CaMn$_2$Sb$_2$: Spin waves on a frustrated antiferromagnetic honeycomb lattice

TL;DR

This study investigates the magnetic excitations in CaMn$_2$Sb$_2$, revealing it as a highly frustrated antiferromagnet near a tricritical point, with spin wave behavior well described by a Heisenberg model including multiple exchange interactions.

Contribution

It provides the first detailed spin wave measurements of CaMn$_2$Sb$_2$ and identifies its proximity to a tricritical point on the honeycomb lattice, highlighting frustration effects.

Findings

Spin wave dispersion measured up to 24 meV.

CaMn$_2$Sb$_2$ is close to a tricritical point on the honeycomb lattice.

Exchange interactions suggest significant magnetic frustration.

Abstract

We present inelastic neutron scattering measurements of the antiferromagnetic insulator CaMn$_2$Sb$_2$, 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 $\approx$ 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 CaMn$_2$Sb$_2$ 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 $\approx$ 4 times larger than the reported N\'eel temperature T$_{N}$ = 85 K, suggesting significant frustration arising from proximity to the tricritical point.