Low frequency and weakly nonlinear spin transport in ferromagnet/insulatorsingle and double junctions

In this paper we apply Buttikers gauge invariant, charge conservation, nonl inear transport theory to explore the spin-polarized tunneling of ferromagn et/insulator (semiconductor) single and double junctions. The Green functio n of spin-polarized tunneling is calculated by the tight-binding approximat ion method. The energy and the angle (between the molecular field and the v ertical axis) dependences of the weakly nonlinear dc transport coefficient and the linear low-frequency ac transport coefficient are investigated. The ac tunneling magnetoresistance is also discussed.