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Nouveaux états quantiques de spin induits par frustration magnétique sur le réseau kagome

Abstract : Magnetic frustration helps destabilizing conventional Néel order at T = 0 in dimensions 2, and therefore allows the emergence of new original quantum phases. The S=1/2 Heisenberg Hamiltonian on the highly frustrated kagome lattice, which is made of corner-sharing triangles, is expected to stabilize such quantum states, including the spin liquid ones which do not break any symmetry even at T = 0. This thesis work focuses on the experimental study of two kinds of S=1/2 (Cu2+) kagome compounds using NMR and μSR local probes as well as thermodynamic measurements (magnetic susceptibility, specific heat).In Mg-herbertsmithite magnetic frustration occurs thanks to a first nearest-neighbor antiferromagnetic interaction J and is responsible for the spin liquid behavior observed down to T = J/10000. In comparison with the formerly known isostructural counterpart Zn-herbertsmithite, we showed that it shares similar physical magnetic properties while allowing sensitive structural refinements and therefore a control of the level of Cu/Mg substitutions defects. Our experiments performed on such well controlled materials allow us to investigate the origin of the dynamical relaxation in these compounds in relation with the existence of interplane spins defects. Kapellasite and haydeite possess both ferromagnetic (J1) and antiferromagnetic (Jd) interactions. They offer the possibility to explore the phase diagram generated by such competing interactions on the kagome lattice. For kapellasite, our μSR experiments evidenced a spin liquid character down to T ≈ J1/1000. We tracked the temperature dependence of the magnetic susceptibility probed by 35Cl-NMR as well as of the specific heat, from which the ratio Jd/J1 = 0.85 can be evaluated. This ratio locates the ground-state of kapellasite to be within an original non-coplanar spin phase described by 12 magnetic sublattices and called cuboc2. Magnetic exchanges in haydeite locate its ground-state within the ferromagnetic phase. Both our local and thermodynamic measurements point to a partial ferromagnetic transition at T = 4 K. This study confirms the relevance of the frustrated quantum kagome lattice to stabilize original quantum phases and suggests the existence of a new spin liquid phase, distinct from the one expected for antiferromagnetically coupled spins.
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Edwin Kermarrec. Nouveaux états quantiques de spin induits par frustration magnétique sur le réseau kagome. Autre [cond-mat.other]. Université Paris Sud - Paris XI, 2012. Français. ⟨NNT : 2012PA112346⟩. ⟨tel-00783605⟩

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