Magnetic structure and dynamics in thealphaandβphases of solid oxygen

We describe a comprehensive neutron scattering study of α- and β-${\mathrm{O}}_2$. In α-${\mathrm{O}}_2$, an inelastic feature at 10 meV is identified with the $b^{\mathrm{*}}$/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-${\mathrm{O}}_2$ has 2D short-range magnetic correlations. These results are discussed in light of a model of magnetoelastic coupling in a frustrated triangular antiferromagnet.