Spin pumping in d-wave superconductor/ferromagnet hybrids*

Spin-pumping across d-wave superconductors such as YBa₂Cu₃O_7-d (YBCO) in ferromagnet/superconductor interfaces provides a new playground for the study of spin polarized supercurrents due to the d-wave nature of the superconducting gap. Here we use wideband ferromagnetic resonance to study spin-pumping effects in bilayers that combine a soft metallic Ni₈₀Fe₂₀ (Py) ferromagnet and YBCO. We evaluate the spin conductance in YBCO by analyzing the magnetization dynamics in Py. We find that the Gilbert damping exhibits a drastic drop as the heterostructures are cooled across the normal-superconducting transition and that well below the critical temperature the damping depends on the surface morphology of the YBCO. In particular, an upturn of the damping appears at low temperatures and can be explained by the large density of quasiparticle bound states characteristic of d-wave superconductivity. Our hypothesis is that those states are accessible via YBCO crystallites at the surface, which directly exposes the YBCO ab plane to the interface with the ferromagnet. These findings open the path for tuning the spin pumping efficiency by engineering the YBCO surface, for example by growing YBCO in different crystallographic directions.