Multiferroic investigations of Aurivillius phase thin films

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Colfer, Louise
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University College Cork
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In recent years, the amount of data being created and processed is growing at a much faster rate than the rate of computational storage technology development. With CMOS technologies reaching their miniaturisation limits, new disruptive materials are needed to increase data storage capabilities. Technological road-maps have identified room temperature, non-volatile magnetoelectric multiferroic materials as promising candidates for memory scaling within future memory storage devices. Although multiferroic memory devices have the potential to revolutionise memory storage technologies, commercial devices successfully utilising multiferroics have not yet come to fruition. The focus of this thesis is to understand and optimise a rare example of a room temperature magnetoelectric multiferroic, Bi6TixFeyMnzO18 (B6TFMO; x = 2.80 to 3.04; Y = 1.32 to 1.52; Z = 0.54 to 0.64). Aurivillius phase materials, (Bi2O2)(An−1BnO3n+1), where ferroelectric perovskite units are interleaved between dielectric [Bi2O2]2+ layers, are flexible scaffolds for technological applications. While earlier studies indicated that B6TFMO is a promising material for future memory devices, my thesis presents significant advances in the characterisation, understanding and optimisation required towards implementing the material in fully realised devices. In this work, correlation between the octahedral tilting and atomic-level structural distortions with functional electronic and magnetic properties of B6TFMO were determined, revealing that crystal field splitting of the Ti4+ octahedra is influenced by its position within the Aurivillius unit cell. Theoretical calculations determined that this is predominantly driven by changes in the extent of tetragonal distortion along the c-direction. Atomic scale mapping of polar displacements reveals this has a direct impact on the ferroelectric properties. Polarisation is largest towards the outer perovskite cells, correlating with an increased extent of local tetragonal distortion of octahedral geometries. Experiments demonstrate that tilting of the BO6 octahedra competes with the extent of tetragonal distortion of the TiO6 octahedra, where the degree of octahedral tilting increases towards the central layers of this Aurivillius system, where the magnetic cations preferentially partition. This work presents the first indication that octahedral tilting might be an important enabler of long-range magnetic interactions and subsequent multiferroic behaviour in B6TFMO. Delving deeper into fundamental understandings of B6TFMO’s antipolar and magnetic behaviour, the purposeful inclusion of structural defects within the layered structure of B6TFMO and how they can impart elastic strain and electrostatic energy changes which in turn influence polar behaviour is explored. The findings show that the vicinal sapphire substrates (mis-cut angle 0.2 o to 10 o) are successful for promoting the propagation of sub-unit-cell defects and disruptions to the periodicity of the Aurivillius phases. This has a marked effect on the film morphology and ferroelectric properties. Macroscopic and local measurements show that defect, crystal grain and ferroelectric domain density increases with increasing substrate mis-cut angle. Atomic resolution polarisation mapping showed that charged domain walls alongside exotic polar vortices are facilitated by OPBs when two OPB defects are spaced 5 nm apart. This work provides insight into methods for successfully controlling defect levels and how polar vortex domain walls and charged domain walls are promoted within layered multiferroics by tailoring the underlying substrate that the film is grown on. Moving on from vicinal sapphire surfaces, patterned sapphire with 3D domes were used to encourage the growth of the Aurivillius grains towards an upright geometry. An increased number of non-(00l) reflections were present in the B6TFMO films on patterned sapphire along with evidence from STEM imaging showing that B6TFMO grains grow along the incline of the patterned sapphire domes. With the growth of the crystal grains towards an upright geometry it would be expected that access to the major a-axis polarisation via out-of-plane measurement would be improved, however with a maximum inclination angle of 60 ° achieved with the 3D dome architectures, the out-of-plane piezoresponse of the samples remained weaker than the in-plane piezoresponse. Studies of the magnetic properties of the films demonstrated that the B6TFMO samples were ferromagnetic at room temperature. These findings provide further evidence of room temperature multiferroic behaviour in B6TFMO. Lastly, the role of bismuth excess and substrate strain were investigated to optimise the epitaxial growth of B6TFMO via DLI-CVD. A single-step deposition method on epitaxial substrates was developed to allow the successful synthesis of continuous 45 nm thick B6TFMO films at thicknesses relevant to applications as piezoelectric actuators, sensors and energy harvesters. These films nucleated via a layer-by-layer growth mode and were found to have a strong in-plane ferroelectric response with isotropic domains. Film purity was enhanced with utilisation of epitaxial substrate with appropriate lattice match to B6TFMO and by optimising the amount of bismuth precursor used. In this work, progress was made towards the optimisation of epitaxially grown B6TFMO films, allowing greater control of film orientation and augmenting strain-induced enhancement of multiferroic properties in future data storage devices. Overall, this research has increased understanding of the fundamental mechanisms governing the ferroelectric and ferromagnetic properties of B6TFMO. The work has elucidated some of the key requirements fundamental to the manifestation of polar topologies and has created strategies for the tailoring of novel polar topologies. This combination of new material understanding and new growth optimisation of room temperature multiferroics contributes to solving the ‘big data’ problem. Application of B6TFMO in future technologies based on ultra-high density, energy efficient memory devices, spintronic devices, multilevel resistance control (memristive and synaptic devices) and energy-efficient neuromorphic “brain inspired” devices are envisioned.
Multiferroic , Aurivillius , Ferroelectric , Characterisation
Colfer, L. A. 2023. Multiferroic investigations of Aurivillius phase thin films. PhD Thesis, University College Cork.
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