$ for $-1\mathrm{eV}⩽E-E_{\mathrm{F}}⩽1\mathrm{eV}$.