Coexistence of ferromagnetism and superconductivity: Magnetization and Mössbauer studies of EuFe₂(As_1-xP_x) ₂

Magnetization and ⁵⁷Fe and ¹⁵¹Eu Mössbauer studies of EuFe₂(As_1-xP_x )₂ (x = 0-1.0) at temperatures (5-300 K) have been performed. The magnetization studies show a decrease of the divalent Eu sublattice antiferromagnetic transition temperature from T_AFM = 20 K for x = 0 to 16 K at x ≈ 0.2. For x > 0.2, the Eu sublattice is ferromagnetically ordered at T_FM, which increases up to 27 K for x = 1.0. For 0.2 < x < 0.5, the system becomes superconducting. ¹⁵¹Eu Mössbauer studies in the antiferromagnetic range show a constant saturation hyperfine field of 26.2 T and that the magnetization is almost perpendicular to the c-axis. On the other hand, in the ferromagnetic range, the hyperfine field increases up to 30.8 T (for x = 1) and the easy axis is almost parallel to the c-axis. In both regions the magnetic axis seems to be tilted from the basal plane or the c-axis by ∼20°. The ⁵⁷Fe Mössbauer studies show no magnetism in the iron site for x > 0.2, yet at 5 K exhibit transferred magnetic hyperfine fields (∼1 T) from the ferromagnetically ordered Eu sublattice, even in the superconducting region. Superconductivity in the presence of ferromagnetism is generally not observable. However, transferred magnetic hyperfine fields in the superconducting state are observed here for the first time.