Vortex penetration severely limits high field performances in bulk niobium RF cavities for accelerators. To reach higher fields, Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] proposed to deposit nanometric layers (d < λ) to partially screen of the field sensed by niobium. Model (NbN/MgO)n (n = 0 to 4) samples have been deposited on thick Nb layers. This paper presents the first complete characterization set (HC1 and RF surface resistance) of a new family of composite nanostructured superconducting layers deposited on Nb, which are liable to bring a breakthrough in the technology of superconducting accelerating RF cavities.