Spin dynamics in the antiferromagnetic phases of the Dirac metals AMnBi(A $=$ Sr, Ca)
ORAL
Abstract
The square Bi layers in AMnBi(A $=$ Sr, Ca) host strongly spin-orbit coupled bands that form highly anisotropic Dirac points. We have mapped the dispersion of Mn spin fluctuations in these materials by triple-axis neutron spectroscopy. The spectra show pronounced spin gaps of 10.(2) meV (Sr) and 8.(8) meV (Ca) and extend to maximum energy transfers of 61-63 meV. For each material, reciprocal space maps of the magnon spectral weight were recorded for both in-plane and out-of-plane momentum transfer. The observed spectra can be accurately reproduced by a simple linear spin-wave model. Detailed global '?fits of the full magnon dispersion allow tight constraints on the magnitude of in-plane and inter-layer exchange parameters as well as on the magnetocrystalline anisotropy constant. We '?find no evidence that the magnetic ground state of these materials is coupled to the topology of the Bi 6p$_{\mathrm{x,y}}$bands.
*This work was supported by the U.K. Engineering and Physical Sciences Research Council (grant no. EP/J017124/1). MCR is grateful to the Oxford University Clarendon Fund for provision of a scholarship.
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