Interplay of the superconductivity and magnetism in Eu_0.7Na _0.3Fe₂As_1.4P_0.6

The resistivity, magnetic susceptibility, magnetization and specific heat under magnetic fields were measured on a Eu_0.7Na_0.3Fe ₂As_1.4P_0.6 single crystal. A decrease in resistivity to zero, which shifted toward lower temperatures under a magnetic field, was observed at T_c ∼ 25K due to a superconducting transition. The electrical and magnetic anisotropy was quite small. In low-field magnetization, helimagnetism appeared at T_HM ∼ 19K, which changed to ferromagnetism in higher fields. Therefore, superconductivity coexists with helimagnetism in a low field but with ferromagnetism in a high field. The specific heat showed anomalies due to the helimagnetic and superconducting transitions as well as the temperature dependence caused by ferromagnetic spin waves in the low temperature regions. The Sommerfeld constant γ was extraordinarily large in the normal and superconducting states. The superconducting and magnetic transitions under magnetic fields with the -plane and -axis configurations were summarized in a phase diagram.