Plonųjų manganitų sluoksnių tyrimas stipriuose impulsiniuose elektriniuose ir magnetiniuose laukuose

Abstract

The main aim of this work was to investigate electrical conductivity of La-Ca(Sr)-MnO thin films at high pulsed electric and magnetic fields and to clear up the possibilities to use these materials for high pulsed magnetic field sensor and fault current limiter applications. The dissertation consists of the preface, six chapters, summary and main conclusions, references, list of publications and abstract (in Lithuanian). The main objectives of the work, scientific novelty, goals, validation of results, and statements for defense are presented in the preface. Chapter 1 presents an introduction and review of previous works on electroresistance (ER) and magnetoresistance (MR) phenomena in manganites. Chapter 2 presents a description of two deposition techniques which were used for preparation of thin manganite films: metal organic chemical vapour deposition (MOCVD) and pulsed laser deposition (PLD). Measurement equipment and methods are described in Chapter 3. The resistance dependence on voltage was investigated using electric pulses with duration of 5–30 ns and amplitude up to 1000 V in the temperature range of 4.2–300 K. For magnetoresistance measurements a pulsed magnetic field generator, which generates magnetic field pulses of 0.6–2 ms duration with amplitude up to 50 T was used. Chapter 4 presents an investigation of surface morphology of prepared films and a characterization of their properties at low electric and magnetic fields. The electric and magnetic properties of epitaxial films – an increase of electrical resistance and a decrease of the resistance maximum temperature with a decrease of film thickness – were explained by the existence of two different phases in ultrathin films. Chapter 5 presents investigations of the electroresistance induced by the nanosecond duration strong electric field pulses in thin La-Ca(Sr)-MnO films. The ER was measured in the temperature range of 4.2–300 K up to electric field strength of 80 kV/cm and its behavior was studied. The highest negative ER (–93 %) was obtained for polycrystalline La-Ca-MnO films grown by the pulsed laser deposition technique. It was concluded that the ER of electronic origin in the polycrystalline and epitaxial films is a result of strong electric field effects at structural imperfections, i.e. grain boundaries (GB). The results were analyzed by using an empirical formula of conductivity for tunneling across grain boundaries and three parameters: σ0 – low-voltage conductivity associated with electron elastic – direct and resonant – tunneling across GB, nonlinear conductivity coefficient σ1, and applied voltage exponent α indicating predominant tunneling mechanism, were obtained for the investigated films. It was found that depending on film preparation conditions and temperature the following charge transport mechanisms could take place: inelastic tunneling through localized states for films grown on lucalox and sapphire, while space charge limited currents or electron-electron interaction in weakly disordered solids for films grown on MgO substrate. Finally, a fast high-power fault current hybrid superconducting-magnetic limiter having low threshold voltages and operating at liquid nitrogen temperature was suggested. Chapter 6 presents an investigation of magnetoresistance (MR) measurements in thin La-Ca(Sr)-MnO films. The MR was measured in the temperature range of 80–320 K in pulsed magnetic fields up to magnetic inductance of 50 T. The negative MR in epitaxial and polycrystalline manganite thin films was analyzed and explained using the modified Mott’s hopping model for both ferromagnetic and paramagnetic phases. It was concluded that a high magnetic field mostly influences the magnetic properties of grain boundary material. In the frame of this model, it was found that the polaron spin moment J in polycrystalline or ultrathin epitaxial films decreases in comparison with thick epitaxial ones and this decrease is most likely related with the increase of structural disorder of the material. The anisotropy of magnetoresistance of La-Sr-MnO films was studied around room temperature in order to use these films for the development of a high pulsed magnetic field sensor. It was found the small MR anisotropy of polycrystalline La0.83Sr0.17MnO3 films in temperatures around room temperature and concluded that it is a result of partial texture of the films. The obtained MR anisotropy of polycrystalline films was significantly smaller in comparison to epitaxial ones of the same doping, which at the room temperature were in ferromagnetic phase. It was concluded that the obtained MR anisotropy of epitaxial films is due to variation of an angle between magnetization and current flow (AMR effect) and demagnetization field anisotropy of thin manganite film (shape effect). Finally, the high pulsed magnetic field sensor measuring the magnitude of magnetic field (B-scalar sensor) at the room temperature, based on thin polycrystalline La0.83Sr0.17MnO3 and La0.67Ca0.33MnO3 films, was developed. Summary and main conclusions present the general conclusions and results of the research and describe their practical applications. References include articles and sources of data and information used in this work. List of publications presents articles in scientific journals and proceedings, in which the main results of the research described in this dissertation have been published.