Fabrication of Small Magnets Using Mono- and Seven-Core (Ba,A)Fe2As2 (A: K, Na) HIP Round Wires

Small magnets using 18-m class mono-core (Ba,Na)Fe₂As₂ and seven-core (Ba,K)Fe₂As₂ round wires were constructed. Wires used in these magnets were fabricated using powder-in-tube method followed by hot isostatic pressing process. Both magnets show zero resistivity at low currents, and strong increase in resistance shows up at 47-53 A. Generated magnetic fields at these currents were 3.1 kOe and 2.7 kOe for magnets using mono-core (Ba,Na)Fe₂As₂ and seven-core (Ba,K)Fe₂As₂ round wires, respectively. Short pieces of the long wires used for the magnets were evaluated independently after disassembling the magnets. It turns out that critical current (I_c) varies significantly along the length of the wire, and the largest I_c reaches 160 A in mono-core (Ba,Na)Fe₂As₂ wire. X-ray tomography observations of the wires revealed severe sausaging effect even in a few mm length scale, which could be the reason for the large variation of I_c along the entire length of the wire. We also evaluated the effect of bending strain on mono-core and multi-core (Ba,Na)Fe₂As₂ wires. Effects of bending strain turns out to be modest, and more than 60% of I_c was sustained even at the bending strain of 0.5%.