Cobalt-doping effects in single crystalline and polycrystalline EuFe2−xCoxAs2 compounds

A series of Co-doped EuFe2−x Co x As2 compounds were prepared in both of single crystalline and polycrystalline forms. The Co-doping effects on the crystal structure, electrical resistivity and magnetic susceptibility were systematically studied. Superconductivity was found in polycrystalline Co-doped samples from zero resistivity effects, with the highest onset superconducting transition temperature at 26 K in the optimum doped EuFe1.84Co0.16As2 compound. While due to the stronger competition between the superconducting order and the Eu2+ magnetic order, the zero resistivity effect is absent in the Co-doped single crystal samples.