Study of Unconventional Triplet Superconductivity and Spin-Valve Effects in Superconductor/Ferromagnet Nanolayered Heterostructures

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In heterostructures of a superconducting (S) and two ferromagnetic (F1,F2) nanolayers an odd in frequency s-wave triplet pairing generation occurs for a non-collinear orientation of the magnetizations (M1, M2) of the F-layers. This pairing channel is of extraordinary long range in the F-layers, because the equal-spin pairing is supported by the magnetized conduction band of the F-metal. The generation of this component significantly affects the transition temperature of both superconducting proximity effect spin-valve designs, F1/S/F2 and S/F1/F2. For the S/F1/F2 system the theory predicts an absolute minimum of the superconducting transition temperature close to the crossed configuration of M1 and M2. This effect is expected to be large (of order 1K between different alignments of the magnetizations) if the F-layers are magnetic alloys. Especially for these materials, we want to investigate the phenomenon in the present work and compare it with the corresponding phenomenon in F1/S/F2 structures. However, exchange biasing of a layer made of a diluted ferromagnetic alloy is a difficult task. Therefore, our recently improved method to exchange bias one of the ferromagnetic layers (especially important for a diluted ferromagnetic alloy) will be applied. In this way we also want to realize a large standard (or inverse) spin-valve effect of order 1K, as expected for the difference in the critical temperatures between parallel and antiparallel alignment of the magnetizations, for which the odd in frequency s-wave triplet pairing generation is not present. Moreover, we expect to solve the long-standing problem to distinguish the spin-valve effects from magnetic stray-field effects. To study the mutual influence of triplet components and the current transport in our systems, the spin-valve effects will be investigated at different applied magnetic fields and currents up to the critical one. Since our recent experiments on memory effects in a F/S/F type spin valve structure have shown evidence of non-equilibrium effects, we finally want to perform a pilot study on non-equilibrium superconductivity in S-F-heterostructures.