# Spindynamics in the antiferromagnetic phases of the Dirac metals $A$MnBi$_2$ ($A=$ Sr, Ca)

## TL;DR

This study measures and models the spin-wave spectra in antiferromagnetic Dirac metal compounds $A$MnBi$_2$ ($A=$ Sr, Ca), finding magnetic dynamics unaffected by Dirac fermions and providing detailed exchange parameters.

## Contribution

It provides the first detailed experimental and theoretical analysis of spin dynamics in $A$MnBi$_2$, revealing magnetic interactions independent of Dirac fermion effects.

## Key findings

- Spin gaps of approximately 8-10 meV observed.
- Magnon dispersion accurately modeled by linear spin-wave theory.
- No detectable influence of Dirac fermions on magnetic dynamics.

## Abstract

The square Bi layers in $A$MnBi$_2$ ($A =$ Sr, Ca) host Dirac fermions which coexist with antiferromagnetic order on the Mn sublattice below $T_\mathrm{N} = 290\,$K (Sr) and $270\,$K (Ca). We have measured the spin-wave dispersion in these materials by triple-axis neutron spectroscopy. The spectra show pronounced spin gaps of 10.2(2)$\,$meV (Sr) and 8.3(8)$\,$meV (Ca) and extend to a maximum energy transfer of 61 - 63$\,$meV. The observed spectra can be accurately reproduced by linear spin-wave theory from an Heisenberg effective spin Hamiltonian. Detailed global fits of the full magnon dispersion are used to determine the in-plane and inter-layer exchange parameters as well as on the magnetocrystalline anisotropy constant. To within experimental error we find no evidence that the magnetic dynamics are influenced by the Dirac fermions.