Oxygen monolayers adsorbed on graphite studied by neutron scattering

The phase diagram of absorbed films of ${\mathrm{O}}_2$ molecules on (002) surfaces of graphite has been measured in the range of densities from submonolayer to 1.6 monolayer. This system is particularly interesting because it orders magnetically below 11.9 K for dense monolayer and higher coverages. This magnetic order is observed both through the antiferromagnetic-superstructure-Bragg reflection as well as through the accompanying magnetostrictive distortion which is seen as a splitting of the nuclear Bragg scattering peak. In all, three distinct two-dimensionally ordered phases have been found, each being incommensurate with the graphite surface lattice. Both first-order and continuous transitions have been observed between these phases.