Magnetic exchange field enhancement of the superconducting density of states in a thin-film d-wave superconductor on a ferromagnetic insulator

At a superconductor/ferromagnet insulator (S/FI) interface, a proximity-induced magnetic exchange interaction spin-splits the quasiparticle density of states and suppresses superconductivity. In an FI/S/FI spin-valve structure, the effect of the magnetic exchange interaction depends on the relative alignment of the FI layers' magnetizations: typically, the suppression of superconductivity is larger in the parallel state than in the antiparallel state. However, in a fully oxide-based spin-valve with a YBa2Cu3O7 (YBCO) superconducting layer sandwiched by a ferromagnetic insulator Pr0.8Ca0.2MnO3, the ground state of an anisotropic d-wave superconducting gap can be modulated, with the quasiparticle density of states being enhanced in the parallel state for specific YBCO thicknesses. To verify this interpretation and offer insights into the nature of the magnetic proximity effect, we present density of states measurements from planar FI/S/FI grain boundary junctions with YBCO, providing direct evidence of an enhancement in the energy associated with the quasiparticle peak in the parallel state.