Local Magnetometer: First Critical Field Measurement of Multilayer Superconductors

S-I-S (Superconductor-Insulator-Superconductor) nanometric superconducting multilayers have been proposed by Gurevich* to increase the maximum accelerating field of Nb RF cavities. This enhancement of HC1 may be done by coating Nb with thin layers of thickness less than the penetration depth. Therefore, it is necessary to find a particular tool, which allows us measuring HC1 directly. In fact, DC magnetometers (e.g. magnetometer SQUID) are largely used for magnetic measurements but these last are strongly influenced by orientation, edge and shape effects, especially in the case of superconductor thin films. For that reason, we developed at Saclay facilities a specific local magnetic measurement of first critical field HC1. The principle of our local magnetometer is based on the third-harmonic voltage method purposed by Claassen**, which is very useful to estimate the first critical field HC1 of superconducting multilayer samples with nondestructive and contactless, but more importantly, without demagnetization effects***. This paper will present the evolution of the magnetometer to overcome all types of difficulties.