The Interplay of Fe and Ce Magnetism in Ca$_{0.71}$ Ce$_{0.29}$(Fe$_{1-x}$Co$_x$)As$_2$ single crystals*

In this talk, we will present the synthesis and characterization of the Ca$_{0.71}$ Ce$_{0.29}$(Fe$_{1-x}$Co$_x$)As$_2$ single crystals. Elastic neutron scattering complemented by resistivity, susceptibility and heat capacity measurements has revealed a paramagnetic-to-antiferromagnetic phase transition of the Fe sublattice at 69K and a monoclinic-to-triclinic structural phase transition at 73 K in Ca$_{0.71}$ Ce$_{0.29}$FeAs$_2$. In addition, Fe spin reorientation and Ce ordering at lower temperatures, reminiscent of the one in REFeAsO (RE=Ce, Pr, Nd) materials, exist. The Co substitution on the Fe sites completely suppresses the ordering of Fe sublattice at x=0.032. However, it only slightly affects the Ce ordering, which prevents the formation of superconductivity in Ca$_{0.71}$ Ce$_{0.29}$(Fe$_{1-x}$Co$_x$)As$_2$.