We study about various superconducting/magnetic multilayers, both theoretically and experimentally. First, we consider the two ferromagnetic layers of which spin orientation could be switched from the same direction to the opposite direction, which was intensively studied in the GMR effects, are in contact with superconductor. Theoretically, we solve the Usadel Eq. including spin-orbit interaction, a result of recent work by Demler et el., to calculate the variation of the critical temperature of superconductor by the change of spin alignment of the ferromagnetic layers. We found that there could be a large difference in the critical temperature, especially, in weak magnetic influence limit, for a thin magnetic layer of about ~$ε_m$/2. This effect is related with the pair breaking effect which is great for the ferromagnetically aligned case. Spin-orbit scattering effect reduces this effect. We expect this large difference in critical temperature, which is up to 40%, can be used for applications, such as superconducting-magnetic RAM.
To verify the above effect, we study about the V/Ni/Cu/Ni, V/Ni/Cu/Fe or V/Ni/Cu/Ni/NiO multilayers, experimentally. We found that the use of Ni/Fe multilayer, especially the use of NiO layer is very effective way to get the orientation controllable magnetic layers. Though we could get good quality V layer, we could not see the desired critical temperature variation. We believe that the fabrication of a well defined, 10~20Å thick magnetic layer and the micro-bridge structure which could avoid magnetic domain effect, will be crucial for future verification.
As a continuation of the above calculation, we theoretically study about the spin-orbit scattering effect on the critical temperature oscillation in superconducting/magnetic multilayers, which is actively debated recently. Our results show that there could be non-oscillatory behaviour, contradictory to the results of Radovic et el., particularly for materials of large spin-orbit scattering effect, such as Gd.
Moreover, the superconducting/spin-glass system was experimantally studied, also. $Au_{1-x}Cr_x$ thin foil was used, which showed clear magnetization difference between ZFC(Zero Field Cool) and FC(Field Cool) case. In opposition to the previous Abrikosov-Gor'kov theory on magnetic impurities, we could not observe the critical temperature difference between the two case. It suggest that there should be a new theoretical approach for the proximity effect between superconductor and spin glasses.
There has been another study on the cylindrically grown YBCO over YSZ substrate, which could find many applications, such as magnetic shield. Especially, to clarify the edge dislocation effect, we measured the R-T curve change by external field. The activation energy, calculated by Tinkham model, is similiar to that of a good quality YBCO thin film or single crystal. The other magnetoresistance effects also show that the edge dislocation effect is negligible.
As a more realistic model for the above YBCO thin film, we consider the Josephson junction array model. In particular, we include the effect of in-plane misalignment which is created during deposition procedure, as well as the mutual and the self inductance of each square lattice. Our model is in agreement with the experimental results of decreasing critical current by increasing in-plane misalignment. This model with the proper improvement, such as 3D array expansion taken into account of the anisotropy of high temperature superconductor and temperature effect, is expected to be useful for understanding much wider experimental results, such as magnetoresistance effect.
