Enhanced Magnetic Valve Effect and Magneto-Coulomb Oscillations in Ferromagnetic Single Electron Transistor

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Abstract
We report on magnetotransport properties of a ferromagnetic single electron transistor (SET) in which tunnel resistance R T changes about 3.6-3.8% by the magnetic valve effect. We find that the magnetic valve effect is enhanced in the Coulomb blockade region: The magnetoresistance (MR) ratio of the off-state of the SET is more than 40%, while that of the on-state is 4.0%. A mechanism of enhancement by the higher-order tunneling process is proposed. Furthermore, we find a monotonic phase shift of the Coulomb oscillations induced by the magnetic field, which results in magneto-Coulomb oscillations with fixed gate voltage. A simple explanation for this effect based on a magnetic-field-induced change in the Fermi energy of the spin-polarized electrons in an island electrode is given.