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Théorie pour les systèmes désordonnés de spins localisés en interaction avec des porteurs itinérants : les semiconducteurs magnétiques dilués

Abstract : This thesis is particularly devoted to the study of ferromagnetism in diluted systems, particularly in diluted magnetic semiconductors (DMS). The formalism, based on finite temperature Green functions, is very general to study magnetism and transport in disordered systems. First, it is shown that the often used mean field - RKKY approach (MF-RKKY) is not suitable to describe in a general way the magnetic properties of diluted systems. A better treatment of the Heisenberg RKKY Hamiltonian within self consistent local RPA theory (SC-LRPA) allowed us to show particularly that MF-RKKY approaches largely overestimate both critical temperatures and extent of the ferromagnetism stability region. Thereafter we study the non-perturbative “V-Jpd” model by exact diagonalization for each disordered configuration and one calculates explicitly the corresponding magnetic exchanges. Apart from the perturbative limit, these exchanges are not of RKKY type. Then the effective Heisenberg Hamiltonian is treated within SC-LRPA. This two step approach shows more precisely (i) the importance of disorder and thermal and transverse fluctuations and (ii) that the Coulomb potential V plays a crucial role to understand the origin of the ferromagnetism in DMS. This minimal model wich takes account of the percolation and multiple scattering of the itinerant carriers over the impurities, gives a unified description of DMS's. Lastly, a numerical study of finite size effects and importance of statistical sampling have pointed out severe insufficiencies of the « full » Monte Carlo treatment of the diluted “V-Jpd” model. This microscopic model fills the gap between oversimplified treatments and approaches based on ab initio calculations.
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Richard Bouzerar. Théorie pour les systèmes désordonnés de spins localisés en interaction avec des porteurs itinérants : les semiconducteurs magnétiques dilués. Matière Condensée [cond-mat]. Université de Picardie Jules Verne, 2008. Français. ⟨tel-00367282⟩

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