Measurement of trapped flux magnetic field near the superconducting film by the dependence of superconducting current through a Josephson junction sensor

Abstract

Two-dimensional (2-D) magnetic field dependences of the Josephson current through a superconducting tunnel junction perpendicularly positioned to the sample niobium film have first been measured. Josephson tunnel junction was used as a magnetic sensor and was used to measure the magnetic field around the niobium superconducting film after adding the vertical magnetic field up to 5000 A/m. The 2-D magnetic field dependences of Ic through the junction without and with the Nb thin-film sample have been compared. Without Nb thin-film sample, the Ic-H dependence of the sensor junction showed the Fraunhofer patters of the same sensitivity in two directions parallel to edges of the sensor junction. With sample niobium film, the Ic-H dependence of the sensor junction showed the Fraunhofer patters of the different sensitivity, where in the direction perpendicular to the Nb thin-film sample the sensitivity becomes half because of the Meissner effect of Nb. Moreover, the magnetic field Hz perpendicular to the sample film has been added in the triangle shape as a follow sequence: (0)-(2000[A/m])-(0)-(3000)-(0)-(4000)-(0)-(5000)-(0). In this sequence, every time the Hz perpendicular field reduced to zero, 2-D Ic-H dependence of the sensor was measured. Significant changes in the measured 2-D Ic -H dependences were observed above a certain threshold value of Hz = 3000 A/m. We consider that the shift of the measured 2-D Ic-H dependences in the Hz minus direction occurred because of the flux quanta were trapped in the sample superconducting niobium film after the Hz value had been added greater than 3000 A/m.