Inhomogeneous vortex-state-driven enhancement of superconductivity in nanoengineered ferromagnet-superconductor heterostructures

Thin film heterostructures provide a powerful means to study the antagonism between superconductivity (SC) and ferromagnetism (FM). One interesting issue in FM-SC hybrids, which defies the notion of antagonistic orders, is the observation of magnetic field-induced superconductivity (FIS). Here, we show that in systems where the FM domains and/or islands produce spatial inhomogeneities of the SC order parameter, the FIS can derive significant contribution from different mobilities of the magnetic flux identified by two distinct critical states in the inhomogeneous superconductor. Our experiments on nanoengineered bilayers of ferromagnetic CoPt and superconducting NbN, where CoPt/NbN islands are separated by a granular NbN, lend support to this alternative explanation of FIS in certain class of FM-SC hybrids.