Evidence for induced magnetization in superconductor-ferromagnet heterostructures: A scanning tunneling spectroscopy study

We performed scanning tunneling spectroscopy of c -axis-oriented YBa2 Cu3 O7-δ films on top of which ferromagnetic SrRuO3 islands were grown epitaxially in situ. When measured on the ferromagnetic islands, the density of states exhibits small gaplike features consistent with the expected short-range penetration of the order parameter into the ferromagnet. However, anomalous split-gap structures are measured on the superconductor in the vicinity of ferromagnetic islands. This observation provides evidence for the recently predicted induced magnetization in the superconductor side of a superconductor/ferromagnet junction and demonstrates its manifestation in the density of states of the superconductor. Surprisingly, the length scale of the effect inside the superconductor was found to be an order of magnitude larger than the superconducting coherence length. This is inconsistent with the theoretical prediction of a penetration depth of only a few superconducting coherence lengths. We discuss a possible origin for this discrepancy.