Quantum phase diagram of the S=1/2 triangular-lattice antiferromagnet Ba3CoSb2O9

The magnetic phases of the ideal spin-1/2 triangular-lattice antiferromagnet Ba3CoSb2O9 are identified and studied using Ba-135,Ba-137 nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30 T, oriented parallel and near perpendicular to the crystallographic ab plane. For both directions, the saturation field is approximately 33 T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to a 1/3 magnetization plateau (M-sat/3), as well as for a higher-field phase transition near to similar to(3/5)M-sat for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semiclassical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the c axis.