We have studied the dependence of the superconducting (SC) transition temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers in the spin valve system CoO(x)/Fe1/Cu/Fe2/Pb. We find that this dependence is nonmonotonic when passing from the parallel to the antiparallel case and reveals a distinct minimum near the orthogonal configuration. The analysis of the data in the framework of the SC triplet spin valve theory gives direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the two magnetizations.

The critical temperature T_c of a superconductor/ferromagnet (SF) bilayer can exhibit nonmonotonic dependence on the thickness d_f of the F layer. SF systems have been studied for a long time; according to the experimental situation, the "dirty" limit is often considered which implies that the mean free path in the layers is the second smallest spatial scale after the Fermi wavelength. However, all calculations reported for the dirty limit were done with some additional assumptions, which can be violated in actual experiments. Therefore, we develop a general method (to be exact, two independent methods) for investigating T_c as a function of the bilayer's parameters in the dirty case. Comparing our theory with experiment, we obtain good agreement. In the general case, we observe three characteristic types of T_c(d_f) behavior: 1) nonmonotonic decay of T_c to a finite value exhibiting a minimum at particular d_f, 2) reentrant behavior, characterized by vanishing of T_c in a certain interval of d_f and finite values otherwise, 3) monotonic decay of T_c and vanishing at finite d_f. Qualitatively, the nonmonotonic behavior of T_c(d_f) is explained by the interference of quasiparticles in the F layer, which can be either constructive or destructive depending on the value of d_f.Comment: 13 pages (including 7 EPS figures), REVTeX 4. Version 2: minor correction

We consider the minigap in a disordered ferromagnet (F) in contact with a superconductor (S) in the situation when the magnetization of the F layer is inhomogeneous in space and noncollinear. If the magnetization is strongly inhomogeneous, it effectively averages out, and the minigap survives up to the exchange field hc ∼ (L/a)E Th , where L is the thickness of the F layer, a is the scale on which the magnetization varies, and E Th is the Thouless energy. Technically, we use the "triplet" version of the Usadel equations, including both singlet and triplet components of the Green's functions. In many cases, the effect of disordered magnetization may be effectively included in the conventional Usadel equations as the spin-flip scattering term. In the case of low-dimensional magnetic inhomogeneities (we consider spiral magnetization as an example), however, the full set of "triplet" equations must be solved.