We report on the first observation of a pronounced reentrant superconductivity phenomenon in a superconductor/ferromagnet layered system. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of $\mathrm{Nb}/{\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x$. The superconducting transition temperature $T_c$ drops sharply with increasing thickness $d_{\mathrm{CuNi}}$ of the ferromagnetic layer, until complete suppression of superconductivity is observed at $d_{\mathrm{CuNi}}\approx 4\mathrm{nm}$. Increasing the ${\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x$ layer thickness further, superconductivity reappears at $d_{\mathrm{CuNi}}\ge 13\mathrm{nm}$. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov-like state in the ferromagnetic layer.

• Received 11 February 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.057004