[WB6] Bulk

11:30 AM - 12:00 PM JST(2:30 AM - 3:00 AM UTC)

[WB6-05-INV] Evidence of multiscale supercurrents in K-doped BaFe₂As₂: intrinsic or extrinsic limitation?

*Chiara Tarantini¹, Shah Alam Limon¹, Keyou Mao¹, Md Rafsun Jani^1,2, Dongyi Qin³, Akiyasu Yamamoto Yamamoto³, Eric E. Hellstrom^1,2,4, Fumitake Kametani^1,4 (1. National High Magnetic Field Laboratory, Florida State University (United States of America), 2. Department of Materials Science and Engineering, FAMU-FSU College of Engineering, Florida State University (United States of America), 3. Department of Applied Physics, Tokyo University of Agriculture and Technology (Japan), 4. Department of Mechanical Engineering, FAMU-FSU College of Engineering, Florida State University (United States of America))

Keywords:

K-doped BaAs2Fe2,Critical Current Density,AC Susceptibility,Microstructure

Although K-doped BaAs₂Fe₂ (K-Ba122) is a promising superconducting material for applications because of its intermediate T_c and high H_c2 with a moderate anisotropy, achieving high critical current density (J_c) in untextured bulk sample is challenging. One of the main reasons is that it is still not fully clear whether the grain-to-grain connectivity in randomly oriented grain suffers from intrinsically current-blocking effects or whether it is affected by extrinsic effects, like grain boundary segregation, chemical inhomogeneity or other defects. In this work we studied the effect of varying the milling energy density (65-200 MJ/kg) in the preparation of K-Ba122 bulk samples. This variation affects the grain and grain boundary microstructures, and we investigate the sample magnetic performance to better understand what causes their different J_c. We determined that in our samples, which all have small grain size, T_c is not directly correlated to J_c. Performing AC susceptibility, we found that in at least one case there are obvious signs of multiscale supercurrents that are not caused by granularity but that inevitably influence the overall J_c performance. The different connectivity quality also induces obvious differences in the irreversibility field. Taking into account the magnetization response and the microstructural features we ascribed the difference in J_c to lack of connectivity on a larger scale caused by nano-cracks at some grain boundaries. These subdivided the samples into macroscopic regions limiting the overall performance [1]. The presence of these extrinsic J_c-limiting features in bulk sample does not however exclude the contribution of intrinsic limitation. To better understand the intrinsic features of K-Ba122 we investigated the transport properties of high quality single and bicrystal thin films evaluating H_c2, H_Irr, J_c, and the pinning performance. Those results provided useful insights into the K-Ba122 phase generating a valuable new prospective for the realization of bulk and wires.
References
1) C. Tarantini et al. Supercond. Sci. Technol. 38, 045023 (2025)