Precise Determination of the Threshold Current for Third-Harmonic Voltage Generation in the ac Inductive Measurement of Critical Current Densities of Superconducting Thin Films

Abstract

In an ac inductive method to measure critical current densities J/sub c/ of superconducting films by detecting third-harmonic voltages V/sub 3/ cos 3/spl omega/t, ac magnetic fields are generated with ac currents I/sub 0/ cos /spl omega/t in a small coil mounted just above the film. J/sub c/ is calculated from the threshold coil current I/sub th/, at which full penetration of magnetic field is achieved and V/sub 3/ starts to emerge. For convenience, I/sub th/ has been usually determined by a constant-voltage criterion, e.g., V/sub 3///spl radic/2 = 50 /spl mu/V. However, V/sub 3/ is proportional to I/sub th/, and the determination of I/sub th/ by the constant-voltage criterion inherently causes a systematic error. We confirmed this by comparing the obtained J/sub c/ with those obtained from dc magnetization and by a transport method. We propose to use a constant-resistance criterion, such as V/sub 3//I/sub 0/ = 2 m/spl Omega/, as an alternative method to determine I/sub th/ accurately.