Asymmetric Splitting of an Antiferromagnetic Resonance via Quartic Exchange Interactions in Multiferroic Hexagonal HoMnO3

Abstract

The symmetric splitting of two spin-wave branches in an antiferromagnetic resonance (AFR) experiment has been an essential measurement of antiferromagnets for over half a century. In this work, circularly polarized time-domain THz spectroscopy experiments performed on the low symmetry multiferroic hexagonal HoMnO_{3} reveal an AFR of the Mn sublattice to split asymmetrically in an applied magnetic field, with an ≈50% difference in g factors between the high and low energy branches of this excitation. The temperature dependence of the g factors, including a drastic renormalization at the Ho spin ordering temperature, reveals this asymmetry to unambiguously stem from Ho-Mn interactions. Theoretical calculations demonstrate that the AFR asymmetry is not explained by conventional Ho-Mn exchange mechanisms alone and is only reproduced if quartic spin interactions are also included in the spin Hamiltonian. Our results provide a paradigm for the optical study of such novel interactions in hexagonal manganites and low symmetry antiferromagnets in general.

Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling

“…The energy transfers of the crystal field excitations from multiple peak fittings are found at 1.7, 3.2, and 6.7 meV, which are consistent with the values reported in Ref. [18]. As we expected in an ideal 2D THA, most features of magnons are well captured by the Heisenberg model.…”

Interplay between spin dynamics and crystal field in the multiferroic compound HoMnO3

“…The THz spectrum of HoMnO 3 was first studied in [31] where the Mn (M) magnon mode as well as several Ho crystal field excitations were observed as a function of external magnetic field. The Mn magnon was more recently investigated under light irradiation [40] while its dependence on the Ho-Mn interaction is reported in [32]. Here, thanks to synchrotron based spectroscopy, we are ) Energy (cm -1 ) Energy (cm -1 ) able to measure with great precision the whole spectra from 0.3 to 1.8 THz (10 to 60 cm −1 ).…”

“…This Mn reorientation transition, unique in the hexagonal manganites, shows up also in the electric properties with a drop in the electric polarisation below T SR which is further recovered at T Ho [30]. Signatures of the rareearth Mn coupling in the spin dynamics in an applied magnetic field have also been reported [31,32], although no rare-earth CF-Mn magnon hybridization has yet been observed.…”

Efficient spin excitation via ultrafast damping-like torques in antiferromagnets

“…All anisotropy parameters can be extracted from a fit of the temperature dependence of ω 0 with a minimal model (see Methods section). (The deviations below 25 K are caused by the incipient ordering of the Ho(4b) moments 24,31,32 , see Supplementary Fig. 3).…”