FERROMAGNETIC SINGLE-ELECTRON TRANSISTOR

We studied magnetotransport properties of a single electron transistor (SET) consist of Ni/NiO/Co ultrasmall tunnel junctions. At high tempe ratures or at high bias conditions,the SET shows magnetoresistance (MR ) of about 4% which originates from the magnetic valve effect. To inve stigate the magnetic valve effect in Coulomb blockade regime, we measu red the zero-bias MR of the SET for on-state (resistance minimum of Co ulomb oscillations) and off-state (resistance maximum of them) as a fu nction of temperature. The MR ratio of the off-state starts growing bl ow 0.4 K and exceeds 40% at 20 mK, while that of the on-state is tempe rature independent (approximate to 4%). This effect is qualitatively e xplained by quantum fluctuation of charge. We also found that the magn etic held affects the phase of the Coulomb oscillations. A qualitative explanation for this effect is given which is based on spin polarizat ions of the island and the lead electrodes. (C) 1998 Elsevier Science Ltd. All rights reserved.