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TUNABLE MULTIFUNCTIONALITIES ACHIEVED IN OXIDE-BASED NANOCOMPOSITE THIN FILMS
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Functional oxide-based thin films have attracted much attention owing to their broad applications in modern society. The multifunction tuning in oxide thin films is critical for obtaining enhanced properties. In this dissertation, four new nanocomposite thin film systems with highly textured growth have been fabricated by pulsed laser deposition technique. The functionalities including ferromagnetism, ferroelectricity, multiferroism, magnetoelectric coupling, low-field magnetoresistance, transmittance, optical bandgap and dielectric constants have been demonstrated. Besides, the tunability of the functionalities have been studied via different approaches.
First, varies deposition frequencies have been used in vertically aligned nanocomposite BaTiO3:YMnO3 (BTO:YMO) and BaTiO3:La0.7Sr0.3Mn3 (BTO:LSMO) thin films. In both systems, the strain coupling effect between the phases are affected by the density of grain boundaries. Increasing deposition frequency generates thinner columns in BTO:YMO thin films, which enhances the anisotropic ferromagnetic response in the thin films. In contrast, the columns in BTO:LSMO thin films become discontinuous as the deposition frequency increases, leading to the diminished anisotropic ferromagnetic response. Coupling with the ferroelectricity in BTO, the room temperature multiferroic properties have been obtained in these two systems.
Second, the impact of the film composition has been demonstrated in La0.7Ca0.3MnO3 (LCMO):CeO2 thin film system, which has an insulating CeO2 in ferromagnetic conducting LCMO matrix structure. As the atomic percentage of the CeO2 increases, enhanced low-field magnetoresistance and increased metal-to-insulator transition temperature are observed. The thin films also show enhanced anisotropic ferromagnetic response comparing with the pure LCMO film.
Third, the transition metal element in Bi3MoMTO9 (MT, transition metals of Mn, Fe, Co and Ni) thin films have been varied. The thin films have a multilayered structure with MT-rich pillar-like domains embedded in Mo-rich matrix structure. The anisotropic magnetic easy axis and optical properties have been demonstrated. By the element variation, the optical bandgaps, dielectric constants as well as anisotropic ferromagnetic properties have been achieved.
The studies in this dissertation demonstrate several examples of tuning the multifunctionalities in oxide-based nanocomposite thin films. These enhanced properties can broaden the applications of functional oxides for advanced nanoscale devices.