Microstructure change in Co46Al19O35 granular thin films by annealing

We have studied changes in microstructures of Co46Al19O35 granular thin films on annealing by transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS). The as-deposited from exhibits a large magnetoresistance (MR) ratio of approximately 9% at room temperature. The films consist of nearly spherical cobalt particles of a few nanometer in diameter embedded in an amorphous aluminum oxide matrix. Although the MR ratio decreases during annealing at 300 degrees C, no visible changes in the microstructure are evident. However, the interparticle distance of the particles estimated from the peak position in the SAXS data shows apparent increase after 10 min annealing. Upon further annealing, the interparticle distance does not change noticeably, but the MR ratio continues to decrease. Electron diffraction results show that a considerable amount of cobalt oxide appears during long term annealing. Thus the degradation of the MR ratio after long term annealing is attributed to oxidation of the Co particles. (C) 1999 Acta Metallurgica Inc.