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 epsilon phase of molecular oxygen at high pressure. Starting with the delta phase-which is a quasi-two-dimensional distorted triangular arrangement of antiferromagnetically ordered molecules-pressure could decrease the bla ratio in the basal planes pushing it toward the ideal triangular value of 1/root 3, thus increasing magnetic frustration. We conjecture that when frustration takes over, the magnetic order may turn into a 120 degrees planar spin-spiral structure inside the epsilon phase, until at higher pressures hand-overlap metallization suppresses magnetization in the zeta 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.