Physical properties of BFO thin film heterostructures under ansiotropic epitaxial strain

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Copyright: Paull, Oliver
This thesis represents an effort to understand the structure of anisotropically strained Bismuth Ferrite (BiFeO3 BFO). This is executed by using anisotropic epitaxy and exploring the structure, magnetism, and electromechanical response in anisotropically strained BFO at various levels of average in-plane strain. This includes in the vicinity of the strain-induced morphotropic phase boundary where large enhancements to the electromechanical performance are identified. Bismuth ferrite (BFO) is a room-temperature magnetoelectric material that is able to easily adapt its crystal structure to accomodate any strain that is applied to it. By utilising high-index crystallographic substrates the effect anisotropic epitaxial strain has been explored using three different substrate materials (SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT), and LaAlO3) each with four orientations. The unit cell parameters of the BFO films behave linearly when weakly compressively strained on SrTiO3, and become more non-linear on LSAT. The strain-driven morphotropic phase boundary in BFO films grown on tilted LaAlO3(310) surfaces is able to stabilise a low-symmetry bridging phase between the well known M_A and M_C symmetries of BFO when deposited on SrTiO3 and LaAlO3 respectively. The anisotropic strain conditions of the substrate miscut force the BFO film to maintain strain along a high-symmetry in-plane direction whilst partially relaxing in the orthogonal low-symmetry in-plane direction. Interferometric displacement sensor (IDS) measurements indicate that the intrinsic piezoresponse of this new phase of BFO is double that of the R'-like version. Moreover we see spectroscopic indications through IDS and band-excitation frequency response measurements that there is a field-induced phase transition occurring under electric field wherein the low-symmetry phase is reversibly interconverted into the tetragonal-like phase creating a giant effective electromechanical response. These observations are fully supported by density functional theory and effective Hamiltonian calculations. When growing thicker films of this soft low-symmetry phase, a rich and detailed phase coexistence between the R', T', and bridging phase arise that is reminiscent of a highly tilted mixed-phase BFO. The topography of these samples also exhibit domain-like periodic stripes that evolve with the crystallography and are intimately linked together. \\ At the end of this thesis a number of neutron scattering experiments are presented on BFO films on YAlO3, LaAlO3, LSAT, and SrTiO3 substrates. Despite calculations and some experimental hints of a C-type antiferromagnetic phase in T'-BFO, there appears to be no evidence of this magnetic phase in BFO//YAO and BFO//LAO. Additionally, a cycloid model has been developed and implemented in order to fit ambiguous cycloidal peaks with a constrained model. This model is applied to two different systems of BFO with the results and interpretations discussed.
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PhD Doctorate
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