Abstract
The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, T-c, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing FeSe using solid-state sintering yields samples exhibiting strong ferromagnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above T-c. Due to the complicated phase diagram, the samples are a mixture of several phases, the superconducting beta -FeSe, and the non-superconducting delta -FeSe and gamma -FeSe. Furthermore, antiferromagnetic Fe7Se8 and ferromagnetic alpha -Fe may be contained, depending directly on the Se loss during the sintering process. Here, we show MHLs measured up to +/- 7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T) measurements. We also performed a thorough analysis of the microstructures in order to establish a relation between microstructure and the resulting sample properties.