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

摘要

We propose an inductive method to measure critical current density $J_c$ inbulk superconductors. In this method, an ac magnetic field is generated by adrive current $I_0$ flowing in a small coil mounted just above the flat surfaceof superconductors, and the third-harmonic voltage $V_3$ induced in the coil isdetected. We present theoretical calculation based on the critical state modelfor the ac response of bulk superconductors, and we show that thethird-harmonic voltage detected in the inductive measurements is expressed as$V_3= G_3\omega I_0^2/J_c$, where $\omega/2\pi$ is the frequency of the drivecurrent, and $G_3$ is a factor determined by the configuration of the coil. Wemeasured the $I_0$-$V_3$ curves of a melt-textured $\rm YBa_2Cu_3O_{7-\delta}$bulk sample, and evaluated the $J_c$ by using the theoretical results.