Synthesis of CaKFe${}_4$As${}_4$ bulk samples with high critical current density using a spark plasma sintering technique

A high density CaKFe${}_4$As${}_4$ bulk sample was successfully synthesized using a spark plasma sintering (SPS) technique. The density of the synthesized sample was 5.02 g cm${}^{-3}$, corresponding to 96.2% of the theoretical density of CaKFe${}_4$As${}_4$. Moreover, a reasonably high Vickers hardness of 1 GPa was measured. The electrical resistivity of the SPS bulk sample was as low as approximately 600 $\mu \mathrm{\Omega }$ cm at 300 K, which is smaller than that of the ordinary sintered polycrystalline sample by nearly one order of magnitude, and exhibited a sharp superconducting transition, with the transition width $\mathrm{\Delta }{\mathit{\text{T}}}_c$ less than 2 K, indicating an improved grain connectivity. The critical current density of the SPS bulk sample, as calculated from the magnetization hysteresis loops (magnetic ${\mathit{\text{J}}}_c$), reached 18 kA cm${}^{-2}$ at 4.2 K under 5 T, which is the highest among the iron-based superconductor polycrystalline samples reported thus far.