Anomalous spin-pumping behavior of half-metallic ferromagnet/d-wave superconductor heterostructures
Spin-pumping experiments in superconductor/ferromagnet heterostructures, which probe spin-sinking by the superconductor, have revealed a variety of complex behaviors. Most studies have focused on conventional s-wave superconductors combined with metallic or insulating ferromagnets. Here, we study a d-wave superconductor paired with a half-metallic ferromagnet, in epitaxial YBa2Cu3O7-d/La0.7Sr0.3MnO3 heterostructures with two crystalline orientations: one in which YBCO is c-axis oriented, and the other in which YBCO grows along the (103) direction. Using ferromagnetic resonance (FMR), we probe the temperature-dependent Gilbert damping coefficient {\alpha}. For (103) heterostructures, {\alpha}(T) initially decreases below Tc, but then increases at lower temperatures, exceeding normal-state levels. This behavior can be understood in terms of the opening of the superconducting gap and spin transport via nodal quasiparticles, which dominate when the ab-plane of YBCO is exposed at the interface. In stark contrast, c-axis heterostructures exhibit a pronounced enhancement of {\alpha}(T) below Tc, peaking at 0.65-0.7Tc before decaying. This anomaly suggests the dominance of interface-bound Andreev states, arising from a locally suppressed superconducting order parameter due to proximity effects with the half-metallic LSMO.