Effect of heat treatment on superconducting properties of FeSe wire fabricated by PIT method
We report on successful fabrication of superconducting FeSe wire using hot gas extrusion (HGE) ex-situ PIT (powder in tube) method. Length of the obtained wire was about 60cm with cross-sectional superconducting core area nearly 2.5x 10^-3 cm^2. For the wire sample we observed superconducting transition temperature, T_onset = 11 K, about 1.2K lower compared
to the preliminary prepared FeSe powder. Heat treatment in argon atmosphere at 350C resulted in transition width decrease from \Delta T(10% - 90%) = 1.75K in sample without heat treatment down to \Delta T = 0.9 K in annealed samples. Estimated derivative of the upper critical eld as a function of temperature of the sample annealed during 72h in argon atmosphere
at 350C is dHc2/dT =2.9 T/K. Applying WHH theory to our data allows to define Hc2(0K) = 0.69Tc x(dHc2/dT )= 19.8T. The untreated wire shows critical current density, Jc = 75 A/cm2 at T=4.0 K in zero field. Increasing annealing time up to 72 hours at 350C in argon atmosphere gives rise to Jc increase of about 60% approaching 120 A/cm2 at T=4.0K and H=0T . Also Jc measurements were made in magnetic fields up to 9T. Our results show applicability of the HGE PIT method for fabrication of superconducting wires based on FeSe compound. Long-range heat treatment is necessary to improve superconducting properties of the samples.