Elastic and electrical anomalies at low-temperature phase transitions in BiFeO₃

Our measured dielectric constant and mechanical response of multiferroic BiFeO₃ indicate four phase transitions below room temperature. Features correlate with those reported at 50K (from a peak in the zero-field-cooled magnetic susceptibility) and 230K (from splitting between field-cooled and zero-field-cooled magnetic data; Singh et al 2008 Phys. Rev. B 77 144403), and those at 140 and 200K (from the magnon light scattering cross section; Singh et al 2008 J. Phys.: Condens. Matter 20 25203). The primary order parameter is not the polarization in any of the low-T transitions. Instead, the transition near 200K shows strong elastic coupling, while that at 50K is fundamentally magnetic, but magnetostrictively coupled to the lattice. The low-T phase transitions display glassy behaviour. A further anomaly at 140K interpreted as spin reorientation (Singh et al 2008 J. Phys.: Condens. Matter 20 25203; Cazayous et al 2008 Phys. Rev. Lett. 101 37601) shows only weakly in dielectric and mechanical studies, indicating that it is predominantly magnetic with little coupling to any of the other order parameters.