The influence of oxygen flux on the tunnelling magnetoresistance effect of Co-Al-O granular thin films

A series of Co-Al-O granular thin films were fabricated by RF reactive sputtering at various oxygen fluxes, and the tunnelling magnetoresistance (TMR), phase composition, microstructure and the conductance mechanism of the films were systematically investigated. X-ray diffraction and transmission electron microscopy revealed that the metal granules in the films gradually change from an AlCo alloy phase to a pure Co phase, and the granules become smaller with increasing oxygen flux. The TMR also varied with an increase of the oxygen flux and exhibited a maximum of -7% at 1.7 vol% oxygen flux. A crossover of the temperature coefficient of resistivity (TCR) from positive to negative with the increase of the oxygen flux is observed, and zero TCR is obtained at 1.0 vol% oxygen flux, implying a change of the conduction mechanism. These behaviours of the TMR and the TCR can be ascribed to the influence of the oxygen flux on the phase composition and the microstructure.