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 ¹⁵¹Eu and ⁵⁷Fe Mössbauer 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 ¹⁵¹Eu Mössbauer data indicate that the Eu²⁺ spins order magnetically below 18 K. ⁵⁷Fe 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 λ-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 (ρ_ab) and out-of-plane susceptibility (χ_c) show superconducting transitions with zero resistance and diamagnetism, respectively. But the in-plane susceptibility (χ_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. (papers)