Measurement of critical current density of YBCO film by a mutual inductive method using a drive coil with a sharp iron core

Recent developments in high-temperature superconductors have facilitated the production of high critical current and large-area films by the PLD method or MOD method. Nondestructive measurement techniques for high quality films are of great importance to estimate superconducting properties, such as the critical current density, JC. The inductive method using the third-harmonic voltage component, V3, is a promising technique for the measurement of the properties of JC. On inductive determination of JC for high-JC thin films and thicker films (thickness d ⩾ 0.1 mm), the drive coil must generate a magnetic field sufficiently large to break the complete field shielding of the film. Therefore, we investigated a mutual inductance technique using a drive coil with a sharp ferromagnetic core for investigation of the JC of the PLD-YBCO film. As a result, an abrupt nonlinear increase in the voltage across the pick-up coil was detected after magnetic full penetration of the film, and the threshold of the drive current Ith was suppressed to 0.23-0.28 times as compared with that measured by the inductive method using the third-harmonic voltage, V3. FEM analysis indicated the diameter of the excitation field region to be 0.6 mm in the case of a 0.4 mm ϕ iron core. The E-J characters measured by the mutual inductive method as a function of frequency agreed well with those measured by the inductive method using V3.