Competition between Fe-based superconductivity and antiferromagnetism of Eu2+ in Eu1-xCaxFe2As2

Single crystals of Eu1-xCaxFe2As2 (x = 0.25, 0.50 and 0.75) are prepared. With increasing x, the Neel temperature of Eu2+ decreases and disappears at x = 0.75, whereas the spin density wave (SDW) transition temperature, TSDW, 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 EuFe2As2, for x = 0.50, the superconductivity seems to overcome the AFM, which is ascribed to the weaker AFM (or lower TN) of Eu2+.