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Dynamique de spin dans les structures semi-conductrices de basses dimensions

Abstract : This work is devoted to the studies of spin dynamics of magnetic Mn ions embedded in low-dimensional CdTe-based nanostructures. A particular emphasis is placed on the system of single CdTe quantum dots with single Mn ions, which give an insight into unperturbed interactions of magnetic ion with its semiconductor environment. A method of all-optical manipulation and readout of the spin state of a single magnetic ion is presented. The experiment exploits the spin-conserving exciton transfer between neighbouring quantum dots to inject polarized exitons into the dot with the Mn ion so as to orient its spin. The dynamics of the orientation process is measured in a time-resolved experiment and described with a simple rate equation model. One of possible orientation mechanisms - the dark excitons recombination - is discussed and shown experimentally. The optical orientation of the Mn ion is also used to investigate the spin-lattice relaxation of an isolated magnetic ion in low and high magnetic field. The work also addresses phenomena occurring in low and zero magnetic field in large systems of magnetic moments - quantum wells and quantum dots containing Mn ions. New experimental technique of fast magnetic pulses is used to investigate the Mn spin dynamics in the absence of magnetic field with nanosecond resolution. After the pulse of magnetic field, a fast decay of the magnetization is observed. The experimental findings are described in terms of hyperfine coupling with the nuclear spin of the Mn ions. The effect is highly sensitive to the presence of an anisotropy, particularly that introduced by hole gas and by strain. Application of the external magnetic field suppresses the fast dynamics in diluted magnetic quantum wells and a well known slow spin-lattice relaxation is observed. Surprisingly, quantum dots with many Mn ions show fast spin dynamics even after application of external field up to 0.5 T. The fast process may origin from the spin-spin interaction between the magnetic ions, especially if their spatial distribution within the dot is not uniform.
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Mateusz Goryca. Dynamique de spin dans les structures semi-conductrices de basses dimensions. Autre [cond-mat.other]. Université de Grenoble; 122 Université de Varsovie, 2011. Français. ⟨NNT : 2011GRENY076⟩. ⟨tel-00647043v3⟩

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