Collective and local excitations in Ba2CoTeO6 : A composite system of a spin-1/2 triangular-lattice Heisenberg antiferromagnet and a honeycomb-lattice J1−J2 Ising antiferromagnet

Abstract

We report the results of multifrequency high-magnetic-field electron-spin resonance (ESR) measurements on the highly frustrated antiferromagnet ${\mathrm{Ba}}_{2}{\mathrm{CoTeO}}_{6}$. This compound is magnetically composed of two subsystems A and B, which are described as a spin-1/2 triangular-lattice Heisenberg antiferromagnet and a honeycomb-lattice ${J}_{1}\ensuremath{-}{J}_{2}$ Ising antiferromagnet, respectively. ${\mathrm{Ba}}_{2}{\mathrm{CoTeO}}_{6}$ undergoes successive magnetic phase transitions at ${T}_{\mathrm{N}1}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}12.0$ K and ${T}_{\mathrm{N}2}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}3.0$ K. For a magnetic field $H$ parallel to the $c$ axis, subsystem B exhibits successive metamagnetic transitions with magnetization plateaus at one-third and one-half of the saturation magnetization. Below ${T}_{\mathrm{N}2}$, we observed collective ESR modes for $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}c$, which are characteristic of a triangular-lattice Heisenberg antiferromagnet with weak easy-plane anisotropy. We also observed a local excitation mode, which can be assigned as a single flip of the Ising-like spin of subsystem B. From a detailed analysis of the collective and local ESR modes, combined with the magnetization process, we determined the magnetic parameters of subsystems A and B, and confirmed that the two subsystems are almost decoupled.