Remnant Magnetization Behavior of a K-Doped Ba122 Polycrystalline Bulk

It is still largely elusive whether K-doped BaFe ₂ As ₂ (K-Ba122) is granular due to the intrinsic blocking effects at the grain boundaries (GBs). We investigated the remnant magnetization characteristics of a K-Ba122 polycrystalline bulk. Remnant magnetization is effective to evaluate the contribution of magnetization from multi-scale current loops particularly if electromagnetic granularity is present due to weakly coupled GBs. The derivative of remnant magnetization of the K-Ba122 sample showed only a single peak, which was markedly dependent on the specimen size and shifted toward lower field as the specimen size decreases, strongly indicating that the current loop is intergrain. However, the high angle annular dark field scanning transmission electron microscope (HAADF-STEM) analysis revealed that this sample still has nano-cracks at GBs, degrading the continuous network of strong intergrain connectivity. Such extrinsically degraded network of intergrain connectivity can also be seen as a size-dependence of remnant magnetization J_c , which indicated the presence of strongly localized regions with much higher J_c than the bulk J_c . Our study suggested that the extrinsic GB nano-cracks are still the major cause of connectivity degradation, and the intrinsic current suppression might not be practically detectable at clean, fully connected GBs.