Anharmonic Phonons and Magnons in BiFeO3
  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. 
Olivier Delaire, Matthew B. Stone, Jie Ma, Ashfia Huq, Delphine Gout,   Craig Brown, Kefeng Wang, and Zhifeng Ren
cond-mat.mtrl-sci cond-mat.other cond-mat.str-el