Magnetic and resistive superconducting transitions in the iron-based compounds FeSe and LaOFFeAs

Using a null-picovoltmeter modified into a highly sensitive magnetometer, for the first time we compare the temperature behavior of magnetic and resistive transitions to the superconducting state for the samples of iron-based compounds FeSe (monocrystalline) and LaOFFeAs (granular) as well as the behavior of those transitions for the conventional single-component superconductor (Pb). It is found that the nature of the temperature behavior of the above transitions in iron-based superconductors, unlike that in Pb, is thermodynamically different, in LaOFFeAs granular samples the difference being radical. It is also found that the magnetic susceptibility of FeSe samples in the non-superconducting state is positive, indicating a ferromagnetic nature of the ordering of the local spins of iron ions. The susceptibility of LaOFFeAs samples in this state remains diamagnetic. The features of the superconducting transitions found are analysed.