Magnetic Phase Diagram of the S=1/2 Triangular-Lattice Heisenberg Antiferromagnet Ba3CoNb2O9

TL;DR

This study maps the magnetic phase diagram of Ba3CoNb2O9, a triangular-lattice antiferromagnet, revealing two-step antiferromagnetic transitions and nearly Heisenberg-type interactions with weak anisotropy.

Contribution

It provides the first detailed low-temperature phase diagram of Ba3CoNb2O9, highlighting the importance of interlayer exchange interactions and weak anisotropy in its magnetic behavior.

Findings

Two-step antiferromagnetic transitions at 1.39 K and 1.13 K

Monotonic decrease of transition temperatures with increasing magnetic field

Exchange interactions are nearly Heisenberg type with weak easy-axis anisotropy

Abstract

We report the results of low-temperature thermal and magnetic measurements on Ba$_3$CoNb$_2$O$_9$ powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. Ba$_3$CoNb$_2$O$_9$ is found to undergo two-step antiferromagnetic transitions at $T_{\rm N1}=1.39~{\rm K}$ and $T_{\rm N2}=1.13~{\rm K}$. As the magnetic field is increased, both $T_{\rm N1}$ and $T_{\rm N2}$ 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 Ba$_3$CoSb$_2$O$_9$ [Susuki {\it et al}., Phys. Rev. Lett. \textbf{110}, 267201 (2013)].