Enhanced magnetoresistance of permalloy/Al-oxide/cobalt tunnel junctions in the Coulomb blockade regime

We report on magnetotransport properties of arrays of cobalt/aluminum-oxide/permalloy tunnel junctions with lateral dimensions in the range of 100×200 ${\mathrm{nm}}^2$. Due to the small capacitances, these devices show simultaneously a pronounced Coulomb blockade behavior at low temperatures and spin-dependent tunneling transport. The Coulomb blockade energies range from 0.1 to 2.6 meV. All tunnel junctions exhibiting Coulomb blockade behavior did also show a strong dependence of the tunnel current on an external magnetic field, i.e., a tunneling magnetoresistance. At temperatures below 100 K, the resistance changes up to 17% in magnetic fields of ∼500 Oe. In addition to the known slight increase of the tunneling magnetoresistance with decreasing bias voltage, we found a strong enhancement in the Coulomb blockade regime. This enhancement is explained by cotunneling effects. Considerations towards possible application in devices are given.