Inductive measurement of in-field critical current density of YBCO thin film on a SrTiO_3 bicrystal substrate using the third harmonic voltage method

摘要

The spatial distribution and the magnetic field dependence of the critical current density J_c for a YBa_2Cu_3Oy (YBCO) thin film deposited on a SrTiO_3 (STO) bicrystal substrate have been investigated using the third harmonic voltage method. When a pick-up coil is mounted on the area without a crystal grain-boundary, the third harmonic voltage V_3 increases monotonically with increasing coil current I_0 over a certain threshold value. On the other hand, when the pick-up coil is mounted on the area with a crystal grain-boundary, the V _3-I_0 curve shows a strange behavior. However, the V _3-I_0 curves show a monotonic increase when a magnetic field over 0.07T is applied to the YBCO thin film, because the shielding current hardly flows across the crystal grain-boundary in the large magnetic field. The magnetic field dependence of J_c is also measured using the four-probe method to clarify the strange behavior of the V_3-I _0 curve in the third harmonic voltage method.