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Multiferroic materials

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Multiferroics are compounds of particular interest for the coupling between the magnetism and ferroelectric properties. Controlling the magnetic properties by electric field is one of the crucial issue. BiFeO3 (BFO) is the most studied multiferroic and it is important to reveal in this material the collective modes that are responsible for ferroelectricity of magnetic origin.

The phonon modes of a BFO single crystal have been assigned using micro-Raman spectroscopy at low temperature which allows us to focus on single ferroelectric domain. We measured a large frequency splitting between the transverse and longitudinal components of the A1 phonon mode in contrast with previous measurements on thin films where the splitting is negligible. Applying an external electric field we induce frequency shifts of the low energy modes related to the Bi-O bonds. These softenings are due to a tensile stress via the piezoelectric effect giving access to the phonon deformation potentials. Although no phonons are expected under 50 cm-1, several low frequency peaks can be observed. We believe that theses polarized modes are of magnetic origin with electric dipole activity. The frequencies of theses electromagnons are in agreement with recent theoretical investigations.

- Polar phonons and spin excitations coupling in multiferroic BiFeO3 crystals
P. Rovillain, M. Cazayous, Y. Gallais, A. Sacuto, R. P. S. M. Lobo, D. Lebeugle, and D. Colson
Phys. Rev. B 79, 180411 (2009)

- Possible Observation of Cycloidal Electromagnons in BiFeO3
M. Cazayous, Y. Gallais, A. Sacuto, R. de Sousa, D. Lebeugle, and D. Colson
Phys. Rev. Lett. 101, 037601 (2008)

- Electric field effect on BiFeO3 single crystal investigated by Raman spectroscopy, M. Cazayous, D. Malka, D. Labeugle, and D. Colson, Appl. Phys. Lett. 91, 71910 (2007).