Magnetic-Field Induced Quantum Phase Transitions in Triangular-Lattice Antiferromagnets

Cs2CuBr4 and Ba3NiSb2O9 are magnetically described as quasi-two-dimensional triangular-lattice antiferromagnets with spin-½ and 1, respectively. We show that both systems exhibit a magnetization plateau at one-third of the saturation magnetization Ms due to the interplay of spin frustration and quantum fluctuation. In Cs2CuBr4 that has a spatially anisotropic triangular lattice, successive magnetic-field induced quantum phase transitions including a magnetization plateau were observed. For Ba3NiSb2O9, we performed exact diagonalization for rhombic spin clusters with up to 21-sites to analyze the magnetization process. The calculated results are in agreement with experimental observations.