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A series of Co x (Cr 2 O 3 ) 1−x granular films with various Co volume fractions x (x = 0.15-0.55) have been fabricated by RF co-sputtering. X-ray diffraction, transmission electron microscopy, energy dispersive x-ray and x-ray photoelectron spectrometer have been employed to characterize the structure, composition and element valence. VSM and the conventional four-point resistance measurement method have been used to investigate the magnetic and electrical properties. It is found that the films consist of Co particles with an average size of about 5 nm distributed in an amorphous Cr 2 O 3 matrix for x = 0.35. The isotropic tunnelling magnetoresistance (TMR) is obtained. TMR increases with decrease in x from 0.55, reaches a maximum of 5.1% at x = 0.35, then decreases with further decrease in Co volume fraction. This behaviour can be explained by the film microstructure variation and conductive mechanism. What is interesting is that TMR becomes negative, −3.5%, for the x = 0.15 sample. This behaviour was also observed in Fe-Cr-O granular films with the lowest Fe volume fraction (x = 0.15). This change in the sign of TMR implies that the barrier thickness of the metal-insulator-metal junction plays an important role in the determination of the spin polarization of tunnelling electrons.

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