The temperature behaviour 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 non zero 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 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.
Categories: cond-mat.supr-con