Paper detail

Nodal superconducting exchange coupling

The superconducting equivalent of giant magnetoresistance, involves placing a thin-film superconductor between two ferromagnetic layers. A change of magnetization-alignment in such a superconducting spin-valve from parallel (P) to antiparallel (AP) creates a positive shift in the superconducting transition temperature (ΔTc) due to an interplay of the magnetic exchange energy and the superconducting condensate. The magnitude of ΔTc scales inversely with the superconductor thickness (dS) and is zero when dS exceeds the superconducting coherence length (ξ) as predicted by de Gennes. Here, we report a superconducting spin-valve effect involving a different underlying mechanism that goes beyond de Gennes in which magnetization-alignment and ΔTc are determined by the nodal quasiparticle-excitation states on the Fermi surface of the d-wave superconductor YBa2Cu3O7-δ (YBCO) grown between insulating layers of ferromagnetic Pr0.8Ca0.2MnO3. We observe ΔTc values that approach 2 K with ΔTc oscillating with dS over a length scale exceeding 100 ξ and, for particular values of dS, we find that the superconducting state reinforces an antiparallel magnetization-alignment. These results pave the way for all-oxide superconducting memory in which superconductivity modulates the magnetic state.