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

We present the anisotropic magnetoresistance (AMR) properties in Fe ₄N, 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 300K. 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 ∼140K in the Co₄N film and was negative below 140K. Diffraction lines corresponding to the (100) plane were clearly observed in the 2θ_χ (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.