Josephson current in superconductor-ferromagnet structures with a nonhomogeneous magnetization
Abstract
We calculate the dc Josephson current I_J 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 % h\tau\ll1 the expression for the Josephson critical current I_c is reduced to that which can be obtained from the Usadel equation (h is the exchange energy, \tau is the momentum relaxation time). In the opposite limit h\tau\gg1 the superconducting condensate oscillates with period % v_F/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 I_J in the case when the F layer presents a nonhomogeneous (spiral) magnetic structure with the period 2\pi /Q. It is shown that for not too low temperatures, the \pi-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 \xi_{T}=% \sqrt{D/2\pi T}. 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 I_{c} depends on the relative orientation of the effective exchange field h of the bilayers. In the case of an antiparallel orientation, I_{c} 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.