From two-dimensional spin vortex crystal to three-dimensional Néel order in the Mott insulator Sr2 F2(Fe1-xMnx)2OS2

We report Mn and Co substitution at the Fe site in the tetragonal checkerboard Mott insulator Sr2F2Fe2OS2, which develops an antiferromagnetic two-k spin vortex crystal at TN=110 K. While Co substitution increases TN to 127 K (x=0.3) in Sr2F2(Fe1-xCox)2OS2, Mn substitution suppresses TN for x up to 0.2 in Sr2F2(Fe1-xMnx)2OS2 replacing the vortex crystal with a spin glass for 0.2<x<0.5. The activation gap determined from resistivity has a local minimum of Eg=0.18 eV for x=0.5 where a new phase appears at TN=45 K. Neutron diffraction data along with corroborating magnetic evidence shows this is a G-type collinear antiferromagnet, with a transition temperature increasing for 0.5<x<0.8. Density functional theory calculations point to the correlated nature of the transition metal d bands: A modest Hubbard energy U=2 eV is sufficient to open a Mott gap around the Fermi level. The calculations also correctly reproduce the ordered spin structure of Sr2F2Fe2OS2 and Sr2F2FeMnOS2. A deeper understanding of the nature of the magnetic ordering is obtained within an effective theoretical model of local moments with doping-dependent competing exchange interactions J1-J2-J2′.