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Étude théorique de l'anisotropie magnétique dans des complexes de métaux de transition : application à des complexes mono- et binucléaires de Ni(II) et Co(II)

Abstract : Single molecule magnets are molecular complexes containing transition metal or lanthanides ions which are able to block their magnetization below a certain blocking temperature Tb. This blocking is caused by an energy barrier separating the two orientations of magnetization leading to two stable magnetization states. These two states can be controlled by an external magnetic field.Therefore, it is theoretically possible to use these molecules as bits which are able to store“classical” information. However, due to the quantum nature of these molecules, the relaxation of magnetization can exist even at low temperatures. This phenomenon is called the quantum tunneling effect and prevents the bistable (classical) behavior of the magnetic properties, as well as their use as classical bits for data strorage.Yet, the quantum tunneling of the magnetization also leads to a particular situation at a low temperature where two levels are separated by an energy related to the non-axial character(rhombic) of the magnetization (when the spinis an integer). Such two-levels system could be used as a quantum bit (qbit) which is the basic unit for quantum information processing. Thus,the design of classical or quantum bits require a precise understanding of magnetic properties and their origin at a microscopic level.The Ph.D work was devoted to the theoretical study of the magnetic anisotropy in mononuclear and binuclear Ni(II) (S=1) and Co(II) (S=3/2) complexes. Ab initio calculations based on the wave function theory were carried out and the spin Hamiltonian parameters were extracted. Model complexes were used to investigate the structural and electronic parameters causing magnetic anisotropy.Calculations were, also, performed on complexes synthesized in the laboratory.Comparison between real and model complexes allowed rationalizing the magnetic properties and imagining new synthesis strategies leading to the desired magnetic properties. Binuclear complexes that can be considered as double qbits and used to build quantum logic gates were also investigated. The calculations were performed by fragmenting the binuclear complexes into two mononuclear units in order to study the local anisotropy of each metal ion.The exchange interaction was investigated using Density Functional theory (DFT). In order to study the influence of an external perturbation on magnetic properties, the magnetic properties of a mononuclear Co(II) complex under an external electric field applied parallel or perpendicular to the axis of easy magnetization were calculated. The application of an electric field can lead to important modifications of magnetic properties. Thereby, offering the possibility to the manipulation of these molecules by external electric fields.
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Submitted on : Friday, December 21, 2018 - 12:04:07 PM
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  • HAL Id : tel-01963458, version 1

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Benjamin Cahier. Étude théorique de l'anisotropie magnétique dans des complexes de métaux de transition : application à des complexes mono- et binucléaires de Ni(II) et Co(II). Chimie de coordination. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLS070⟩. ⟨tel-01963458⟩

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