Study of polycrystalline bulk Sr${}_3$OsO${}_6$ double-perovskite insulator: comparison with 1000 K ferromagnetic epitaxial films

Polycrystalline Sr${}_3$OsO${}_6$, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 ${}^{\circ }$C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr${}_3$OsO${}_6$ toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr${}_3$OsO${}_6$ films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr${}_3$OsO${}_6$ 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 ($<$17 K). This value is remarkably high, suggesting the presence of possible fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr${}_3$OsO${}_6$ share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.