Research: Magnon-polaron excitations in noncollinear antiferromag­nets

Magnons and phonons, which are quanta of spin waves and crystal-lattice vibrations in ordered materials, respectively, are a central topic in many areas of solid state research. The compound Mn₃Ge belongs to a family of antiferromagnets with noncollinear arrangements of the magnetic moments, which demonstrate strong hybridizations of the magnon and phonon modes. Detailed inelastic neutron scattering measurements of Mn₃Ge revealed the magnetic excitations that have a 5-meV gap and display an anisotropic dispersive mode reaching ≃ 90 meV at the boundaries of the magnetic Brillouin zone. The spectrum at the zone center shows two additional resonances that demonstrate characteristics of both magnons and phonons. The ab initio lattice-dynamics calculations show that these can be associated with the magnon-polaron modes resulting from the hybridization of the spin fluctuations and the low-energy optical phonons. The observed magnetoelastic coupling agrees with the previously found negative thermal expansion in this compound and resembles the features reported in the spectroscopic studies of other antiferromagnets with the similar noncollinear spin structures.

A. S. Sukhanov, M. S. Pavlovskii, Ph. Bourges, H. C. Walker, K. Manna, C. Felser, D. S. Inosov,
Magnon-polaron excitations in the noncollinear antiferromagnet Mn₃Ge,
Phys. Rev. B 99, 214445 (2019) (arXiv)