Depression of the superconducting critical temperature and finite-size scaling relation in YBa2Cu3O7−δ/La2/3Ca1/3MnO3 bilayers

High-quality YBa2Cu3O7−δ/La2/3Ca1/3MnO3 (YBCO/LCMO) bilayers were fabricated on (0 0 1)-oriented SrTiO3 (STO) substrates by dc-sputtering technique. Bottom layer was always LCMO since it grows better on STO than on YBCO. The thickness of the ferromagnetic layer varied between 5 and 35 monolayers (∼2–13 nm) and that of the top YBCO was fixed at 10 monolayers (∼12 nm). The transport properties of the YBCO layers as well as the magnetic properties of the LCMO counterparts were studied as a function of the LCMO layer thickness. A sizeable depression of the Curie temperature (TC) of the LCMO layers from the bulk to lower temperatures is observed when decreasing their layer thickness dLCMO, which might be ascribed to intrinsic dimensionality effects or strain-induced phenomena. On the contrary, the superconducting critical temperature of the YBCO layer TS displays a sudden strong decrease at a critical LCMO thickness of ∼12 nm. Since no dramatic change of the structural and morphological quality of the YBCO top layers with increasing dLCMO is observed, the suppression of TS of the YBCO layers should take place via proximity effect due to the increasing magnetization strength in the LCMO layer. However, extrinsic factors like interface strain, interdiffusion of cations between YBCO and LCMO or injection of spin-polarized carriers from the magnetic into the superconducting layer could also play an important role or even to be directly responsible for the observed depressed TS.