Inductive measurements of third-harmonic voltage and critical current density in bulk superconductors

We propose an inductive method to measure critical current density J(c) in bulk superconductors. In this method, an ac magnetic field is generated by a drive current I-0 flowing in a small coil mounted just above the flat surface of superconductors, and the third-harmonic voltage V-3 induced in the coil is detected. We present theoretical calculation based on the critical state model for the ac response of bulk superconductors, and we show that the third-harmonic voltage detected in the inductive measurements is expressed as V-3=G(3)omegaI(0)(2)/J(c), where omega/2pi is the frequency of the drive current, and G(3) is a factor determined by the configuration of the coil. We measured the I-0-V-3 curves of a melt-textured YBa2Cu3O7-delta bulk sample, and evaluated the J(c) by using the theoretical results. (C) 2003 American Institute of Physics.