Condensed-Matter Physics & Materials Science Seminar

The antagonism between superconductivity and ferromagnetism has generated a considerable degree of interest in recent years. Thin film heterostructures of a variety of superconducting (SC) and ferromagnetic (FM) materials permit experimental verification of such diverse phenomena as-phase shift, triplet pairing and enhanced flux pinning by magnetic inhomogeneities. We have been working on FM-SC-FM and SC-FM-SC trilayers of conventional materials such as NbN and CoPt, and the exotic systems comprising of YBa2Cu3O7 (YBCO) and La1-xSrxMnO3 (LSMO) deposited on lattice matched substrates. Transport measurements on LSMO-YBCO-LSMO system show clear oscillations in critical current (Ic) as the thickness of the LSMO layers is scanned from ~ 50 Å to ~ 1100 Å. In the light of existing theories of FM-SC heterostructures, this observation suggests a long range proximity effect in the manganite, modulated by its weak exchange energy (~2 meV). One interesting issue in FM-SC hybrids that defies the notion of antagonistic orders is the magnetic field induced superconductivity (FIS). We have shown that in systems where the FM domains/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 nano-engineered 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.

This research has been supported by grants from the Department of Science and Technology (DST) and the Board for Research on Nuclear Sciences (BRNS), Government of India.