Anharmonic phonons and magnons in BiFeO <inf>3</inf>

The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO 3 were measured for temperatures 200≤T≤750K, 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. A clear anomaly is seen in the T dependence of Bi-dominated modes across the Néel transition. These results highlight the importance of spin-phonon coupling in this material. © 2012 American Physical Society.