S= 12 triangular-lattice antiferromagnets Ba3CoSb2O9 and CsCuCl3: Role of spin-orbit coupling, crystalline electric field effect, and Dzyaloshinskii-Moriya interaction

We have performed the detailed investigations of the magnetization of the S=12 triangular-lattice antiferromagnets Ba3CoSb2O9 and CsCuCl3 with a 120 spin structure in the ab plane. In Ba3CoSb2O9, the magnetic susceptibility (χ) exhibits a broad maximum above the Néel temperature (TN) as is expected in the low-dimensional antiferromagnet (AFM). In CsCuCl3, χ exhibits a continuous increase down to TN as if it is the three-dimensional AFM. This is induced by the strong ferromagnetic (FM) interaction along the c axis. The magnetic phase diagrams are also very different. Although the transition field from the umbrella to the 2-1-coplanar phase (Hu-c) for Hc is almost independent of temperature in Ba3CoSb2O9, it shows a considerable decrease with increasing temperature in CsCuCl3. The temperature independent Hu-c in Ba3CoSb2O9 originates from the magnetic anisotropy from the van Vleck contribution, which does not depend so much on the temperature. The temperature dependent Hu-c in CsCuCl3 originates from the magnetic anisotropy from the Dzyaloshinskii-Moriya (DM) interaction, which decreases with increasing temperature. For Hab, the clear transition from the Y-coplanar to the up-up-down (uud) phase was observed in Ba3CoSb2O9 but not in CsCuCl3. While the reentrant behavior of TN originating from the thermal and quantum spin fluctuations is observed in both compounds, it is pronounced in Ba3CoSb2O9 but small in CsCuCl3. These differences originate from the existence or nonexistence of the DM interaction. The DM interaction in CsCuCl3 suppresses those fluctuations in the ab plane, leading to the less pronounced reentrant behavior of TN and the broad crossover in place of the phase transition. We analyzed the anisotropic magnetization of Ba3CoSb2O9 in the paramagnetic region by the mean field calculation. The spin-orbit (SO) coupling, the uniaxial crystalline electric field, and the isotropic exchange interaction were taken into account. We could estimate the anisotropy ratio of the exchange interaction J/Jz=1.19 and g/g=1.13 with g=4.45 and g=3.95 in the ground state. We emphasized that although the isotropic exchange interaction was used, the above anisotropies at low temperatures are induced simultaneously through the SO coupling and the uniaxial crystalline electric field and they are closely associated with each other.