Study of Polycrystalline Bulk Sr₃OsO₆ Double-Perovskite Insulator: Comparison with 1000 K Ferromagnetic Epitaxial Films

Polycrystalline Sr₃OsO₆, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 °C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr₃OsO₆ toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr₃OsO₆ films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr₃OsO₆ is under the influence of interfaces. A specific heat capacity of 39.6(9) × 10^–3 J mol^–1 K^–2 is found at low temperatures (<17 K). This value is remarkably high, suggesting the presence of possible Fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr₃OsO₆ share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.