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Étude du rôle joué par le réseau cristallin dans le couplage magnéto-électrique des matériaux multiferroïques

Abstract : In this thesis, we present an infrared spectroscopy study of the magneto-electric coupling of multiferroic materials. We first measured the temperature dependent infrared phonon spectrum of the antiferromagnetic compound MnF2 in order to find the changes induced by the magnetic phase transition on the phonons parameters. Then we measured the phonon spectra of TbMnO3 and MnWO4, which are both multiferroic. These materials behave very differently across the phase transitions. On one hand, the phonon spectrum of TbMnO3 undergoes a renormalisation which is similar to that of MnF2 at the antiferromagnetic transition. In this material, there is also an observable effect of the ferroelectric transition on one phonon polarised in the direction of the spontaneous ferroelectric polarisation, indicating a link between the ferroelectricity and the atomic motions of this particular mode. On the other hand, the phonon spectrum of MnWO4 does not exhibit any detectable change at the magnetic and ferroelectric phase transitions. This is possibly related to a purely electronic origin of the ferroelectricity in this material. We have also studied the magnetic excitations of these materials. In MnF2, the infrared transmission measurements show an absorption which is due to the electric activity of a zone edge bimagnon. This excitation is similar to the electromagnons of multiferroic materials because it is a magnetic excitation activated by an oscillating electric field. In TbMnO3, we measured the infrared absorption of the most intense electromagnon around 60 cm−1 and we showed that its whole spectral weight comes from two phonons polarised in the a direction. The absence of electromagnon in the absorption spectrum of MnWO4 indicates that the existence of an electromagnon in the specturm of multiferroic materials is closely related to a strong coupling between the phonons and the magnetic order.
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Submitted on : Monday, January 24, 2011 - 2:20:16 PM
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  • HAL Id : pastel-00558878, version 1


Renaud Schleck. Étude du rôle joué par le réseau cristallin dans le couplage magnéto-électrique des matériaux multiferroïques. Science des matériaux [cond-mat.mtrl-sci]. Université Pierre et Marie Curie - Paris VI, 2010. Français. ⟨pastel-00558878⟩



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