Josephson current in superconductor-ferromagnet structures with a nonhomogeneous magnetization

Abstract

We calculate the dc Josephson current IJ for two types of superconductor-ferromagnet (S/F) Josephson junctions. The junction of the first type is a S/F/S junction. On the basis of the Eilenberger equation, the Josephson current is calculated for an arbitrary impurity concentration. If hτ≪1, the expression for the Josephson critical current Ic is reduced to that which can be obtained from the Usadel equation (h is the exchange energy, and τ is the momentum relaxation time). In the opposite limit hτ≫1 the superconducting condensate oscillates with period vF/h and penetrates into the F region over distances of the order of the mean free path l. For this kind of junctions we also calculate IJ in the case when the F layer presents a nonhomogeneous (spiral) magnetic structure with the period 2π/Q. It is shown that for not too low temperatures, the π state which occurs in the case of a homogeneous magnetization (Q=0) may disappear even at small values of Q. In this nonhomogeneous case, the superconducting condensate has a nonzero triplet component and can penetrate into the F layer over a long distance of the order of ξT=D/2πT. The junction of the second type consists of two S/F bilayers separated by a thin insulating film. It is shown that the critical Josephson current Ic depends on the relative orientation of the effective exchange field h of the bilayers. In the case of an antiparallel orientation, Ic increases with increasing h. We establish also that in the F film deposited on a superconductor, the Meissner current created by the internal magnetic field may be both diamagnetic or paramagnetic.

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