Fabrication of Small Magnets Using (Ba,A)Fe₂As₂ (A: Na, K) Round Wire with Large Critical Current

Since the discovery of superconductivity in LaFeAs(O,F) in 2008 [1], extensive studies have been performed on iron-based superconductors (IBSs) for exploration of both fundamental physics and applications. One of salient features of IBSs is that they have large upper critical field over 500 kOe at low temperatures [2] and have good potential to replace conventional Nb-based superconductors. To apply superconducting materials into practical use at low temperatures and high fields, good critical current characteristics is indispensable. In single crystalline form, critical current density (<i>J</i>_c) in IBSs at low temperatures exceeds 0.1 MA/cm² [3,4] and it can be enhanced further by introducing artificial defects [5,6]. With such excellent characteristics, developments of superconducting wires and tapes have been conducted. Among various materials systems in IBSs, such as 1111, 122, and 11 systems, 122 system with only one kind of anion with modest <i>T</i>_c turns out to be an excellent candidate for such purpose. <i>J</i>_c of the wire exceeded 1x10⁴ A/cm² at 4.2 K under self-field quickly [7], and it exceeds 1x10⁴ A/cm² even at 100 kOe by introducing hot-isostatic-press (HIP) process [8].<br/>In the present study, we fabricated small magnets using 10 m-class (Ba,<i>A</i>)Fe₂As₂ (<i>A</i>: Na, K) round wires prepared by powder-in-tube (PIT) method. Magnets are processed under high pressure using HIP technique after glass-fiber insulations are installed [9]. Critical current (<i>I</i>_c) of the coils at 4.2 K reach 60 A and 66 A under the self-field, and magnetic fields of 2.6 kOe and 2.5 kOe are successfully generated at the center of the coil, using (Ba,Na)Fe₂As₂ and (Ba,K)Fe₂As₂ wires, respectively. After the characterization of the coils, wires are unwound and short segments are picked out from different locations of the wire for further characterizations. The largest transport <i>I</i>_c at 4.2 K under the magnetic field of 100 kOe for (Ba,Na)Fe₂As₂ wire reaches 51.8 A, which corresponds to <i>J</i>_c = 54 kA/cm². This <i>J</i>_c value is larger than the record value of previous IBS superconducting round wires by more than 20 %. Texturing of grains in the core of the wire due to the improvement of the wire drawing process plays a key role for the enhancement of <i>J</i>_c. After this success, we attempted to increase <i>I</i>_c further by making the cross section of the core larger. We also prepared such wires as long as 10 m, and constructed small coils using react-and-wind method. However, the resulting <i>I</i>_c of the wire was fairly low. Inspection of the wire core after unwinding the coil makes it clear that high density cracks are introduced in the wire due to small bending diameter in spite of large core of the wire.<br/>References<br/>[1] Y. Kamihara <i>et al</i>., J. Am. Chem. Soc. <b>130</b>, 3296 (2008).<br/>[2] N. Ni <i>et al</i>., Phys. Rev. B <b>78</b>, 014507 (2008).<br/>[3] R. Prozorov <i>et al</i>., Phys. Rev. B <b>78</b>, 224506 (2008).<br/>[4] Y. Nakajima, T. Taen, and T. Tamegai, J. Phys. Soc. Jpn. <b>78</b>, 023702 (2009).<br/>[5] Y. Nakajima <i>et al</i>., Phys. Rev. B <b>80</b>, 012510 (2009).<br/>[6] T. Tamegai <i>et al</i>., Supercond. Sci. Technol. <b>25</b>, 084008 (2012).<br/>[7] K. Togano, A. Matsumoto, and H. Kumakura, Appl. Phys. Express <b>4</b>, 043101 (2011).<br/>[8] J. D. Weiss <i>et al</i>., Nat. Mater. <b>11</b>, 682 (2012).<br/>[9] S. Pyon <i>et al</i>., Supercond. Sci. Technol. <b>34</b>, 105008 (2021).