Nodal superconducting exchange coupling

A superconducting spin valve consists of a thin-film superconductor between two ferromagnetic layers. A change of magnetization alignment shifts the superconducting transition temperature (ΔΤ_c) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of ΔΤ_c scales inversely with the superconductor thickness (d_S) and is zero when d_S exceeds the superconducting coherence length (ξ). Here, we report a superconducting spin-valve effect involving a different underlying mechanism in which magnetization alignment and ΔΤ_c are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa₂Cu₃O_7-δ sandwiched between insulating layers of ferromagnetic Pr_0.8Ca_0.2MnO₃. We observe ΔΤ_c values that approach 2 K with the sign of ΔΤ_c oscillating with d_S over a length scale exceeding 100ξ and, for particular values of d_S, the superconducting state reinforces an antiparallel magnetization alignment. These results pave the way to all-oxide superconducting memory in which superconductivity modulates the magnetic state.