Magnetic structure and dynamics in thealphaandβphases of solid oxygen

We describe a comprehensive neutron scattering study of {α}- and {β}-O₂. In {α}-O₂, an inelastic feature at 10 meV is identified with the b*/2 zone boundary, enabling the first measurement of the intrasublattice exchange in this system. We find the intrasublattice exchange constant to be -1.2±{}0.1 meV. Taken in conjunction with the intersublattice exchange, -2.4 meV determined from magnetic susceptibility, we find that the system is very close to a magnetic instability. The {α}-phase staggered magnetization drops with increasing temperature, extrapolating to a purely magnetic transition at 31 K, which is 7 K above the {α}-{β} transition. This is only 40% of the mean-field ordering temperature, implying that the magnetic couplings are predominantly two-dimensional (2D). {β}-O₂ has 2D short-range magnetic correlations. These results are discussed in light of a model of magnetoelastic coupling in a frustrated triangular antiferromagnet.