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Theses

Etude des Langasites magnétiques:
De la frustration magnétique au multiferroïsme

Abstract : This PhD thesis presents the results of synthesis, structural and physical studies of Langasites with magnetic arrays. On one hand, on the compound Pr3Ga5SiO14 (which magnetic network is topologically equivalent to a kagomé net) it was shown by crystal electric field calculations that properties such as specific heat, energy levels measured by inelastic neutron scattering, and magnetic susceptibility (change of anisotropy at 130K) are driven by single-ion physics (without interactions). The origin of the dynamical magnetic signal observed at 50mK in spin echo spectroscopy remains unclear. The study of the Sm-Langasite is more difficult due to the high neutron absorption by the nuclei of samarium and the proximity of fundamental multiplet with excited levels. On the other hand, it was shown that compounds A3BFe3D2O14 (A = Ba, Sr, Ca, B = Nb, Ta, Sb, D = Si, Ge) with triangular array of Fe3+ triangles are magnetically ordering at 35K for compounds with Sb, or 26K without Sb, with magnetic moments lying in the plane of the triangles, oriented at 120 degrees from each other in a triangle, and helically modulated perpendicular to the plane of the triangles. Mean field calculations showed that the chiral crystal structure, resulting in twisted exchange paths, involves a direct correlation between magnetic chirality and helicity. The polarized neutron diffraction on single crystal showed additionally that a single triangular chirality together with a single helicity is stabilized. Measurements of the thermal variation of the dielectric constant indicate a magnetoelectric effect at the magnetic transition.
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https://tel.archives-ouvertes.fr/tel-00351575
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Submitted on : Friday, January 9, 2009 - 3:23:15 PM
Last modification on : Thursday, November 19, 2020 - 1:00:55 PM
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Karol Marty. Etude des Langasites magnétiques:
De la frustration magnétique au multiferroïsme. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2008. Français. ⟨tel-00351575⟩

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