Electron transport in ferromagnetic small tunnel junctions

Both the single-electron charging effect and spin-dependent tunneling are important for electron transport in small tunnel junctions made of ferromagnetic metals. In this paper, we review our experiments on such systems. The hysteretic negative magnetoresistance at low magnetic fields, known as the tunnel magnetoresistance (TMR), is largely enhanced when the Coulomb blockade takes effect at low temperatures. Although such enhancement is expected to some extent by the quantum fluctuation of charge at strong tunneling regime, the quantitative agreement has not been obtained yet. The resistance of ferromagnetic single-electron transistors (SET) oscillates at high magnetic fields. A shift of the chemical potential in magnetic fields by the Zeeman effect causes the migration of electrons between the electrodes, which appears as resistance oscillations. Experimental results for various structures are consistent with such a model.