Flux distribution in Fe-based superconducting materials by magneto-optical imaging

Abstract

Abstract"This paper presents the magnetic flux distributions in Fe-based superconducting materials including single crystal of Ba(Fe 1.9 Ni 0.1 )As 2 and Ba(Fe 1.8 Co 0.2 )As 2 , as well as polycrystalline SmFeO 0.75 F 0.2 As by means of magneto-optical imaging (MOI) technique. The single crystals were grown out of FeAs flux while polycrystalline sample was grown by hot-press. A MOI film with in-plan magnetization was used to visualize flux distributions at the sample surface. A series of magneto-optical images was taken when the samples were zero-field cooled and field cooled. The flux behavior, including penetration into and expelling from the samples, as well as pinning properties were studied. When external fields increase, flux is completely shielded from the crystals, then, gradually penetrates toward the crystal center from the edge. For polycrystalline sample, Meissner state was observed at very low field. With increasing the field further, flux penetrates into the sample easily along grain boundary, then into grain. Compared with high-T c cuprates, it is found that the flux distributions in Fe-based superconducting materials are very similar to that in high-T c cuprates with strong pinning strength. This paper presents the magnetic flux distributions in Fe-based superconducting materials including single crystal of Ba(Fe 1.9 Ni 0.1 )As 2 and Ba(Fe 1.8 Co 0.2 )As 2 , as well as polycrystalline SmFeO 0.75 F 0.2 As by means of magneto-optical imaging (MOI) technique. The single crystals were grown out of FeAs flux while polycrystalline sample was grown by hot-press. A MOI film with in-plan magnetization was used to visualize flux distributions at the sample surface. A series of magneto-optical images was taken when the samples were zero-field cooled and field cooled. The flux behavior, including penetration into and expelling from the samples, as well as pinning properties were studied. When external fields increase, flux is completely shielded from the crystals, then, gradually penetrates toward the crystal center from the edge. For polycrystalline sample, Meissner state was observed at very low field. With increasing the field further, flux penetrates into the sample easily along grain boundary, then into grain. Compared with high-T c cuprates, it is found that the flux distributions in Fe-based superconducting materials are very similar to that in high-T c cuprates with strong pinning strength.

Enhanced transport critical current density in Sn-added SmFeAsO1−xFxtapes prepared by the PIT method

“…Further studies indicate that Sn additions can significantly enhance the intergranular current density of polycrystalline Sm1111 [19,20]. Besides the restriction of intergranular current density by poor grain connectivity [21,22], the post-rolling heat treatment for Sm1111 tapes has always been difficult. Indeed, the optimized heat treatment time at 1100 °C is limited to 20-50 s [18], and the longer time heat treatment causes severe J c degradation in the Sm1111 tapes.…”

Inhomogeneous field calibration of a magneto-optical indicator film device

“…(11) Here, the N (x/y/z) are the entries of the demagnetization tensor of the thin film MOIF material in main axis representation and φ u and φ c are the orientation of the uniaxial and cubic anisotropy axes, respectively, in the sample plane. Combining equations ( 6) to (11) allows to derive an expression relating the field angle φ to the measured FMR frequency as a function of the anisotropy parameters of the material and of its magnetization:…”

“…The drawback of the MOIF technique is a reduced spatial resolution due to the sensor-sample distance and the thickness of the sensor film. The capabilities of the MOIF technique were already demonstrated by, for example, the quantitative analysis of thin hard magnetic samples [9] and investigation of vortex dynamics in superconductors [10,11]. Furthermore, first approaches for quantitative MOIF measurements and calibration methods were introduced.…”