Magneto-Optical Characterization of Iron-Based Superconducting Wires and Tapes

The critical current density, J_c, in high-quality single crystals of iron-based superconductors (IBS) has reached larger than 10⁵ A/cm² at 4.2 K. This large value of J_c urged the development of potential superconducting wires or tapes based on IBS. We have fabricated superconducting wires and tapes of IBS using several techniques and made an extensive characterization by structural, transport, and magnetic measurements. The powder-in-tube method is used for (Ba, K)Fe₂As₂ and Ba(Fe, Co)₂As₂ wires and the diffusion method is used for FeSe tape. Intra- and inter-granular J_c´s have been evaluated by direct transport and indirect magnetization measurements as well as using magneto-optical (MO) imaging. The intergranular J_c´s at 4.2 K under self-field for (Ba, K)Fe₂As₂ and Ba(Fe, Co)₂As₂ wires and FeSe tapes are 1.3 × 10⁴ A/cm², 3.8 × 10³ A/cm², and 600 A/cm², respectively. On the other hand, MO images have demonstrated that the intragranular J_c is at least one order of magnitude larger than the intergranular J_c, which is consistent with the magnetic evaluation.