Magnetoelastic and magnetoelectric couplings across the antiferromagnetic transition in multiferroic BiFeO3

Clear anomalies in the lattice thermal expansion (deviation from linear variation) and elastic properties (softening of the sound velocity) at the antiferromagnetic-to-paramagnetic transition are observed in the prototypical multiferroic BiFeO3 using a combination of picosecond acoustic pump-probe and high-temperature x-raydiffraction 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 TN 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.