Possible antiferroquadrupolar order in the Kondo semiconductor CeOs4Sb12

We report the results of dc magnetization, thermal expansion, and magnetostriction measurements of the Kondo semiconductor CeOs4Sb12. These results provide a magnetic-field-temperature phase diagram for the three principal axes of the cubic crystal [100], [110], and [111] and reveal the magnetic anisotropy of the three ordered phases A, B, and C. The magnetic anisotropy of the transition temperature TC from the paramagnetic phase to the C phase is TC[111]>TC[110]>TC[100]. At low temperatures, the magnetization has a distinct magnetic anisotropy in high magnetic fields: M[100]>M[110]>M[111]. This ratio of magnetization is in good agreement with that of the magnetically anisotropic Γ67 quartet. The observation indicates that the crystalline electric field ground state is a Γ67 quartet and that the c-f hybridization effect is nearly isotropic. In addition, comparison of the present experimental results with numerical calculations of a two-sublattice model using the mean-field approximation suggests that the C phase is a Γ5-type antiferroquadrupolar ordered state despite the presence of strong c-f hybridization effects. In this case, the increase in TC due to the external field is mainly caused by the Γ2-type antiferro-octupolar interaction.