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 the first 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 N\'eel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveal a mean field ordering temperature ≈ 4 times larger than the reported N\'eel temperature TN = 85 K, suggesting significant frustration arising from proximity to the tricritical point.
@article{arxiv.1505.06153,
title = {CaMn$_2$Sb$_2$: Spin waves on a frustrated antiferromagnetic honeycomb lattice},
author = {D. E. McNally and J. W. Simonson and J. J. Kistner-Morris and G. J. Smith and J. E. Hassinger and L. De-Beer-Schmitt and A. I. Kolesnikov and I. A. Zaliznyak and M. C. Aronson},
journal= {arXiv preprint arXiv:1505.06153},
year = {2015}
}