Iron-based superconductors (IBSs) possess relatively high superconducting transition temperature (T_c) exceeding 50 K, as well as large and nearly isotropic upper critical field (H_c2), i.e. about 100 T at 4.2 K. Due to these characteristics, IBSs are considered promising candidates for the high magnetic field applications, either in the form of wire-wound magnets or as trapped field magnets. Among various IBSs, 122 materials such as Ba_1-xK_xFe₂As₂ have been intensively studied owing to their small anisotropy. Meanwhile, recent studies demonstrated the high application potentiality of CaKFe₄As₄ (CaK1144) [1-3]. Here, we report the synthesis of a CaKFe₄As₄ bulk sample using the spark plasma sintering (SPS) technique. The SPS technique is known as a useful technique to obtain dense bulk samples through a simple procedure. The SPS uses pulsed currents that pass through conductive dies (typically made of graphite) as well as samples themselves that produce Joule heating. We demonstrate that the bulk sample fabricated using SPS method has a high density, reaching 96% of theoretical density. Moreover, the critical current density of the SPS bulk sample, as calculated from the magnetization hysteresis loops (magnetic J_c), reached 18 kA cm^-2 at 4.2 K under 5 T [4].

Fig. Magnetic field dependence of the magnetic Jc of the CaK1144 bulk sample synthesized using SPS technique measured at 4.2 K (black), 10 K (blue), 15 K (green), 20 K (orange), 25 k (red), and 30 K(grey). The inset shows the photograph of the bulk.

 [1] S. J. Singh, et al. Phys. Rev. Mater. 2, 074802 (2018).
[2] S. Pyon, et al. Phys. Rev. B 99, 104506 (2019).
[3] S. Ishida, et al. npj Quantum Materials, 4, 27 (2019).
[4] S. Ishida, et al. Supercond. Sci. Technol. 33, 094005 (2020).

Keywords: Iron-based superconductors, CaKFe4As4 bulk, Spark plasma sintering, Critical current density