Anharmonic Phonons and Magnons in BiFeO3

TL;DR

This study investigates the temperature-dependent phonon and magnon spectra of BiFeO3 using inelastic neutron scattering, revealing strong anharmonicity and spin-phonon coupling without evidence of a spin gap.

Contribution

It provides the first detailed measurement of phonon and magnetic excitations in BiFeO3 across a wide temperature range, highlighting anharmonic effects and spin-phonon interactions.

Findings

Strong temperature-dependent broadening of phonon DOS

No evidence of a spin gap in magnetic spectrum

Large-amplitude Bi and O ion motions indicating anharmonicity

Abstract

The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO$_3$ were measured for temperatures $200 \leq T \leq 750\,$K, using inelastic neutron scattering (INS). Our results indicate that the magnetic spectrum of BiFeO$_3$ closely resembles that of similar Fe perovskites, such as LaFeO$_3$, despite the cycloid modulation in BiFeO$_3$. We do not find any evidence for a spin gap. A strong $T$-dependence of the phonon DOS was found, with a marked broadening of the whole spectrum, providing evidence of strong anharmonicity. This anharmonicity is corroborated by large-amplitude motions of Bi and O ions observed with neutron diffraction. These results highlight the importance of spin-phonon coupling in this material.