Magnetic structure and dynamics in the<i>alpha</i>and<i>β</i>phases of solid oxygen

We describe a comprehensive neutron scattering study of \ensuremath{\alpha}- and \ensuremath{\beta}-${\mathrm{O}}_{2}$. In \ensuremath{\alpha}-${\mathrm{O}}_{2}$, 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\ifmmode\pm\else\textpm\fi{}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 \ensuremath{\alpha}-phase staggered magnetization drops with increasing temperature, extrapolating to a purely magnetic transition at 31 K, which is 7 K above the \ensuremath{\alpha}-\ensuremath{\beta} transition. This is only 40% of the mean-field ordering temperature, implying that the magnetic couplings are predominantly two-dimensional (2D). \ensuremath{\beta}-${\mathrm{O}}_{2}$ has 2D short-range magnetic correlations. These results are discussed in light of a model of magnetoelastic coupling in a frustrated triangular antiferromagnet.

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