Sign of the spin-polarization in cobalt-iron nitride films determined by the anisotropic magnetoresistance effect

We present the anisotropic magnetoresistance (AMR) properties in Fe4N, Co3FeN and Co4N epitaxial thin films grown on SrTiO3(001) substrates using molecular beam epitaxy. A negative AMR effect was observed in the Fe4N and Co3FeN films below 300 K. This behavior was attributed to the negative spin-polarizations of both the electrical conductivity and density of states at the Fermi level. The sign of the AMR ratio changed at ∼140 K in the Co4N film and was negative below 140 K. Diffraction lines corresponding to the (100) plane were clearly observed in the ϕ-2θχ (in-plane) x-ray diffraction patterns for the Fe4N and Co3FeN films but not for the Co4N film. This indicated that nitrogen atoms were present at the body center of each unit cell in the Fe4N and Co3FeN films, whereas they were lacking in the Co4N film. This assumption is discussed with energetic calculations using a spin density-functional theory.