Determination of the properties of a superconducting FeSe single crystal using EPR spectroscopy

Using EPR spectrometer, the properties of FeSe single crystal were studied in magnetic fields up to 6000 Oe at temperatures from 3.5 to 8 K in the superconducting state and at temperatures above 8 K and up to 25 K in the normal state. It was shown that by measuring non-resonant spectrometer signals in zero and weak magnetic fields up to 30 Oe, it is possible to determine critical temperature Tc, the superconducting transition width, and the value of the first critical magnetic field Hc1 in FeSe single crystal. Resonant EPR signals are observed at fields of 1400 and 3400 Oe, which correspond to the paramagnetism of doubly ionized iron ions (Fe2+) in the FeSe crystal. Non-resonant EPR signals in a magnetic field up to 6000 Oe at temperatures from 3.5 to 8 K are a nonlinear function of the applied field and correspond to the superconducting mixed state of the FeSe, but at temperatures above 8 K (up to 25 K) they are field-independent.