  The electronic structure and magnetic exchange interactions in pure and
V-doped Fe16N2 are studied within the framework of density functional theory.
The Curie temperatures were obtained with both mean field approximation (MFA)
as well as Monte Carlo (MC) calculations. The Curie temperature (TC) for pure
Fe16N2 obtained within MFA are significantly larger than the experimental
value, suggesting the importance of thermal fluctuations in these systems, and
has a resemblance of a lower dimensional spin system. We also briefly discuss
about the various possible factors which may lead to a large magnetic moment in
this material. The calculated magnetic susceptibility at zero field shows sharp
peak at T=TC which resemble a local moment system. From the nature of exchange
interactions we try to figure out the nature of the Fesites which might contain
localized d-states. Finally, we point out that Fe16N2 can also act as a good
spin injector for the III-V semiconductors in addition to its well promised
application as permanent magnet since it has a very high spin polarization
(larger compared to elemental ferromagnets) as well as quite smaller lattice
mismatch (compared to half-metallic Heusler alloys) with the conventional III-V
semiconductors such as GaAs or InGaAs. We further demonstrate this through our
calculations for Fe16N2(001)/InGaAs(001) heterostructures which shows the
non-negligible spin polarization in the semiconductor (InGaAs) region implying
a long spin diffusion length.
