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Magnetically coupled superconductor-ferromagnet systems have been studied by low temperature Scanning Tunneling Microscopy and Spectroscopy. The stray field of the ferromagnet induces a non-uniform superconducting state characterized by a local superconducting critical temperature Tc and a non monotonic behavior of Tc vs H close to the critical temperature [1,2].

We studied Pb/[Co/Pd] systems and we visualized the emergence of superconductivity in regions above the separation between adjacent magnetic domains, as well as reverse domain wall superconductivity. Moreover, deep in the superconducting state vortices of opposite polarity are induced by the stray field of the ferromagnet in zero applied external field and they are strongly confined on the stripe of the same polarity. The nucleation of spontaneous vortex-antivortex strongly depends on the domain width [3].

Our results demonstrate that such S/F structures are attractive model systems that offer the possibility to control the strength and the location of the superconducting nuclei.

[1] Yang Z., Lange M., Volodin A., Szymczak R., and Moshchalkov V.V., Nature Mater. 3, 793 (2004)
[2] Aladyshkin A. Yu., Buzdin A. I., Fraerman A.A., Melnikov A.S., Ryzhov D.A., and Sokolov A. V., Phys. Rev. B 68, 184508 (2003)
[3] M. Iavarone, S.A. Moore, J. Fedor, S. T. Ciocys, G. Karapetrov, J. Pearson, V. Novosad, and S. D. Bader, under Review Nature Communications 5, 4766 (2014)