Characterization of Local Critical Current Density in Fe(Se,Te) Bulk and Film by Magnetic Microscopy
東川 甲平, 細谷 築, 井上 昌睦, 木須 隆暢 （九大）； SALA Alberto, BRACCINI Valeria, BELLINGERI Emilio, PUTTI Marina （CNR-SPIN）
Abstract:For the performance improvement of iron-based superconductors, it should be effective to clarify the relationship between local superconducting properties and the corresponding microstructure in a sample. However, general transport measurement or magnetization measurement only can estimate averaged properties such as critical current or averaged critical current density. In this study, local critical current density distribution was visualized by means of low-temperature scanning Hall-probe microscopy. This measurement system can cool a sample down to 5 K and apply external magnetic field up to 5 T. Measuring in-plane distribution of magnetic field just above a sample, that of local critical current density could be estimated by an inverse problem of the Biot-Savart law. As a result, it was found that many grain boundaries blocked current flow in a Fe(Se,Te) bulk superconductor and they limited macroscopic critical current in the sample. On the other hand, several dozen times higher current density was also observed on a grain boundary. This means that to investigate the difference of such grain boundaries will be very effective to clarify the performance limiting factor in the bulk superconductor. Furthermore, it was also found that Fe(Se,Te) film deposited on a CaF2 single crystal also had very inhomogeneous distribution of critical current density. The locally maximum value was estimated to be 3 MA/cm2 at 4 K and 1 microvolt/cm in electric field criterion while 1 MA/cm2 class was obtained by transport method. This demonstrates very high potential of iron-based superconductor, and means that the improvement of spatial homogeneity will still be important even for a film deposited on a single crystal. From these findings, it could be concluded that characterization of local critical current density by means of magnetic microscopy would be very powerful technique for further performance improvement of iron-based superconductors.
This work was supported by JST as a project of