Magnetization Process and Quantum Many-Body Effects in Spin-1/2 Triangular-Lattice Antiferromagnet(Research)

Abstract

Hexagonal compound Ba_3CoSb_2O_9 approximates the two-dimensional S=1/2 triangular-lattice Heisenberg antiferromagnet, in which a magnetic Co^<2+> ion has an effective spin-1/2. We have performed high-field magnetization measurements on Ba_3CoSb_2O_9 using powdered sample and single crystals. For powdered sample, we found that the entire magnetization curve including the one-third quantum magnetization plateau is in excellent quantitative agreement with the results of theoretical calculations. However, for single crystals, we observed anisotropy in magnetization process. A one-third quantum plateau is clearly observed for H‖ab, whereas for H‖c, the magnetization curve exhibits a cusp near one-third of the saturation field. The suppression of the one-third plateau for H‖c is ascribed to the easy-plane anisotropy. For H‖ab, we found an small magnetization step above the plateau range indicative of a new high-field quantum phase.