Identifying the Magnetoelectric Modes of Multiferroic BiFeO$_3$

TL;DR

This study identifies and explains the magnetoelectric modes in multiferroic BiFeO$_3$ using THz spectroscopy, highlighting the effects of anisotropy and spin state distortions on mode splitting and activation.

Contribution

The paper provides the first detailed identification of three magnetoelectric modes in BiFeO$_3$ and explains their behavior considering anisotropy and cycloidal spin state distortions.

Findings

Three magnetoelectric modes identified via THz spectroscopy.

Anisotropy causes splitting and activation of specific modes.

Results align with Raman and neutron-scattering data.

Abstract

We have identified three of the four magnetoelectric modes of multiferroic BiFeO$_3$ measured using THz spectroscopy. Excellent agreement with the observed peaks is obtained by including the effects of easy-axis anisotropy along the direction of the electric polarization. By distorting the cycloidal spin state, anisotropy splits the $\Psi_{\pm 1}$ mode into peaks at 20 and 21.5 cm$^{-1}$ and activates the lower $\Phi_{\pm 2}$ mode at 27 cm$^{-1}$ (T=200 K). An electromagnon is identified with the upper $\Psi_{\pm 1}$ mode at 21.5 cm$^{-1}$. Our results also explain recent Raman and inelastic neutron-scattering measurements.