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Theses

Systèmes magnétiques à frustration géométrique: approches expérimentale et théorique

Abstract : In this thesis, the magnetic properties of geometrically frustrated systems have been studied, using experimental and theoretical approaches. In the manuscript, the study of magnetic lattices based on corner-sharing triangles is reported in three different parts. The first part concerns the La3Cu2VO9 compound, formed by weakly coupled frustrated planar clusters, each one being constituted of 9 coupled spins 1/2. In this system, different regimes are successively stabilized when the temperature is decreased: the high temperature paramagnetic regime of individual spins is followed by a paramagnetic regime of collective pseudo-spins 1/2 associated to each cluster below 20 K. Finally, short range inter-cluster correlations emerge below 2 K, indicating a hierarchical rise of the correlations. The following parts are dedicated to the study of the dynamical properties of the kagome lattice. We first show that the langasite compound Nd3Ga5SiO14, in which the anisotropic magnetic moments carried by the Nd3+ ions form a kagome lattice, does not present any magnetic ordering down to 2 K, despite a Curie-Weiss temperature of a few tens of Kelvin. Moreover, we could observe a slowing down of the spin fluctuations below 300 K. Finally, we present a numerical study of the spin dynamics of the Heisenberg antiferromagnet on the kagome lattice. We show that unexpected collective excitations develop below T/J=0.01, in spite of the very short spin-spin correlation length in the system. In addition, some of the excitations are characterized by a non-uniform distribution of spectral weight. This is understood as an effect of the particular geometry of this network.
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https://tel.archives-ouvertes.fr/tel-00257237
Contributor : Julien Robert <>
Submitted on : Monday, February 18, 2008 - 5:51:54 PM
Last modification on : Friday, December 18, 2020 - 1:08:04 PM
Long-term archiving on: : Thursday, May 20, 2010 - 10:43:00 PM

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  • HAL Id : tel-00257237, version 1

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Julien Robert. Systèmes magnétiques à frustration géométrique: approches expérimentale et théorique. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2007. Français. ⟨tel-00257237⟩

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