Optical coupling to spin waves in the cycloidal multiferroic BiFeO3

TL;DR

This paper explores the unique spin wave spectrum and optical responses of BiFeO3, revealing how its multiferroic properties enable electrical control and complex magneto-dielectric resonances.

Contribution

It introduces a phenomenological Landau theory analysis of BiFeO3's magnon and phonon spectrum, highlighting distinct features due to incommensurate structure and coupling effects.

Findings

Distinct spin wave spectrum due to zone folding and soft mode anisotropy

Electrical control of spin wave propagation via ferroelectric reorientation

Multiple magneto-dielectric resonances from phonon-magnon coupling

Abstract

The magnon and optical phonon spectrum of an incommensurate multiferroic such as BiFeO3 is considered in the framework of a phenomenological Landau theory. The resulting spin wave spectrum is quite distinct from commensurate substances due to soft mode anisotropy and magnon zone folding. The former allows electrical control of spin wave propagation via reorientation of the spontaneous ferroelectric moment. The latter gives rise to multiple magneto-dielectric resonances due to the coupling of optical phonons at zero wavevector to magnons at integer multiples of the cycloid wavevector. These results show that the optical response of a multiferroic reveals much more about its magnetic excitations than previously anticipated on the basis of simpler models.