We study the critical temperature T_c of SFF trilayers (S is a singlet
superconductor, F is a ferromagnetic metal), where the long-range triplet
superconducting component is generated at noncollinear magnetizations of the F
layers. We demonstrate that T_c can be a nonmonotonic function of the angle
\alpha between the magnetizations of the two F layers. The minimum is achieved
at an intermediate \alpha, lying between the parallel (P, \alpha=0) and
antiparallel (AP, \alpha=\pi) cases. This implies a possibility of a "triplet"
spin-valve effect: at temperatures above the minimum T_c^{Tr} but below T_c^{P}
and T_c^{AP}, the system is superconducting only in the vicinity of the
collinear orientations. At certain parameters, we predict a reentrant
T_c(\alpha) behavior. At the same time, considering only the P and AP
orientations, we find that both the "standard" (T_c^{P} < T_c^{AP}) and
"inverse" (T_c^{P} > T_c^{AP}) switching effects are possible depending on
parameters of the system.Comment: 5 pages (including 4 EPS figures
We have investigated the differential resistance of hybrid planar Al-(Cu/Fe)-Al submicron bridges at low temperatures and in weak magnetic fields. The structure consists of a Cu/Fe bilayer forming a bridge between two superconducting Al electrodes. In the superconducting state of Al electrodes, we have observed a double-peak peculiarity in differential resistance of the S-(N/F)-S structures at a bias voltage corresponding to the minigap. We claim that this effect (the splitting of the minigap) is due to an electron spin polarization in the normal metal which is induced by the ferromagnet. We have demonstrated that the double-peak peculiarity is converted to a single peak at a coercive applied field corresponding to zero magnetization of the Fe layer.
We demonstrate that Josephson devices with nontrivial phase difference 0 < ϕ g < π in the ground state can be realized in structures composed from longitudinally oriented normal metal (N) and ferromagnet (F) films in the weak link region. Oscillatory coupling across F-layer makes the first harmonic in the current-phase relation relatively small, while coupling across N-layer provides negative sign of the second harmonic. To derive quantitative criteria for a ϕ-junction, we have solved two-dimensional boundary-value problem in the frame of Usadel equations for overlap and ramp geometries of S-NF-S structures. Our numerical estimates show that ϕ-junctions can be fabricated using up-to-date technology.
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