Magnetic and Structural Phases of MonolayerO2on Graphite

Neutron diffraction studies of ${\mathrm{O}}_2$ thin films physisorbed on the basal plane of graphite show three distinct two-dimensional crystalline phases, all incommensurate with the substrate lattice. The low-temperature monolayer phase has a distorted triangular structure analogous to the closest-packed plane of bulk $\alpha$-${\mathrm{O}}_2$. As in $\alpha$-${\mathrm{O}}_2$, a magnetic superlattice reflection persisting up to $T=10\mathrm{}$ K indicates a well-developed antiferromagnetic order. Above $T=10\mathrm{}$ K the structure is similar to the closest-packed plane of $\beta$-${\mathrm{O}}_2$.