*Article* **Microstructure and Flux Pinning of Reacted-and-Pressed, Polycrystalline Ba0.6K0.4Fe2As2 Powders**

#### **Michael R. Koblischka 1,2,\*, Anjela Koblischka-Veneva 1,2, Jörg Schmauch 1 and Masato Murakami 2**


Received: 7 June 2019; Accepted: 2 July 2019; Published: 6 July 2019

**Abstract:** The flux pinning properties of reacted-and-pressed Ba0.6K0.4Fe2As2 powder were measured using magnetic hysteresis loops in the temperature range 20 K ≤ *T* ≤ 35 K. The scaling analysis of the flux pinning forces (*Fp* = *jc* × *B*, with *jc* denoting the critical current density) following the Dew-Hughes model reveals a dominant flux pinning provided by normal-conducting point defects (*δl*-pinning) with only small irreversibility fields, *H*irr, ranging between 0.5 T (35 K) and 16 T (20 K). Kramer plots demonstrate a linear behavior above an applied field of 0.6 T. The samples were further characterized by electron backscatter diffraction (EBSD) analysis to elucidate the origin of the flux pinning. We compare our data with results of Weiss et al. (bulks) and Yao et al. (tapes), revealing that the dominant flux pinning in the samples for applications is provided mainly by grain boundary pinning, created by the densification procedures and the mechanical deformation applied.

**Keywords:** iron-based superconductors; critical currents; flux pinning; microstructure
