Magnetic order in the CaFe_1−xCo_xAsF (x=0.00,0.06,0.12) superconducting compounds

A neutron powder-diffraction experiment has been performed to investigate the structural phase transition and magnetic order in CaFe_1−xCo_xAsF superconductor compounds (x=0.00,0.06,0.12). The parent compound CaFeAsF undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the magnetic order in the form of a spin-density wave (SDW) sets in at 114(3) K. The antiferromagnetic structure of the parent compound has been determined with a unique propagation vector k=(1,0,1) and the Fe saturation moment of 0.49(5)μ_B, aligned along the long a axis. With increasing Co doping, the long-range antiferromagnetic order has been observed to coexist with superconductivity in the orthorhombic phase of the underdoped CaFe_0.94Co_0.06AsF with a reduced Fe moment [∼0.15(5)μ_B]. Magnetic order is completely suppressed in optimally doped CaFe_0.88Co_0.12AsF. We argue that the coexistence of SDW and superconductivity might be related to mesoscopic phase separation.