Magnon-polaron excitations in the noncollinear antiferromagnet Mn3Ge

Abstract

We present the detailed inelastic neutron scattering measurements of the noncollinear antiferromagnet Mn3Ge. 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 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 ab initio latticedynamics 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. arXiv:1906.07640v1 [cond-mat.str-el]

Observation of Magnon Polarons in a Uniaxial Antiferromagnetic Insulator

“…Understanding how magnon-phonon coupling and magnon polarons affect the generation and transport of spin currents in AFMs becomes increasingly important to antiferromagnetic spintronics. In general, AFM magnon dispersion lies above that of acoustic phonons [27]. One exception is the noncollinear AFMs in which the low-lying magnon excitation modes can hybridize with the acoustic phonon modes.…”

Complex magnetic structure and spin waves of the noncollinear antiferromagnet Mn5Si3

“…4), we observe almost an uniform intensity throughout the whole measured energy range thus not requiring an energy-dependent broadening in the calculated spin-wave spectra. Returning to the model results, the acoustic spin-wave modes are centered around the AFM zone centers and have the characteristic V-shape typically observed in several systems [4][5][6][38][39][40] . An optic mode with an energy minimum of about 12 meV is in good agreement with the one determined experimentally with INS (see Fig.…”

“…Characteristic examples are the recent observation of large anomalous transport properties near room temperature, such as the anomalous Nernst 1 and Hall 2,3 effects, in the metallic noncollinear antiferromagnetic systems Mn 3 X (with X = Sn, Ge). In turn, these discoveries have initiated spectroscopic studies that have provided new insights of the intimate coupling between the various degrees of freedom, i.e., spin, lattice and electronic, which could explain the interesting anomalous phenomena in these materials [4][5][6] . Therefore, it is evident that an experimental study of the spin dynamics and a comparison with theoretical models is crucial for understanding the origin of noncollinear spin arrangements and the peculiar properties that arise in ordered solid materials.…”

Momentum-Dependent Magnon Lifetime in the Metallic Noncollinear Triangular Antiferromagnet CrB2

“…Meanwhile, it is also important to discuss how the contribution of the Stoner continuum to the observed magnon spectra is less dominant in CrB 2 , despite its prevalence in metallic antiferromagnets as a primary factor of magnon damping. One possible reason is that the observed magnon branches are mostly in the low energy region (< 15 meV) over the large q space, different from steep V-shape magnon dispersion seen in other metallic magnets [15,[28][29][30]35,36,41,42]. Since the Stoner continuum DOS is proportional to the energy transfer, those magnon branches may suffer less from the Stoner continuum than those in usual metallic antiferromagnets whereas its influence comes into effect for the high-q magnon modes with higher energies.…”

“…Another reason is the nonzero ordering wave vector of CrB 2 , enabling its magnon branches to avoid the dense region of the Stoner continuum near the Γ point [20,43]. As an example, recent studies on the INS spectra of Mn 3 ðSn; GeÞ, which host both steep V-shape dispersion up to 70 meV and a zero or nearby ordering wave vector, have shown severely damped magnon spectra from the Stoner continuum [15,41,42].…”