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Proprietes de basse energie et anisotropies d'interactions de systemes magnetiques geometriquement frustres

Abstract : The work presented in this thesis is divided in two parts, both related to geometrically frustrated spins systems. In some highly frustrated lattices (pyrochlore,...) with an antiferromagnetic interaction between nearest neighbour S=1/2 spins, the ground state as well as the first excited states are singlets (S=0). Furthermore, the spin-spin correlations in these spin-liquids are short ranged. In order to study the low temperature behaviour of these systems we derive an effective hamiltonian restricted to the singlet sector and which takes into account the local caracter of the singlets as well as the lattice geometry. Solving this hamiltonian within the mean field approximation, we are able to extract the singlet structure of the ground state. The kagome antiferromagnet has been widely studied for its peculiar magnetic properties and disordered ground state. However, some experimental realizations of the kagome lattice (jarosites compounds) have an ordered low temperature magnetic structure. Taking into account the geometry of the kagomé lattice (or the related jarosites compounds), we show that Dzyaloshinsky-Moriya interactions are relevant for these systems, and a microscopic derivation of this interaction is done. On the other hand, we show by mean-field calculations as well as Monté Carlo simulations that this interaction explains the magnetic structure of the jarosites. Monté Carlo simulations are also used to study the magnetic properties at finite temperature. Quantum fluctuations are considered through a spin-wave expansion. A similar work is done for the pyrochlore lattice. We explicit the Dzyaloshinsky-Moriya interactions allowed by the symmetry of this lattice and the magnetic ordering which appears. We propose that the ordering of some pyrochlore compounds might be due to Dzyaloshinsky-Moriya interactions.
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Contributor : Maged Elhajal <>
Submitted on : Wednesday, November 27, 2002 - 6:32:34 PM
Last modification on : Friday, December 18, 2020 - 1:22:02 PM
Long-term archiving on: : Friday, April 2, 2010 - 6:22:50 PM

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Maged Elhajal. Proprietes de basse energie et anisotropies d'interactions de systemes magnetiques geometriquement frustres. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2002. Français. ⟨tel-00002039⟩

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