Enhanced tunneling magnetoresistance of Fe-Al2O3 granular films in the Coulomb blockade regime

Tunneling giant magnetoresistance (MR) effects in Fe-Al2O3 granular films that were prepared by cosputtering have been reported. MR of the samples have been measured over a wide range of volume fractions of Fe (x) and it takes a maximum of 4.4% at room temperature for the film with x = 0.45 when a magnetic field of 10 kQe was applied parallel to the film plane. The small tunneling MR at room temperature can be explained by the results of spin-hip processes. However, the observed temperature dependence of MR reveals an enhanced tunneling MR effect in the Coulomb blockade regime. The MR for x = 0.45 remarkably increases with decreasing temperature resulting in MR = 20.1% at 5 K, which is larger than the theory value for Fe-based granular systems. This enhancement of MR effect in the Coulomb blockade regime seems to arise not only from the suppression of the spin-flip process with decreasing temperature, but also from the spin dependent cotunneling in the Coulomb blockade regime. [S0163-1829(99)01238-2].