Anomalous critical fields and the absence of Meissner state in Eu(Fe0.88Ir0.12)2As2 crystals

Abstract

We report electrical and magnetic measurements of an optimally doped Eu(Fe _0.88 Ir _0.12 ) _2 As _2 crystal which shows superconductivity at T _sc = 22 K and magnetic ordering of the Eu ^2+ spins below 20 K. The results suggest that the Eu ^2+ spins lie flat ferromagnetically in the ab plane at T ^ab _m = 20 K, and then tilt toward the c -axis at T ^tilt _m = 17.4 K. The isothermal magnetization loop at low temperatures shows both ferromagnetic behavior and superconducting characteristics, unambiguously demonstrating the coexistence of ferromagnetism and superconductivity. The upper critical field measured is remarkably reduced, as compared to other Fe-based superconductors with the same T _sc , and it exhibits abnormal temperature dependence featured by the existence of an inflection point around T ^ab _m , where the anisotropy ratio γ (≡ H ^∥ ^ab _c2 / H ^∥ ^c _c2 ) shows a minimum value smaller than 1.0. These observations can be explained by a ferromagnetic exchange field of ∼30 T which tilts its direction toward the c -axis below T ^tilt _m . The strong internal field, much higher than the intrinsic lower critical field expected, leads to the absence of Meissner state, which is confirmed by the magnetic measurements under ultra-low fields.