Infrared-active phonon modes in monoclinic multiferroic MnWO4

TL;DR

This study provides a comprehensive polarization-resolved infrared analysis of MnWO4, identifying all phonon modes and their temperature dependence, clarifying previous controversies, and revealing weak magnetoelastic coupling.

Contribution

It offers the first full polarization analysis of infrared-active phonons in MnWO4, including overlooked modes, and clarifies their temperature behavior with implications for magnetoelastic interactions.

Findings

All infrared-active phonon modes identified unambiguously.

No anomalies observed above 20K, contrary to previous reports.

Weak coupling between lattice dynamics and magnetism in MnWO4.

Abstract

We report on polarized infrared reflectivity measurements of multiferroic, monoclinic MnWO4 between 10K and 295K. The full dielectric tensor and the frequency dependence of the orientation of the principal axes have been determined in the frequency range of the phonons. All infrared-active phonon modes (7 A_u modes and 8 B_u modes) are unambiguously identified. In particular the strongest B_u modes have been overlooked in previous studies, in which the monoclinic symmetry was neglected in the analysis. Using a generalized Drude-Lorentz model, we determine the temperature dependence of the phonon parameters, including the orientation of the B_u modes within the ac plane. The phonon parameters and their temperature dependence have been discussed controversially in previous studies, which do not include a full polarization analysis. Our data does not confirm any of the anomalies reported above 20K. However, in the paramagnetic phase we find a drastic reduction of the spectral weights of the weakest A_u mode and of the weakest B_u mode with increasing temperature. Below 20K, the parameters of the A_u phonon modes for E||b show only subtle changes, which demonstrate a finite but weak coupling between lattice dynamics and magnetism in MnWO4. A quantitative comparison of our infrared data with the quasi-static dielectric constant epsilon_b indicates that the spectral weight of the electromagnon is tiny for E||b, in particular much smaller than the weak changes of the spectral weight of the phonons.