Magnetic proximity effect and superconducting triplet correlations at the heterostructure of cuprate superconductor and oxide spin valve

Abstract

We report on studies of heterostructure made of a cuprate superconductor YBa2Cu3O7-d, a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve, and thin gold film (Au). It is shown that a magnetic moment is excited in the cuprate superconductor due to magnetic proximity effect, at the same time magnetic moment is suppressed in the ruthenate/manganite part. The measurements showed that magnetic moment penetration depth significantly exceeds the coherence length of the cuprate superconductor. The induced magnetic moment could be attributed to coupling of the Cu and Mn atoms by a covalent chemical bond resulting in a strong hybridization and orbital reconstruction. The mesa-structures with micrometer sizes were prepared by adding superconducting niobium film (Nb) adjacent to the gold, forming a second superconducting electrode. The DC superconducting current flowing across the mesa-structure was observed even in the case when interlayer thicknesses were much greater than the coherence lengths of the ferromagnets in heterostructure. The maximum of the critical current took place when the thicknesses of ferromagnetic films in spin valve were near to the coherence lengths of the ferromagnets. Obtained data agree with the theoretical predictions for occurrence of the spin-triplet pairing. We measured superconducting current when applied magnetic field was by two orders greater than the field level required for one magnetic flux quantum nucleation in the mesa-structure. Although theory for long-range spin-triplet pairing predicts a dominance of the second harmonic, our estimation of the second harmonic amplitude in the current-phase relation of superconducting current did not exceed 50% of the first one.