Elaboration et étude de couches minces de manganites à valence mixte

Abstract : This thesis reports on properties of Mn oxides of the type L1-xAxMnO3 (L=La, A=Ca, Sr) with x=1/3. These compounds exhibit a paramagnet-to-ferromagnet transition at Tc close to 300 K. In the paramagnetic regime, the electric resistance reveals a semi-conducting behaviour that changes to metallic when the temperature becomes lower than TC. In other words, the magnetic transitition is accompanied by an insulator-to-metal transition. The application of an external magnetic field favours the ferromagnetic ordering of Mn moments which leads to a shift of Tc to higher temperatures which in turn produces an important decrease of the intrinsic resistivity close to this transition, giving rise to a strong magnetoresistive effect called colossal magnetoresistance. In these compounds, many interactions have similar energy scales and compete to determine the ground state, making manganites very interesting materials from the basic point of view (related to the study of strongly correlated systems) as well as for the development of new devices for spin electronics.
Within this frame, this thesis is focused on the three following points :
1. The biggest effort in the experimental work has been dedicated to the setup and automatisation of a radiofrequency sputtering system for the fabrication of thin flms of magnetic
oxides. This systems includes a high vacuum chamber whose atmosphere is controlled by two pumps, electrovalves and fluxmeters. The different elements of the system (substrate
heaters and holders, magnetrons, etc) are computer-controlled, which allows the growth of films and heterostructures in a fully automated fashion.
2. The second part of the work consisted in controlling the growth of thin films of mixed valence manganites, mainly La2/3Ca1/3MnO3, at the nanoscopic scale. The influence of several parameters (temperature, growth rate, etc) on the different key characteristics of the material (cell parameters, transition temperatures, residual resisitivity, surface roughness, etc) has been extensively studied and the reproducible fabrication of high-quality films has been achieved.
3. The main part of the work is focused on the role of interfaces on the physical properties of manganite thin films. It is well know that the interaction responsible for ferromagnetism in these compounds is very sensitive to structural distortions which, combined with a high polarisation of the charge carriers, leads to a strong low-field magnetoresistance in nanometric ceramic powders. This high polarisation also gives rise to a large low-field
response in tunnel junctions. Our contribution consists in the fabrication of artificial interfaces of several types (epitaxial films with in-plane misoriented crystallites, laser-patterned
interfaces and bicrystalline junctions) and the correlation of magnetic and magnetotransport results with data from structural analysis. The influence of the interface with the
substrate (SrTiO3, LaAlO3, NdGaO3) has also been studied by growing fully strained epitaxial films with thicknesses ranging from 2.4 nm to 180 nm. Among the most dramatic effects observed, one can mention a marked decrease of TC accompanied by an increase of
the residual resistivity when reducing the thickness of the films, as well as a multiphase separation between ferromagnetic-metallic, ferromagnetic-insulating and non-ferromagnetic-insulating regions. Apart from this study about the properties of interfaces in manganites, fundamental transport phenomena such as anisotropic magnetoresistance and anomalous Hall effect have been assigned have also been investigated. Their magnitude and temperature dependence have been analysed and a semi-quantitative explanation involving the spin-orbit interaction and the electronic texture close to Tc has been proposed.
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Manuel Bibes. Elaboration et étude de couches minces de manganites à valence mixte. Matière Condensée [cond-mat]. INSA de Toulouse, 2001. Français. ⟨tel-00005931⟩

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