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 (Delta T-c) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of Delta T-c scales inversely with the superconductor thickness (d(s)) and is zero when d s exceeds the superconducting coherence length (xi). Here, we report a superconducting spin-valve effect involving a different underlying mechanism in which magnetization alignment and Delta T-c are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa2Cu3O7-delta, sandwiched between insulating layers of ferromagnetic Pr0.8Ca0.2MnO3. We observe Delta T-c values that approach 2 K with the sign of Delta T-c oscillating with d(s) over a length scale exceeding 100 xi 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.