Effect of Reversible Flux Motion on the Estimation of Critical Current Density in Thin Superconductors Using the Third Harmonic Voltage Method

A significant overestimation of the critical current density Jc was discovered in a measuring method that utilizes induced third harmonic voltage for thin superconductors with a thickness less than 1 μm. This is speculated to be caused by the reversible motion of flux lines in the pinning potential, which reduces nonlinear properties such as hysteresis loss or third harmonic voltage. It is known that the effect of reversible flux motion is prominent for superconductors comparable to or smaller than the pinning correlation length at the sub-micrometer order called Campbell's AC penetration depth. In this paper, the effect of reversible flux motion on the estimation of Jc utilizing this method is investigated using a simplified Campbell's model for the force-displacement characteristics of flux lines, and the factor of overestimation of Jc is discussed.