Negative anisotropic magnetoresistance resulting from minority spin transport in Ni_xFe_4−xN (x = 1 and 3) epitaxial films

We grew 50 nm-thick Ni_xFe_4−xN (x = 1 and 3) epitaxial films on a SrTiO₃(001) single-crystal substrate by molecular beam epitaxy and measured their anisotropic magnetoresistance (AMR) ratios rAMR in the temperature range of 5-300 K with current directions set along either Ni_xFe_4−xN [100] or [110]. A negative rAMR was obtained up to 200 K or higher. Their magnitude | rAMR | increased with decreasing temperature. From the negative AMR effect and the negative spin-polarization of density of states for Ni_xFe_4−xN at the Fermi level, it can be stated that the minority spin transport is dominant in Ni_xFe_4−xN, similar to Fe₄N and Co₃FeN. The rAMR depends on the current direction that arises from the current direction dependence of s-d scattering. In the case of Ni₃FeN, the rAMR decreased to nearly zero at 260 K. This temperature agreed well with the Curie temperature determined from the temperature dependence of magnetization. The AMR curves were reproduced well by using both cos2ϕ and cos4ϕ components below 100 K, whereas a cos2ϕ component was enough to fit those obtained above 100 K. It is assumed that the tetragonal crystal field was enhanced at low temperatures (<100 K) similar to Fe₄N (<50 K).