Characterization of superconducting nanometric multilayer samples for superconducting rf applications: First evidence of magnetic screening effect

The best rf bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H 200 mT close to the thermodynamic critical field H c. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of H c > H c Nb for magnetic shielding of bulk niobium to increase the breakdown magnetic field of superconducting rf cavities. Depositing good quality layers inside a whole cavity is rather difficult, so as a first step, characterization of single layer coating and multilayers was conducted on high quality sputtered samples by applying the technique used for the preparation of superconducting electronics circuits. The samples were characterized by x-ray reflectivity, dc resistivity (PPMS), and dc magnetization (SQUID) measurements. Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating exhibit different behaviors and clearly show an enhancement of the magnetic penetration field. Because SQUID measurements are influenced by edge and shape effects, we propose to develop a specific local magnetic measurement of H C1 based on ac third harmonic analysis in order to reveal the true screening effect of multilayers.