Magnons and spinons in Ba2CoTeO6 : A composite system of isolated spin- 12 triangular Heisenberg-like and frustrated honeycomb Ising-like antiferromagnets

Abstract

We report the neutron scattering results for magnetic orderings and excitations in ${\mathrm{Ba}}_{2}{\mathrm{CoTeO}}_{6}$ composed of two subsystems, A and B, which are described as an $S=1/2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. We found that the excitation spectra of both subsystems are well separated and independent of each other. Stripy ordering of Ising-like subsystem B was confirmed below ${T}_{\mathrm{N}1}=12.0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, whereas the elastic scattering for subsystem A displays sharp streaks along $(1/3,1/3,L)$ and $(2/3,2/3,L)$ at 0.30 K ($\ensuremath{\ll}{T}_{\mathrm{N}2}=3.0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). These findings reveal that subsystems A and B are almost perfectly decoupled. The excitation spectrum of subsystem A is composed of two single-magnon branches with rotonlike minima at the $M$ point and a clearly structured intense continuum, which is similarly observed in ${\mathrm{Ba}}_{3}{\mathrm{CoSb}}_{2}{\mathrm{O}}_{9}$. Dispersion curves for subsystem B can be described by linear spin wave theory within the third-neighbor exchange interaction.