Noncolinear spin polarization from frustrated antiferromagnetism: A possible scenario for molecular oxygen at high pressure

We perform density-functional calculations of the magnetic properties of a simplified structure aimed at capturing some of the features of the elusive ε phase of molecular oxygen at high pressure. Starting with the δ phase---which is a quasi-two-dimensional distorted triangular arrangement of antiferromagnetically ordered molecules---pressure could decrease the $b/a$ ratio in the basal planes pushing it toward the ideal triangular value of 1/√3, thus increasing magnetic frustration. We conjecture that when frustration takes over, the magnetic order may turn into a 120^∘ planar spin-spiral structure inside the ε phase, until at higher pressures band-overlap metallization suppresses magnetization in the ζ phase. This conjecture is substantiated by calculations that also represent the attempt to apply state-of-the-art pseudopotential techniques to the magnetic properties of a frustrated antiferromagnet.