Field-induced low-temperature magnetic phase in alkali-metal sesquioxides

Open p-shell anionic compounds are model systems where the interplay between charge, lattice, orbital, and spin degrees of freedom leads to complex magnetic behavior. In the superoxide NaO2 and the mixed valence sesquioxide Rb4O6, both featuring O2- entities with spin S=1/2 and low-dimensional (D) magnetism, we have recently observed by muSR an unconventional field-induced static magnetic phase out of a dynamical magnetic zero-field ground state. Several questions emerged from these studies, e.g., whether the field-induced phase is generally intrinsic to O2- molecules, whether charge ordering plays a role and whether low-D cooperative magnetism due to orbital order is required to induce it. To address these questions we propose a muSR study of both, the tetragonal and the quenched cubic phases of Rb4O6 and Cs4O6. Contrary to Rb4O6, Cs4O6 reveals charge ordering but no low-D magnetism.