Double re-entrance of superconductivity in superconductor/ferromagnet bilayers

Abstract

We report on the first observation of a double suppression of superconductivity in a superconductor/ferromagnet layered system. The result was obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu41Ni59 with dNb ≊ 6.2 nm. As the thickness of the ferromagnetic alloy gradually increases, the superconducting transition temperature Tc drops sharply until a complete suppression of superconductivity is observed at dcuNi ≊ 2.5 nm. At further increase of the Cu41Ni59 layer thickness, superconductivity restores at dcuNi ≊ 24 nm. Then, with a subsequent increase of dcuNi, superconductivity vanishes again at dcuNi ≊ 38 nm. Our experiments give evidence for the realization of the quasi-one dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state in the ferromagnetic alloy layer.