Electrotransport and magnetic measurements on bulk FeSe superconductors

The superconducting properties of bulk, polycrystalline FeSe samples were characterized through magnetic and electric transport measurements. In order to improve the superconducting properties, the sintering temperature was varied up to 900 °C and to improve the grain connectivity, silver was added (0 to 7 wt.-%). Electric properties of the samples were investigated by the four point probe method (R-T measurements and V-I characteristics). Generally, the sample with 4 wt.-% Ag-addition showed the highest critical transition temperature among all the samples fabricated with the same preparation parameters. The critical currents were obtained from V-I measurements in various applied magnetic fields up to 6 T. Via Arrhenius plots, the pinning potential U ₀ was determined for all samples studied. The magnetic properties (M-T and M-H) of the samples were measured using an extraction magnetometer in a Quantum Design PPMS with applied magnetic fields up to 7 T. The scaling of the normalized volume pinning force versus the reduced field indicated a peak position at ∼0.2 for the Ag-added sample, which points to a pinning at the grain boundaries. (paper)