CaMn₂Sb₂: Spin waves on a frustrated antiferromagnetic honeycomb lattice

We present inelastic neutron scattering measurements of the antiferromagnetic insulator CaMn₂Sb₂, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the H and L directions in reciprocal space, with a maximum excitation energy of ≈24 meV. These excitations are well described by spin waves in a Heisenberg model, including first- and second-neighbor exchange interactions J₁ and J₂ in the Mn plane and also an exchange interaction between planes. The determined ratio J₂/J₁≈1/6 suggests that CaMn₂Sb₂ 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 ≈4 times larger than the reported Néel temperature T_N=85 K, suggesting significant frustration arising from proximity to the tricritical point.