Change of the effective dimensionality of an Nb/CuNi bilayer in an external magnetic field.

A dimensional crossover of superconducting fluctuations in an external magnetic field, applied parallel to the layers, has been found for superconductor/ferromagnet bilayers of Nb/Cu(41)Ni(59). By lowering the temperature, a reduction of the superconducting nuclei size occurs. As soon as the size of the nuclei becomes smaller than the thickness of the superconducting bilayer structure, the dimensionality changes. The temperature dependence of the fluctuation conductivity exhibits a 2D behaviour in zero and weak magnetic fields in the vicinity of the critical temperature, switching to a 3D behaviour in a strong magnetic field at low temperatures.