Magnon-polaron excitations in the noncollinear antiferromagnet Mn${}_3$Ge

Abstract

We present the detailed inelastic neutron scattering measurements of the noncollinear antiferromagnet Mn${}_3$Ge. Time-of-flight and triple-axis spectroscopy experiments were conducted at the temperature of 6~K, well below the high magnetic ordering temperature of 370~K. The magnetic excitations have a 5-meV gap and display an anisotropic dispersive mode reaching $\simeq 90$~meV at the boundaries of the magnetic Brillouin zone. The spectrum at the zone center shows two additional excitations that demonstrate characteristics of both magnons and phonons. The \textit{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.

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• Complex magnetic structure and spin waves of the noncollinear antiferromagnet Mn5Si3
• Anisotropic magnetic interactions in hexagonal AB-stacked kagome lattice structures: Applications to ${\mathrm{Mn}}_3\mathrm{X}$ ($\mathrm{X}$ = $\mathrm{Ge}$, $\mathrm{Sn}$, $\mathrm{Ga}$) compounds
• Magnon Polarons induced by a magnetic field gradient
• Momentum-dependent magnon lifetime in the metallic non-collinear triangular antiferromagnet CrB2
• Magnetic interactions in AB-stacked kagome lattices: magnetic structure, symmetry, and duality