We consider novel unusual effects in superconductor-ferromagnet (S/F) structures. In particular we analyze the triplet component (TC) of the condensate generated in those systems.This component is odd in frequency and even in the momentum, which makes it insensitive to non-magnetic impurities. If the exchange field is not homogeneous in the system the triplet component is not destroyed even by a strong exchange field and can penetrate the ferromagnet over long distances. Some other effects considered here and caused by the proximity effect are: enhancement of the Josephson current due to the presence of the ferromagnet, induction of a magnetic moment in superconductors resulting in a screening of the magnetic moment, formation of periodic magnetic structures due to the influence of the superconductor, etc. We compare the theoretical predictions with existing experiments.

We analyze the proximity effect in a superconductor/ferromagnet (S/F) structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. We demonstrate that not only the singlet but also the triplet component of the superconducting condensate is induced in the ferromagnet due to the proximity effect. Although the singlet component of the condensate penetrates into the ferromagnet over a short length ξ h = D/h (h is the exchange field in the ferromagnet and D the diffusion coefficient), the triplet component, being of the order of the singlet one at the S/F interface, penetrates over a long length D/ǫ (ǫ is the energy). This long-range penetration leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature Tc.In recent experiments on S/F structures a considerable increase of the conductance below the superconducting critical temperature T c was observed [1][2][3]. Although in a recent work [4] it was suggested that such an increase may be due to scattering at the S/F interface, a careful measurement of the conductance demonstrated that the entire change of the conductance was due to an increase of the conductivity of the ferromagnet [1,2].Such an increase would not be a great surprise if instead of the ferromagnet one had a normal metal N. It is well known (see for review [5,6]) that in S/N structures proximity effects can lead to a considerable increase of the conductance of the N wire provided its length does not exceed the phase breaking length L ϕ . However in a S/F structure, if the superconducting pairing is singlet, the proximity effect is negligible at distances exceeding a much shorter length ∼ ξ h . This reduction of the proximity effect due to the exchange field h of the ferromagnet is clear from the picture of Cooper pairs consisting of electrons with opposite spins. The proximity effect is not considerably affected by the exchange energy only if the latter is small h < T c . As concerns such strong ferromagnets as F e or Co used in the experiments [1,2], whose exchange energy h is by several orders of magnitude larger than T c , a singlet pairing is impossible due to the strong difference in the energy dispersions for the two spin bands. At the same time, an arbitrary exchange field cannot destroy a triplet superconducting pairing because the spins of the electrons forming Cooper pairs are already parallel. A possible role of the triplet component in transport properties of S/F structures has been noticed in Refs. [7,8], where the triplet component arose only as a result of mesoscopic fluctuations. However, in both cases the corrections to the conductance are much smaller than the observed ones.In this paper, we suggest a much more robust mechanism of formation of the triplet pairing in S/F structures, which is due to a local inhomogeneity of the magnetization M in the vicinity of the S/F interface. We show that the inhomogeneity generates a triplet component of the superconducting order parameter with an amplitude co...

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