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 10
⁵
A/cm
²
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)Fe
₂
As
₂
and Ba(Fe, Co)
₂
As
₂
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)Fe
₂
As
₂
and Ba(Fe, Co)
₂
As
₂
wires and FeSe tapes are 1.3 × 10
⁴
A/cm
²
, 3.8 × 10
³
A/cm
²
, and 600 A/cm
²
, 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.