Critical current and flux pinning near T c of a YBa2Cu3O7 thin film

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Abstract
The voltage V versus current I of a high‐quality YBa2Cu3O7−x thin film with zero‐resistance temperature equal to 90.8 K was measured at temperatures near Tc (85, 87, and 90 K, respectively) under different magnetic fields (0–7 T). A significant result is that the critical‐current density of the film reached 1.37×104 A/cm2 (zero field) even at 90 K, implying that strong pinning centers exist in our sample. However, a small applied magnetic field will diminish the critical‐current densities remarkably. The pinning‐force densities are found to follow Kramer’s scaling law in both perpendicular and parallel directions of the magnetic fields to the c axis of the film. A possible influence of thermally activated flux creep on the pinning mechanism is confirmed.