Magnetic structures of δ-O2 resulting from competition of interplane exchange interactions

Solid oxygen is a unique molecular crystal whose phase diagram is mostly imposed by the magnetic ordering, i.e., each crystal phase (except γ-O2) has a specific magnetic structure. However, recent experiments showed that high-pressure δ phase is realized in different magnetic structures. In the present paper we study the role of interplane exchange interactions in formation of the magnetic structures with the different stacking sequences of the close-packed planes. We show that the temperature-induced variation in intermolecular distances can give rise to compensation of the exchange coupling between the nearest close-packed planes and result in the phase transition between different magnetic structures within δ-O2. Variation in the magnetic ordering is, in turn, accompanied by stepwise variation in interplane distance governed by spatial and angular dependence of the interplane exchange constants