Structure and ferroelectricity at the atomic level in perovskite oxides.

Abstract

Ferroelectricity is a phenomenon that has been studied for nearly 100 years, forming one arm of the large field of ferroics that includes ferromagnetism and ferroelasticity. Ferroelectric materials have shown promise in a wide range of devices such as non-volatile memory devices, micro mechanical actuators and infrared sensors. Further potential can be realised by combining the ferroelectric properties with other ferroic properties to form more complex devices. This thesis aims to leverage the power of transmission electron microscopy to examine ferroelectricity down to the atomic scale. First, the polarisation in ferroelectric tunnel junction devices, consisting of an ultrathin ferroelectric between two electrodes, is studied in context of the depolarisation field created by bound charges in the ferroelectric. This drives the formation of vortex-type structures in the polarisation, however, the free charges in the electrodes act to screen the depolarisation field. This thesis investigates whether curling polarization structures are innate to ferroelectricity or induced by the absence of electrodes. Here it is shown that in unpoled (La0:7Sr0:3)MnO3/PbTiO3/Co ferroelectric tunnel junctions, the polarization in active PbTiO3 layers 3.6 nm thick forms Kittel-like domains, while for 2.4 nm of PbTiO3 there is a complex flux-closure curling behaviour resembling an incommensurate phase. Reducing the thickness to 1.2 nm, there is an almost complete loss of switchable polarization associated with an internal gradient. Additionally, the polarisation for thicker PbTiO3 films extends into the (La0:7Sr0:3)MnO3 whilst the 1.2 nm PbTiO3 film is dominated by the substrate. Next, the relaxor-like ferroelectric Pb2ScTaO6 has been examined, a highly dynamic system that is typified by the presence of polar nano regions that fluctuate rapidly in time. The properties of Pb2ScTaO6 are strongly dependent on the presence of ordering of the Ta and Sc ions into a chequerboard pattern. Here, highly ordered (> 85 %) Pb2ScTaO6 is examined in terms of its local ordering, where small, local fluctuations in the order are accompanied by disorder at anti phase boundaries. Furthermore, evidence for a phase transition at 220 K to the ferroelectric state is found from electron diffraction and dielectric data. The polar fluctuations are also examined, directly using dark field imaging and from scattering phenomena in atomic resolution scanning transmission electron microscopy. It is found that the polar fluctuations do not change in frequency or size as a function of temperature though electron beam effects cannot be excluded. Finally, the application of geometric phase analysis, used to measure strain, is considered when applied to atomic resolution, Z-contrast images. It is demonstrated that, in such image, an additional phase can be present. If the structure changes from one area to another in the image (e.g. across an interface), the change in this additional phase will appear as a strain in conventional geometric phase analysis, even if there is no lattice strain.The origin of the artefact is formalised and strategies to avoid this pitfall are outlined