Spin-triplet superconducting current in metal-oxide heterostructures with composite ferromagnetic interlayer

Abstract

Superconducting heterostructures fabricated from oxide superconductor $\text{YBa}_{2}\text{Cu}_{2}\text{O}_{7-\delta}$ and a composite ferromagnet La0.7Sr0.3MnO3/SrRuO3 interlayer and Au/Nb counter electrode were studied experimentally. Superconducting current was observed at magnetic field $H$ raised up to 2000 Oe, which is greater than saturation magnetic field of manganite La0.7Sr0.3MnO3 (of order 100 Oe) and greater by a few orders than the value of magnetic field corresponding to penetration of one magnetic flux quantum. Microwave measurements of integer and half-integer Shapiro steps in conditions when relatively low external magnetic field $H was applied showed that the second harmonic in the current-phase relation of superconducting current becomes as big as the first harmonic. Fourier analysis of $I_{C}(H)$ dependence allows extracting the components of fractional periods in $I_{C}(H)$ function that also confirms a deviation from the sinusoidal current-phase relation. The obtained experimental data are explained by theoretical models that predict a huge enhancement of the second harmonic of the spin-triplet component in the superconducting current. The current-phase relation could be controlled by an external magnetic field, changing the directions of magnetization in the composite bilayer ferromagnet, which is inserted between two spin-singlet superconductors.