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Antiferromagnétisme frustré à deux dimensions : de la géométrie triangulaire dans NaCrO2 à la géométrie kagomé dans Cu3Zn(OH)6Cl2

Abstract : The object of this thesis is the experimental study of bi-dimensional geometrically frustrated antiferromagnetic compounds. In these systems, the magnetic structure composed of triangles prevents from satisfying simultaneously all the antiferromagnetic interactions and can generate original ground states.

We present the µSR and NMR study of NaCrO2, an edge sharing triangular compound that appears as a model Heisenberg system of spin 3/2. We show that it presents a very wide fluctuating regime much below a specific heat maximum, signature of original excitations typical for this class of systems.

We also present the study of Herbertsmithite ZnCu3(OH)6Cl2, the first experimental realisation of a perfect kagomé structure with spin 1/2. We show by µSR that in the larger Paratacamite family ZnxCu4-x(OH)6Cl2, when x increases from 0 to 1, the system evolves at low temperatures from a long range ordered state to a fluctuating, spin liquid one for x>0.66. NMR measurements using the oxygen 17 allowed us to distinguish the intrinsic contribution from that of defects issued from the Cu/Zn substitution. The evolution of the intrinsic susceptibility, as well as the dynamical measurements demonstrate the absence of a gap in the spectrum of magnetic excitations. Our measurements suggest that these excitations have the same weight over the whole Brillouin zone, in agreement with theoretical results. The evolution of the defect contribution is in qualitative agreement with the theoretical image of localised non magnetic singlets created around the impurity.
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Submitted on : Monday, March 31, 2008 - 2:24:11 PM
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Areta Olariu. Antiferromagnétisme frustré à deux dimensions : de la géométrie triangulaire dans NaCrO2 à la géométrie kagomé dans Cu3Zn(OH)6Cl2. Matière Condensée [cond-mat]. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00268177⟩

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