Invariable Simultaneous Emergence of Antiferromagnetic Order and Tetragonal Deformation in CoO Nanocrystals

It remains unclear why type-II antiferromagnetic order with a propagation vector along a trigonal axis and a tetragonal Jahn–Teller deformation coexist in a low-temperature phase of CoO. Nanocrystals generally have a lower magnetic transition temperature but crystal size reduction does not affect structural transition locally induced by a crystal field. On this basis, we investigated the magnetic and structural properties of CoO nanocrystals. Both transition temperatures were equally reduced in nanocrystals, which indicates that magnetic order triggers the tetragonal deformation. This observation is consistent with a model that the Jahn–Teller deformation is assisted by spin–orbit energy splitting of t2g electrons in the high-spin Co2+ ion, proposed by Kugel and Khomskii.