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Dynamique lente des systèmes magnétiques désordonnés

Abstract : This PhD work presents an experimental study, using Squid magnetometry at low temperatures, of the slow out-of-equilibrium dynamics of several disordered and/or frustrated magnetic systems. In a first part, some spin glass systems are investigated as model disordered and frustrated magnetic systems for the study of aging phenomena. The focus is set on the rejuvenation and memory phenomena which appear in these systems when they are submitted to temperature variations during their isothermal aging. This original behavior of spin glasses, which contrasts with that of more conventional glassy materials (structural glasses, polymers,...) is studied in detail in several samples with various anisotropies. A reduction of the sensibility of aging to thermal variation is observed as the anisotropy is increased, which points out the relevance of this parameter to understand the out-of-equilibrium properties of these systems. The second part of this study is dedicated to aging in disordered ferromagnets. The observation of rejuvenation and memory effects in these systems is difficult to reconcile with the idea that aging can be a priori well described in terms of domain growth processes. An alternative interpretation in terms of hierarchical reconformation dynamics of domain walls pinned by the local disorder is proposed and accounts well for the experimental observations. Finally, in a last part, the slow out-of-equilibrium dynamics of geometrically frustrated antiferromagnets (Heisenberg spins on the kagome and pyrochlore lattices) is studied in order to investigate the respective roles of disorder and frustration in the out-of-equilibrium properties. Whereas isothermal aging in these systems can be well described by the same scaling laws as in spin glasses, a very weak sensibility of aging to thermal variations is observed in these non disordered systems.
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https://tel.archives-ouvertes.fr/tel-00002623
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Submitted on : Monday, March 24, 2003 - 6:54:43 PM
Last modification on : Monday, February 10, 2020 - 6:12:21 PM
Long-term archiving on: : Friday, April 2, 2010 - 6:57:05 PM

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  • HAL Id : tel-00002623, version 1

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Vincent Dupuis. Dynamique lente des systèmes magnétiques désordonnés. Analyse de données, Statistiques et Probabilités [physics.data-an]. Université Paris Sud - Paris XI, 2002. Français. ⟨tel-00002623⟩

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