ENHANCED MAGNETIC VALVE EFFECT AND MAGNETO-COULOMB OSCILLATIONS IN FERROMAGNETIC SINGLE-ELECTRON TRANSISTOR

We report on magnetotransport properties of a ferromagnetic single ele ctron transistor (SET) in which tunnel resistance RT changes about 3.6 -3.8% by the magnetic valve effect. We find that the magnetic valve ef fect is enhanced in the Coulomb blockade region: The magnetoresistance (MR) ratio of the off-state of the SET is more than 40%, while that o f 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, whic h results in magneto-Coulomb oscillations with fixed gate voltage. A s imple explanation for this effect based on a magnetic-field-induced ch ange in the Fermi energy of the spin-polarized electrons in an island electrode is given.