Tunnel Magnetoresistance in Mesoscopic Ni/Co/Ni Magnetic Double Junctions with AlO_x Tunnel Barriers

We have studied the electrical transport properties of a ferromagnetic single electron transistor consisting of a Ni/Al₂O ₃/Co/Al₂O₃/Ni double junction with side control gates. The dc current-voltage characteristics of the double junction show a clear Coulomb blockade effect at low temperatures and the tunnel current exhibits periodic Coulomb oscillations when the gate voltage is varied. The tunnel magnetoresistance curves show characteristic hysteretic peaks due to the different coercivities of the Ni and the Co films. In the Coulomb blockade region, the tunnel magnetoresistance ratio depends sensitively on the bias voltage and temperatures. The magnetoresistance ratio is enhanced by a factor of more than 2 in the Coulomb blockade region, which is consistent with the theoretical prediction based on the co-tunneling process of electrons.