Influence of Strain and Anisotropy on Structure of BiFeO3 Epilayers and their Utilization as Interface Driven Heterostructures for Multiferroic Device Applications

M., Saj Mohan M and Ramadurai, Ranjit (2019) Influence of Strain and Anisotropy on Structure of BiFeO3 Epilayers and their Utilization as Interface Driven Heterostructures for Multiferroic Device Applications. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Bismuth ferrite (BFO) a room temperature multiferroic material has been studied extensively in recent years due to the interesting features it exhibits with a ferroelectric transition temperature (TC~1103 K) and a canted antiferromagnetic spin cycloid (period ~ 64nm) type ordering with a Neel temperature (TN~ 643 K). At room temperature BFO stabilizes in rhombohedral structure with R3c symmetry in bulk ceramics. It was demonstrated that the substrate strain and orientations can be used for controlling the structural domain variants, polarization directions, for which there have been sufficient experimental as well as theoretical evidences based on manipulation of the ferroelectric domain structures by using vicinal substrate or strain tuning. A minor change in c/a ratio was observed and the BFO layer was expected to possess a tetragonal symmetry with out-of-plane polarization components. The epitaxial BFO grown on other substrates including YAlO3, LaAlO3, LaSrAlTiO4, etc. with a high c/a ratio has shown significant change in the ferroelectric behavior of the material. More over the high epitaxial strain will cause the ferroelectric Curie temperature to come down further toward the Neel temperature of BFO, which is expected to improve the magneto-electric coupling properties. The strain driven structural transformations with varying thickness and temperature driven transformation of the BFO layer on these substrates were studied extensively. These transformations happen through different types of monoclinic distortions associated with the transition. These structural transformations were observed, using the different substrate strain, chemical pressure, and thickness of the epitaxial BFO film. And these structural phases coexist together in the film it can mimic the morphotropic phase boundary like behavior with high enhancement in the piezoelectric coefficients by the mechanisms explains by polarization extension and polarization rotation. First part of this work, we address the tuning the structure of BFO using a template assisted strain varying method, which is achieved previously by different means of changing the substrates, thickness of BFO and compositional change. We also able to mimic the ix morphotropic phase boundary like region in BFO by altering the epitaxial strain condition on the same substrate, with same thickness of the BFO film. In the second part the effects of structural distortions, interface roughness; anisotropy etc. on exchange bias is studied. Chapter 1 begins with the brief introduction to the perovskites materials, piezoelectric materials, materials with high d33 values in their morphotropic phase boundaries etc. Then classifications of multiferroic materials, followed by the literature reviews on the phase instability of epitaxial Bismuth ferrite and its properties are explained. The exchange bias phenomenon and the literature review on the bismuth ferrite based heterostructures were detailed. Chapter 2 deals with the experimental techniques and the working principles of each instrument are explained. The solid state synthesis route and coprecipitation route for target preparation are detailed. The working principles and the methodology of pulsed laser deposition, different scanning methods of high resolution X-ray diffraction and piezoresponse force microscopy techniques are then detailed. Chapter 3 deals with the optimization of the epitaxial bismuth ferrite on different substrates and different orientations are detailed. The growth parameters of the bismuth ferrite are first optimized by depositing the films in different conditions of temperature and partial pressure of oxygen. The optimized parameters were used for depositing the BFO films in different strained conditions and orientations. These films are characterized using HRXRD techniques to confirm the phase, orientations and strain in the films. Further the ferroelectric and piezoelectric characterization is performed using the PFM, to map the ferroelectric domain pattern, local piezoelectric coefficients etc. The films were showing good ferroelectric and piezoelectric characteristics. Chapter 3 deals with the tuning of the different phase of bismuth ferrite using a template layer to stabilize in rhombohedral, tetragonal and mixed phase of both these phase. A La0.77Sr0.33MnO3 (LSMO) buffer layer is used to vary the strain conditions of 20nm thick epitaxial BFO layer grown on LAO substrate. Lowering x the thickness (~2nm) of LSMO, a completely strained layer is able to transfer the strain to BFO layer and stabilize the tetragonal phase with an out-of-plane strain of ~ 16% altering the c/a ratio of BFO to ~1.2. Increasing the thickness of LSMO layer to 10nm results in a mixed phase rhombohedral (R) and tetragonal (T) domains with monoclinic distortion. Further increment of LSMO layer thickness to 20 nm stabilizes rhombohedral phase of BFO. The structural phase confirmation and the monoclinic distortion were confirmed using X-ray diffraction and reciprocal space mapping studies. In addition, the domain studies carried out with Piezoresponce force microscopy (PFM) reveals that the tetragonal phase with weak monoclinic distortion possessed 180 domains with dominant out-of-plane components of polarization. However, the mixed phase (R+T with monoclinic distortion) clearly revealed the various plausible polarization components in both out-of-plane and in-plane components. In the pure rhombohedral phase, the domain pattern transformed into fractal type domains which is commonly observed in BFO epilayers. Further, we have implemented a thermodynamically consistent model based on phase field approach to investigate the role of epitaxial strain on BFO layer to study the formation of domain patterns with various plausible polarization components. The piezoelectric d33 coefficient is found to be ~46 pm/V for the 20nm mixed phase BFO, which is relatively larger when compared to the single phase epitaxial films of given thickness (~20nm). Chapter 4 deals with the dependence of the structural distortion, interface roughness, and anisotropy in exchange bias phenomena between antiferromagnetic BFO and soft ferromagnetic layers. An exchange bias of 510 Oe was observed in the tetragonal phase of BFO with a ‘c/a’ ratio of 1.22, which is comparable with the exchange bias shown by the (111) oriented rhombohedral phase (360 Oe). Both the tetragonal (001) and rhombohedral (111) layers possess ferroelectric polarization normal to the sample surface and so the domain walls are mostly 180o oriented which is expected to have a minimum effect on the exchange bias. However, the weak strain induced structural variants in the (111) oriented rhombohedral BFO and the monoclinic distortion present in tetragonal BFO is expected to play a key xi role in defining the ferroelectric domain wall nature and thereby exhibiting exchange bias characteristics. The sample with low interface roughness exhibited a high exchange bias of ~222 Oe, than that of the sample with high interface roughness. . The Ferroelectric domain size of (001) oriented BFO is twice as large as in (011) oriented sample, and comparatively large domain size is observed in (111) oriented sample, confirming the presence of much higher domain wall density in the sample oriented in (011) direction. This confirms the higher exchange bias in (011) oriented sample. The coercivity enhancement has been observed in all the samples. Further, the optimizing the roughness in epitaxialy oriented film, controlling the ferroelectric domains with 71o ferroelectric domain wall, with a control of modulated spin cycloid vector will be the key for device based application as proposed for the reversible control of exchange bias using external electric field. Chapter 6 details the summary and conclusion of the thesis work and the future prospects of the work are also discussed.

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