Magnetic correlations in γ-oxygen: A neutron polarisation study

The magnetic scattering of γ-oxygen was measured by polarised neutrons with polarisation analysis. Its main feature is a diffuse peak around q = 1 Å^-1, essentially broader than that observed for β-oxygen but quite similar to that found in liquid oxygen. This peak is characteristic of antiferromagnetic correlations. The analysis was carried out for different possible correlations between the spin and the axis of the oxygen molecule. The best fit is obtained for an X - Y model (spin perpendicular to the molecular axis), as for the magnetic structures of the α- and β-phase. The magnetic correlations are negative and very strong between the nearest neighbours (the nearest O_II-molecules in the chains), and much weaker between the next-nearest neighbours, leading to a quasi-one-dimension short-range magnetic order.