First-principles study of the complex magnetism in Fe₁₆N₂

Magnetic exchange interactions in pure and vanadium (V)-doped Fe₁₆N₂ are studied within the framework of density functional theory (DFT). The Curie temperatures were obtained via both mean field approximation (MFA) and Monte Carlo (MC) calculations based on interactions that were obtained through DFT. The Curie temperature (T_C) for pure Fe₁₆N₂ that was obtained under MFA is substantially larger than the experimental value, suggesting the importance of thermal fluctuations. At zero field, the calculated magnetic susceptibility shows a sharp peak at T = T_C that corresponds to the presence of localized d-states. From the nature of the exchange interactions, we have determined the reason for the occurrence of the giant magnetic moment in this material, which remained a mystery for decades. Finally, we posit that Fe₁₆N₂ can also act as a satisfactory spin injector for III-V semiconductors, in addition to its application as a permanent magnet, since it has very high spin polarization (compared to elemental ferromagnets) and smaller lattice mismatch (compared to half-metallic Heusler alloys) with conventional III-V semiconductors such as GaAs and InGaAs. We demonstrate this application in the case of Fe₁₆N₂(001)/InGaAs(001) hetero-structures, which exhibit substantial spin polarization in the semiconductor (InGaAs) region. PACS number: 82.65.My, 82.20.Pm, 82.30.Lp, 82.65.Jv.