Coulomb blockade and higher order tunneling effect in ultrasmall ferromagnetic double tunnel junction

A microscopic theory for ultrasmall ferromagnetic double tunnel junctions i s proposed, which selfconsistently describes the higher order tunneling pro cesses as well as the charged states of the islands for arbitrary environme ntal impedance. Such a theory is indispensable to understanding of the phys ics of this system, since stability of Coulomb blockade here is strongly de pendent on applied magnetic field. Tunneling currents and enhancement of ne gative magnetoresistance due to the Coulomb blockade are briefly discussed.