Magnetic phase diagram of theS=12triangular-lattice Heisenberg antiferromagnetBa3CoNb2O9

Abstract

We report the results of low-temperature thermal and magnetic measurements on ${\mathrm{Ba}}_{3}\mathrm{Co}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. ${\mathrm{Ba}}_{3}\mathrm{Co}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ is found to undergo two-step antiferromagnetic transitions at ${T}_{\mathrm{N}1}=1.39\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=1.13\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. As the magnetic field is increased, both ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$ monotonically decrease. The magnetic field vs temperature phase diagram indicates that the exchange interactions are nearly of the Heisenberg type with weak easy-axis anisotropy and that the exchange interaction between triangular lattices is crucial, in contrast to the case of the quasi-two-dimensional TLAF ${\mathrm{Ba}}_{3}\mathrm{Co}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9}$ [Susuki et al., Phys. Rev. Lett. 110, 267201 (2013)].