Growth and Characterization of Bi3.25La0.75Ti3O12-based Multiferroic Three-dimensional Nanocup Films

Abstract

Over the past decades, the ability to synthesize self-assembled nanocomposite films of complex oxides has paved the way for creating new physical phenomena. Until now, these self-assembled nanocomposite films have demonstrated a variety of fascinating physical phenomena, which include enhanced flux pinning in high-temperature superconductors, strain-enhanced ferroelectricity and multiferroics, enhanced ferromagnetism, magnetoresistance, novel electronic/ionic transport, and coupling of dielectric and optical effects. Furthermore, many other areas remain to be explored for potential practical applications in various materials systems. Among them, self-assembled nanocomposite films containing simultaneously ferroelectric and ferromagnetic phases have attracted considerable attention because they can be used in next-generation electronic devices such as high-density memories, highly sensitive magnetic sensors, and electrically tunable spintronics. For the realization of these devices, the most urgent need is the achievement of a strong multiferroicity and magnetoelectric (ME) coupling effect at room temperature. Compared to the weak room-temperature multiferroicity of the single-phase multiferroics due to chemical contraindication between ferroelectricity and ferromagnetism related to d orbitals, the multiferroic nanocomposites exhibit stronger multiferroicity at room temperature. However, the ME coupling effect in these nanocomposites is transferred from ferroelectric to ferromagnetic phase or vice versa at their interface using piezoelectricity and magnetostriction, so that the combining and control of these simultaneous ferroic orderings are challenging. Many studies have been conducted on multiferroic nanocomposite films to improve this, the most common nanocomposites can be summarized as three types; particulate nanocomposite (0-3 type), laminated nanocomposite (2-2 type), and vertically aligned nanocomposite (1-3 type). Unlike the 0-3 and 2-2 types, 1-3 type nanopillar films have reduced clamping effect of the substrate and efficient strain coupling by the larger interfacial area, which is caused by the intrinsic three-dimensional (3D) heteroepitaxy between two phases. Thus, 1-3 type nanopillar films exhibit a relatively large ME coupling effect compared to the others. Nevertheless, it is still challenging to obtain a strong and genuine ME coupling effect due to the leakage issue of the nanopillar film, resulting from the low resistance of the magnetic pillars penetrating the film matrix. Therefore, novel nanocomposite designs to overcome the structural limitation of conventional nanocomposite films are urgently needed. In the first part of this thesis, we proposed a newly-designed multiferroic three-dimensional (3D) nanocup film. To establish the unique architecture, a heavily Co, Fe-doped ferroelectric Bi3.25La0.75Ti3O12 (BLT) target was used during the growth of BLT thin films via PLD. Employing the density functional theory (DFT) calculations, it was confirmed that CFO could arise from the Co, Fe doped BLT due to the epitaxial strain relaxation. Comprehensive structural investigations revealed that nanocup-shaped ferromagnetic CFOs embedded in the BLT thin film were fabricated. This 3D nanocup BLT-CFO film exhibits room-temperature reversible magnetoelectric switching as well as multiferroicity. Such a reversible ME switching could be achieved because the CFO nanocups were formed on the already-grown 10 nm thick BLT film unlike the conventional nanopillar film so that the leakage path connecting the CFO to the lower electrode was suppressed. These results strongly suggest that the multiferroic 3D nanocup film significantly improves the ME coupling effect. In the second part of this thesis, we demonstrate that the exsolution process, which is modulated by simple doping control, is an efficient method to achieve tunable magnetism and the morphology of nanocups in perovskite ferroelectric BLT thin films. The multimodal structural and magnetic analyses verify that the co-doping of Co and Fe reinforces the phase-separated unique nanocup formation, which is revealed to be a CFO spinel ferrimagnet. By employing density functional theory calculations, we show that different doping combinations change the relative exsolution energies of Co and Fe, which may result in the selective formation of nanocup structures. This development of the multiferroic 3D nanocup films proposes a new paradigm in the architecture design of self-assembled nanocomposite films for diverse multifunctional devices. Moreover, our study presents a simple and effective methodology for the engineering of nanostructure and multifunctionality in nanocomposite films.