Large magnetoresistance observed in facing-target sputtered Ni-doped CNx amorphous composite films

A series of amorphous Ni-doped CNx films with ∼23 at.% Ni were fabricated using facing-target sputtering at different nitrogen partial pressures (PN). The films were composed of ∼1–4 nm Ni-rich particles embedded in a CNx matrix and turn from ferromagnetic at low temperatures to superparamagnetic at room temperature. The largest negative magnetoresistance (MR = [R(H) − R(0)]/R(0)) reaches −59% at a PN of 4% and a temperature of 3 K. With a decrease of PN from 4% to 0%, the electrical transport mechanism changes from tunneling to variable-range hopping and the maximum MR drops from ∼59% to ∼3.8%. The MR–H curves show a weak saturation trend in a high-field regime and the MR–T curves follow the relation of log|MR| ∝ −T below 20 K for all the films, despite the difference in transport mechanism. The origin of the large MR (−59%) can be ascribed to a spin-related high-order tunneling process.