Spin-dependent tunneling phenomena in insulating granular systems

We studied giant magnetoresistance (GMR) and the related properties in Co-Al-O insulating granular films prepared by reactive sputtering. Microstructural observation revealed that the films were composed of Co base metallic granules and Al2O3 base insulating network-like boundaries. The GMR varied as a function of oxygen content (or Co/Al2O3 ratio) and it took a maximum of 8% at room temperature and 17% at 4.2 K for a Co52Al20O28 film, accompanying huge specific electrical resistivities of the order of 10(5)-10(6) mu Omega cm. It has been considered that the observed GMRs are attributed to a spin-dependent tunneling and that the spin diffusion of electrons during tunneling plays an important role of the composition dependence of GMR. In addition, it has been noted that the temperature dependence of GMR is smaller than the T-1 dependence predicted theoretically by Helman et al. but rather close to the theory by Inoue et al.