Abstract
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 approximate to 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) approximate to 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 Neel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveals a mean field ordering temperature approximate to 4 times larger than the reported Neel temperature T-N = 85 K, suggesting significant frustration arising from proximity to the tricritical point.
