Microstructure change in Co46Al19O35 granular thin films by annealing

We have studied changes in microstructures of Co46Al19O35 granular thin fil ms on annealing by transmission electron microscopy (TEM) and small angle x -ray scattering (SAXS). The as-deposited from exhibits a large magnetoresis tance (MR) ratio of approximately 9% at room temperature. The films consist of nearly spherical cobalt particles of a few nanometer in diameter embedd ed in an amorphous aluminum oxide matrix. Although the MR ratio decreases d uring 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 m in annealing. Upon further annealing, the interparticle distance does not c hange noticeably, but the MR ratio continues to decrease. Electron diffract ion results show that a considerable amount of cobalt oxide appears during long term annealing. Thus the degradation of the MR ratio after long term a nnealing is attributed to oxidation of the Co particles. (C) 1999 Acta Meta llurgica Inc.