Magnetic order in the CaFe1-xCoxAsF (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 CaFe1-xCoxAsF 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)mu(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 CaFe0.94Co0.06AsF with a reduced Fe moment [similar to 0.15(5)mu(B)]. Magnetic order is completely suppressed in optimally doped CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity might be related to mesoscopic phase separation.