We present inelastic neutron scattering measurements of the antiferromagnetic insulator ${\mathrm{CaMn}}_2{\mathrm{Sb}}_2$, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the $\mathbf{H}$ and $\mathbf{L}$ directions in reciprocal space, with a maximum excitation energy of $\approx 24$ meV. These excitations are well described by spin waves in a Heisenberg model, including first- and second-neighbor exchange interactions $J_1$ and $J_2$ in the Mn plane and also an exchange interaction between planes. The determined ratio $J_2/J_1\approx 1/6$ suggests that ${\mathrm{CaMn}}_2{\mathrm{Sb}}_2$ is an example of a compound that lies very close to the mean field tricritical point, known for the classical Heisenberg model on the honeycomb lattice, where the Néel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveals a mean field ordering temperature $\approx 4$ times larger than the reported Néel temperature $T_N=85$ K, suggesting significant frustration arising from proximity to the tricritical point.

• Received 20 January 2015
• Revised 7 May 2015
• Corrected 28 May 2015

DOI:https://doi.org/10.1103/PhysRevB.91.180407