Electron Irradiation of Polycrystalline Bulk FeSe Superconductors

Pieces of bulk, superconducting FeSe samples prepared by solid-state sintering were irradiated with 2.5 MeV electrons (T = 23.5 K) at SIRIUS facility using two different fluences, 2×10^19 electrons/cm2 and 4×10^19 electrons/cm2. In contrast to previous work performed on FeSe single crystals, the bulk FeSe samples are polycrystalline without specific texture and exhibit a density of ~4 g/cm3 (= 78% of theoret. density) due to pores. Furthermore, the samples contain magnetic impurities which lead to a ferromagnetic signal of the sample. Such bulk FeSe samples are intended as base material for superconducting trapped field magnets operating at 5 K, with the benefit of a higher upper critical field, Hc2, and reduced flux jump probability as compared to MgB2. The electron irradiation introduced point defects to the FeSe grains. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%. We discuss the changes of Tc, of the magnetic background and the superconducting properties of the samples before and after the electron irradiation.