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 (ΔΤc) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of ΔΤc 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 ΔΤc are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa_2Cu_3O_(7–δ) sandwiched between insulating layers of ferromagnetic Pr_0.8Ca_0.2MnO_3. We observe ΔΤc values that approach 2 K with the sign of ΔΤc 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.