Magnetic and superconducting properties of Ir-doped EuFe₂As₂

The magnetic and superconducting properties of 14% Ir-doped EuFe2As2 are studied by means of dc and ac magnetic susceptibilities, electrical resistivity, specific heat and Eu-151 and Fe-57 M ssbauer spectroscopy (MS) measurements. Doping of Ir in EuFe2As2 suppresses the Fe spin density wave transition and in turn gives rise to high temperature superconductivity below 22.5 K with a reentrant feature at lower temperature. Magnetization and Eu-151 M ssbauer data indicate that the Eu2+ spins order magnetically below 18 K. Fe-57 MS studies show a line broadening in the absorption spectra below 18 K due to transferred hyperfine field from the magnetically ordered Eu sublattices. A pronounced lambda-shape peak in the specific heat supports a second-order phase transition of Eu2+ magnetic ordering with a strong ferromagnetic component, as confirmed by the magnetic field dependences of the transition. For a single crystal, the in-plane resistivity (p(ab)) ab and out-of-plane susceptibility (chi(c)) show superconducting transitions with zero resistance and diamagnetism, respectively. But the in-plane susceptibility (chi(ab)) does not show any diamagnetic shielding against external fields. The observed non-zero resistance in the temperature range 10-17.5 K, below the superconducting transition temperature, suggests the possible existence of a spontaneous vortex state in this superconductor