Spin-wave induced phonon resonance in multiferroic BiFeO$_3$

TL;DR

This study observes a unique phonon resonance in BiFeO3 linked to magnetic order, revealing a novel spin-lattice coupling mediated by the Dzyaloshinskii-Moriya interaction, with potential implications for multiferroic materials.

Contribution

It provides the first direct observation of a spin-wave induced phonon resonance in BiFeO3, demonstrating a new type of spin-lattice interaction in multiferroics.

Findings

Resonance mode appears near the zone center in BiFeO3.

Resonance intensity increases below the Néel temperature.

External magnetic fields can modify the resonance.

Abstract

We report the direct observation of a "resonance" mode in the lowest-energy optic phonon very near the zone center around (111) in the multiferroic BiFeO$_3$ using neutron scattering methods. The phonon scattering intensity is enhanced when antiferromagnetic (AFM) order sets in at T$_N = 640$~K, and it increases on cooling. This "resonance" is confined to a very narrow region in energy-momentum space where no spin-wave excitation intensity is expected, and it can be modified by an external magnetic field. Our results suggest the existence of a novel coupling between the lattice and spin fluctuations in this multiferroic system in which the spin-wave excitations are mapped onto the lattice vibrations via the Dzyaloshinskii-Moriya (DM) interaction.