Enhanced tunneling magnetoresistance of ${\mathrm{F}\mathrm{e}-\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$ granular films in the Coulomb blockade regime

Tunneling giant magnetoresistance (MR) effects in ${\mathrm{F}\mathrm{e}-\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$ 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 $x=0.45\mathrm{}$ when a magnetic field of 10 kOe was applied parallel to the film plane. The small tunneling MR at room temperature can be explained by the results of spin-flip processes. However, the observed temperature dependence of MR reveals an enhanced tunneling MR effect in the Coulomb blockade regime. The MR for $x=0.45\mathrm{}$ remarkably increases with decreasing temperature resulting in $\mathrm{MR}=20.1\%$ 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.