Competition between Fe-based superconductivity and antiferromagnetism of Eu²⁺ in Eu_1-xCa_xFe₂As₂

Single crystals of Eu_1-xCa_xFe₂As ₂ (x 0.25, 0.50 and 0.75) are prepared. With increasing x, the Néel temperature of Eu²⁺ decreases and disappears at x = 0.75, whereas the spin density wave (SDW) transition temperature, T_SDW, is almost unchanged. For x = 0.50, the pressure-induced superconductivity emerges below 20 K and the superconductivity is slightly suppressed by the antiferromagnetic order (AFM) of Eu²⁺. Compared with EuFe ₂As₂, for x = 0.50, the superconductivity seems to overcome the AFM, which is ascribed to the weaker AFM (or lower T_N) of Eu²⁺.