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Exploring the elasticity of multiferroic materials

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Multiferroics are materials that simultaneously display a ferroelectric and a magnetic order, which in some cases interact. The combination of these properties holds promises for future hybrid computing architectures that transform information from one state into another, such as spin excitations into charge, phonons or photons. Among the various multiferroics, bismuth ferrite (BiFeO3) is a material with exceptional properties that develop at room temperature. It has demonstrated its applicative potential in nanoelectronics, photovoltaics, and for its photostriction properties with the development of optically triggered piezotransducers. However, despite being a very studied compound, the elastic properties of BiFeO3 (BFO), essential for the integration of this material into devices, remained incomplete.

To remedy this, the nanosciences acoustics team of INSP at Sorbonne University, the Process and Materials Sciences laboratory at Sorbonne Paris Nord University and the SQUAP team at the MPQ laboratory have determined all the elastic constants Cij of BFO.

For this, the researchers implemented a picosecond scale pump-probe experiment on a BFO single crystal to measure the sound speeds of acoustic waves. From a set of initial elastic constants calculated by Density Functional Theory, the resolution of the Christoffel equation gives access to the first theoretical speeds. The minimization of the difference between the experimental and theoretical speeds makes it possible to determine the six independent elastic constants Cij. Comparison of the directional dependence of experimental and theoretical sound speeds allows identifying the longitudinal, fast and slow transverse acoustic modes as well as the generalized Rayleigh surface wave.

Figure 1: Polar representation of the experimental (symbols) and calculated (lines) sound velocities of bulk and surface acoustic waves in a BFO crystal.

Figure 2: Animation showing the propagation of surface acoustic waves from the epicenter during the acoustic pump-probe experiment on a BFO crystal.

Maximilien Cazayous (

Elastic properties assessment in the multiferroic BiFeO3 by pump and probe method, P. Hemme, P. Djemia, P. Rovillain, Y. Gallais, A. Sacuto, A. Forget, D. Colson, E. Charron, B. Perrin, L. Belliard, M. Cazayous, Appl. Phys. Lett. 118, 062902 (2021).