Effect of Co substitution on the magnetic order in Ca(Fe 1 − x Co x)2 As 2 single crystals studied by neutron diffraction

Abstract

Ca(Fe 1 − x Co x ) 2 As 2 single crystals with a concentration x = 0 . 032 , 0.039, 0.051, 0.056, and 0.063 were prepared and investigated by means of magnetic susceptibility and neutron diffraction combined with in situ electrical resistivity. The x - T phase diagram has been constructed. With increasing Co substitution, the anomaly marking the tetragonal to low-temperature orthorhombic structural phase transition temperature T S , the antiferromagnetic phase transition temperature T N , and the ordered magnetic moment gradually decrease. For samples with 0 . 039 ⩽ x ⩽ 0 . 056 , both transitions split and the T N appears at increasingly lower temperatures than T S . There is a small but finite difference between T S determined upon cooling and warming. For samples with x = 0 . 063 , no orthorhombic distortion and no long-range magnetic order is observed. For x ⩾ 0 . 051 superconductivity appears at low temperatures and the onset of the superconducting phase transition temperature T SC decreases slightly with increasing Co content. For the limited concentration range 0 . 051 ⩽ x ⩽ 0 . 056 , both long-range antiferromagnetism and superconductivity coexist. Comparison with Ba(Fe 1 − x Co x ) 2 As 2 indicates a more abrupt disappearance of magnetic order in Ca(Fe 1 − x Co x ) 2 As 2 . We find a direct correspondence between the magnitude of the ordered magnetic moment and the degree of orthorhombic splitting.