First critical field measurements of superconducting films by third harmonic analysis

The temperature behavior of the first critical field B C1 of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the sample, a nonzero third harmonic voltage V 3 is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of B C1 using this technique are detailed. As a validation test, different type II superconductors Nb, NbN, MgB 2 , and Y 1 Ba 2 Cu 3 O 7-d under the form of thin films have been measured. The comparison between experimental results, data presented in literature, and theoretical predictions is presented and discussed.