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The defence will be in English.
Abstract: This thesis focuses on the study of cobalt-based compounds with a honeycomb lattice formed by Co2+ ions that can realize Kitaev physics, an exactly solvable model that can lead to a quantum spin liquid state. Two isomorphic compounds were studied in single crystal form: BaCo2(AsO4)2 and BaCo2(PO4)2. The first study focuses on the compound BaCo2(AsO4)2, where the description by a spin Hamiltonian initiated more than 50 years ago remained incomplete. Our measurements of magnetization and AC magnetic susceptibility at very low temperatures confirm the presence of collinear magnetic order with defects in the arrangement of ferromagnetic spin chains. Inelastic neutron scattering measurements reveal the presence of a gap in the low-energy branch of magnetic excitations, implying the need to go beyond a purely planar anisotropy. Actually, Monte Carlo and linear spin-wave simulations demonstrate that a spin Hamiltonian including anisotropic Kitaev-type interactions can indeed explain the spin-wave excitations, magnetic structure, and magnetization data under magnetic field, providing a comprehensive understanding of this compound. In comparison, we experimentally studied the much less studied compound BaCo2(PO4)2. Of which only very small single crystals are available. The magnetization and AC susceptibility measurements show similarities with BaCo2(AsO4)2. However, neutron scattering measurements reveal a commensurate propagation vector unlike BaCo2(AsO4)2. Our results suggest that this compound has the same type of magnetic order with fewer defects, also explained by the presence of Kitaev interactions. These results establish that these two compounds are good candidates for Kitaev physics and that these anisotropic interactions produce original physics.
