Magnetoelastic and magnetoelectric couplings across the antiferromagnetic transition in multiferroic BiFeO 3

Clear anomalies in the lattice thermal expansion (deviation from linear variation) and elastic properties (soft-ening of the sound velocity) at the antiferromagnetic-to-paramagnetic transition are observed in the prototypical multiferroic BiFeO 3 using a combination of picosecond acoustic pump-probe and high-temperature x-ray-diffraction experiments. Similar anomalies are also evidenced using second-principles calculations supporting our experimental findings. Those calculations, in addition to a simple Landau-like model we also developed, allow us to understand the elastic softening and lattice change at T N as a result of magnetostriction combined with electrostrictive and magnetoelectric couplings, which renormalize the elastic constants of the high-temperature reference phase when the critical T N temperature is reached.