Electron transport in ferromagnetic small tunnel junctions

Both the single-electron charging effect and spin-dependent tunneling are i mportant for electron transport in small tunnel junctions made of ferromagn etic 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 blocka de takes effect at low temperatures. Although such enhancement is expected to some extent by the quantum fluctuation of charge at strong tunneling reg ime, the quantitative agreement has not been obtained yet. The resistance o f ferromagnetic single-electron transistors (SET) oscillates at high magnet ic fields. A shift of the: chemical potential in magnetic fields by the Zee man effect causes the migration of electrons between the electrodes, which appears as resistance oscillations. Experimental results for various struct ures are consistent with such a model. (C) 2001 Elsevier Science B.V. All r ights reserved.