Nodal superconducting exchange coupling

Abstract

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 ({\Delta}Tc) due to an interplay of the magnetic exchange energy and the superconducting condensate. The magnitude of {\Delta}Tc scales inversely with the superconductor thickness (dS) and is zero when dS exceeds the superconducting coherence length ({\xi}) 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 {\Delta}Tc are determined by the nodal quasiparticle-excitation states on the Fermi surface of the d-wave superconductor YBa2Cu3O7-{\delta} (YBCO) grown between insulating layers of ferromagnetic Pr0.8Ca0.2MnO3. We observe {\Delta}Tc values that approach 2 K with {\Delta}Tc oscillating with dS over a length scale exceeding 100 {\xi} 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.