Strain Controlled Functionalities in Lightly Doped Manganite Perovskite Epitaxial Thin Films Grown by Pulsed Laser Deposition PDF Download
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Author: Richard Teboh Mbatang
Publisher:
ISBN:
Category :
Languages : en
Pages : 188
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Functional Oxides have been widely studied in the last decades because of their fascinating properties as ferromagnetism, ferroelectricity, superconductivity and multiferroicity. The widely studied complex oxides are the perovskite and the hexagonal oxides. Intriguing phenomena have been observed in epitaxial thin films of manganite perovskites, which greatly differed from those of bulk material. Some of the interesting properties of lightly doped manganite oxide thin films, especially La0.9Sr0.1MnO3 thin films, are an enhanced magnetoresistance (MR) and temperature coefficient of resistance (TCR). The physical properties of these films are related to the effects that occur at the interface. Some of these interfacial effects includes defect formation, charge transfer, exchange coupling, strain, interfacial reconstruction and cation intermixing. The fast development of new characterization techniques have made the study of interfacial effects easy. Fabrication techniques such as Pulsed Laser Deposition (PLD) have been employed to produce interfaces with great atomic precision. Selecting materials with the right lattice mismatch and optimization of the growth conditions as laser repetition rate, temperature, oxygen pressure, substrate-target distance and laser energy are crucial in growth of epitaxial thin films for functional device application. In addition to these, selecting the right proposition of the various phase is important in achieving the right microstructure for composite films. The main focus of this dissertation is to investigate the relationship between structure and magneto-transport properties in lightly doped manganite perovskite thin films with the concentration on 10% strontium doped lathanum manganese oxide, La0.9Sr0.1MnO3 (LSMO) grown by Pulsed Laser Deposition (PLD). We grew films of different thicknesses (15 nm, 30 nm, 75 nm) and on different substrates ((001) LaAlO3 (LAO, (001) SrLaGaO4 (LSGO), (110), GdScO3 (DSO) and (110) GaScO3). We found that thinner films show highly enhanced magnetic and transport properties while thicker films exhibit low transport and magnetic properties due to strain relaxation. We also observed that compressive strain enhanced magnetic (saturation magnetization, Curie temperature (Tc)) and transport properties (metal-insulating transition (MIT), magnetoresistance (MR), temperature coefficient of resistance (TCR)). The enhancement of magnetic and transport properties were attributed to the suppression of Jahn-Teller distortion, electron-phonon coupling and the enhancement of double exchange coupling. In summary, strain can be used to tune the physical properties of expitaxial thin films for technological applications.
Author: Richard Teboh Mbatang
Publisher:
ISBN:
Category :
Languages : en
Pages : 188
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Book Description
Functional Oxides have been widely studied in the last decades because of their fascinating properties as ferromagnetism, ferroelectricity, superconductivity and multiferroicity. The widely studied complex oxides are the perovskite and the hexagonal oxides. Intriguing phenomena have been observed in epitaxial thin films of manganite perovskites, which greatly differed from those of bulk material. Some of the interesting properties of lightly doped manganite oxide thin films, especially La0.9Sr0.1MnO3 thin films, are an enhanced magnetoresistance (MR) and temperature coefficient of resistance (TCR). The physical properties of these films are related to the effects that occur at the interface. Some of these interfacial effects includes defect formation, charge transfer, exchange coupling, strain, interfacial reconstruction and cation intermixing. The fast development of new characterization techniques have made the study of interfacial effects easy. Fabrication techniques such as Pulsed Laser Deposition (PLD) have been employed to produce interfaces with great atomic precision. Selecting materials with the right lattice mismatch and optimization of the growth conditions as laser repetition rate, temperature, oxygen pressure, substrate-target distance and laser energy are crucial in growth of epitaxial thin films for functional device application. In addition to these, selecting the right proposition of the various phase is important in achieving the right microstructure for composite films. The main focus of this dissertation is to investigate the relationship between structure and magneto-transport properties in lightly doped manganite perovskite thin films with the concentration on 10% strontium doped lathanum manganese oxide, La0.9Sr0.1MnO3 (LSMO) grown by Pulsed Laser Deposition (PLD). We grew films of different thicknesses (15 nm, 30 nm, 75 nm) and on different substrates ((001) LaAlO3 (LAO, (001) SrLaGaO4 (LSGO), (110), GdScO3 (DSO) and (110) GaScO3). We found that thinner films show highly enhanced magnetic and transport properties while thicker films exhibit low transport and magnetic properties due to strain relaxation. We also observed that compressive strain enhanced magnetic (saturation magnetization, Curie temperature (Tc)) and transport properties (metal-insulating transition (MIT), magnetoresistance (MR), temperature coefficient of resistance (TCR)). The enhancement of magnetic and transport properties were attributed to the suppression of Jahn-Teller distortion, electron-phonon coupling and the enhancement of double exchange coupling. In summary, strain can be used to tune the physical properties of expitaxial thin films for technological applications.
Author:
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Category :
Languages : en
Pages : 12
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The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. We found that this is of special importance in the systems with multiple competing interactions. We have used in situ scanning tunneling microscopy to image surfaces of La5/8Ca3/8MnO3 films grown by pulsed laser deposition. The atomically resolved imaging was combined with in situ angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O2 leads to mixed-terminated film surfaces, with B-site (MnO2) termination being structurally imperfect at the atomic scale. Moreover, a relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La, Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction.
Author: Libin Jin
Publisher:
ISBN: 9781361025710
Category :
Languages : en
Pages :
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This dissertation, "Epitaxial Thin Films and Heterojunctions of Electron Doped Manganite La1-xHfxMnO3" by Libin, Jin, 靳立彬, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled EPITAXIAL THIN FILMS AND HETEROJUNCTIONS OF ELECTRON DOPED MANGANITE La Hf MnO 1-x x 3 Submitted by Jin Libin For the Degree of Master of Philosophy at The University of Hong Kong in October 2015 The strong electron correlated system like perovskite oxides in transition metal oxides have been studied for many years. Most efforts have been focused mainly 3+ 4+ on the hole-doped manganites with Mn /Mn double exchange interaction. Recently, the tetravalent ions doping has attracted considerable attention to make electron doping LaMnO compounds. Such electron-doped manganites may be applied for fabrication of all-manganites devices potentially. The sensitivities to magnetic fields, currents, electric fields, photo illumination and mechanical strain make the electron-doped manganites of great interests for relevant applications. In this thesis, thin films and heterojunctions of tetravalent hafnium doped perovskite manganite La Hf MnO were fabricated. The hafnium shows a 1-x x 3 unique tetravalent state as well as an ion radius closed to trivalent lanthanum ion. These properties make the investigation on La Hf MnO system relatively 1-x x 3 simple and reliable. The structural and physical properties of La Hf MnO (x=0.1, 0.2, 0.3) thin 1-x x 3 films were studied systematically. The crystal structure and epitaxy of La 1- Hf MnO thin films were examined by X-ray diffraction. The temperature x x 3 dependent resistance and magnetization of different samples were investigated in detail. Both temperature dependent resistance and field dependent magnetization indicated the evident transition between paramagnetic and ferromagnetic states. The electronic structures of epitaxial La Hf MnO were measured by X-ray 0.9 0.1 3 Photoelectron Spectroscopy. Both spectrum of Mn 3s and Mn 2p demonstrated 2+ 3+ the composition of Mn /Mn ions. Such evidence strongly suggested the electron-conductive mechanism in La Hf MnO systems. 0.9 0.1 3 The heterojunctions composed of oxygen-deficient SrTiO and 3-δ La Hf MnO thin films have been studied. These heterojunctions demonstrate 0.9 0.1 3 good rectifying characteristics with very low leakage current and high breakdown voltage in a wide temperature range. X-ray diffraction and electron backscattering diffraction measurements reveal that both SrTiO and La Hf MnO layers 3-δ 0.9 0.1 3 have single crystal nature and [001]-orientation. Transmission electron microscope images show an excellent epitaxial growth of SrTiO and 3-δ La Hf MnO layers. Such all-oxides junctions utilizing the bi-layer 0.9 0.1 3 heterostructure may be applied in microelectronic devices. The n-i-p heterojunctions have been fabricated by depositing an n-type La Hf MnO layer on p-type Si with a thin SrTiO intermediate layer. These 0.9 0.1 3 3 junctions exhibit excellent rectifying properties for temperature from 20 K to 300 K. Under illumination of 630 nm light a remarkable photocurrent has been observed. The photosensitivity is over 1200% under -3 V bias and illumination of the light at room temperature, demonstrating very a pronounced photocurrent effect. The injection of photo-carriers could be responsible for the observed phenomenon. The obtained results in this thesis demonstrate the electron-conductive mechanism in La Hf MnO system. The La Hf MnO based heterojunctions 1-x x 3 0.9 0.1 3 show remarkable characteristics and may
Author: Binod Paudel
Publisher:
ISBN:
Category : Heterostructures
Languages : en
Pages : 0
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The perovskite 3d- transitional metal complex oxides offer opportunities to construct artificial heterostructures and chemical doping of atoms with different atomic radii resulting in similar crystal structures but diverse functionalities. The discovery of such functionalities has been possible because of the advances in synthesis, which enables fabrication of oxide heterostructures in the form of thin films, thus providing a rich ground to study their functional response with external stimuli, such as strain, electric field, and chemical pressure. In this dissertation, I investigate how such parameters can modulate different functionalities magnetic (interfacial and bulk), transport, and optical, in the three-class of heterostructures, namely: lightly doped manganite, multiferroic heterostructures, and rare-earth chromites.In the first work, I study the magneto-electric coupling at the heterostructure interface of a ferroelectric PbZr0.2Ti0.8O3 (PZT) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) layers grown on an Nb-doped SrTiO3 (001) substrate utilizing polarized neutron reflectometry (PNR) in consort with ab initio-based density functional theory (DFT) calculations. The working conditions of the functional device are mimicked by gating the heterostructure with a Pt top electrode to apply an external electric field, which alters the magnitude and switches the direction of the ferroelectric (FE) polarization, across the PZT layer. PNR results show that the gated PZT/LSMO exhibits interfacial magnetic phase modulation attributed to ferromagnetic (FM) to A-antiferromagnetic (A-AF) phase transitions resulting from the accumulation of holes. When the net FE polarization points towards the interface (positive), the interface does not undergo a magnetic phase transition and retains its global FM-ordered state. In addition to changes in the interfacial magnetic ordering, the global magnetization of LSMO increases. while switching the polarization from positive to negative and it decreases vice versa. DFT calculations indicate that this enhanced magnetization also correlates with an out-of-plane tensile strain, whereas the suppressed magnetization for positive polarization is attributed to out-of-plane compressive strain. These calculations also show the coexistence of FM and A-AF -phases at zero out of plane strain. Charge modulations throughout the LSMO layer appear to be unaffected by strain, suggesting that these charge-mediated effects do not significantly change the global magnetization. Such results verify that the interfacial magnetic modulations are due to co-action of strain- and charge-mediated effects, which dominate at different length scales. In the second work, I study the microstructural evolution induced magnetism and transport properties of ferroelastic La0.9Sr0.1MnO3 (LSMO) epitaxial thin films grown on SrTiO3 (001) substrates with different miscut angles. The substrate miscut angle plays a critical role in controlling the in-plane magnetic anisotropy. The microscopic origin of such magnetic anisotropy is attributed to the formation of anisotropic stripe domains along the step terraces at the surface. The Curie temperature is found to be decreased and magnetoresistance gradually increases with the increase in miscut angles. The magnetization in the LSMO films was found to be selectively modulated by the antiferrodistortive phase transition of the SrTiO3 substrate. This phenomenon has been qualitatively explained by a strain-modified Stoner–Wohlfarth model. We conclude that the magnetization modulation by the SrTiO3 phase transition depends on h, the ratio of the applied magnetic field to the saturation field. Such modulation is only visible with h
Author: Mark Charles Monti
Publisher:
ISBN:
Category :
Languages : en
Pages : 310
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We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We studied the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO2 terminated SrTiO3 (STO) substrates and have grown epitaxial thin films of LaAlO3 (LAO), LaGaO3 (LGO), and RAlO3 on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R3+ magnetism on the interface between RAlO3 and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 1014 cm[superscript -2] and 4.5 x 1014 cm[superscript -2], respectively. Rare-earth aluminate films, RAlO3, with R = Ce, Pr, Nd, Sm, Eu, Gd, and Tb, were also grown on STO. Conducting interfaces were found for R = Pr, Nd and Gd, and the results indicate that for R [does not equal] La the magnetic nature of the R3+ ion causes increased scattering with decreasing temperature that is modeled by the Kondo effect. Epitaxial strain between the polar RAlO3 films and STO appears to play a crucial role in the transport properties of the metallic interface, where a decrease in the R3+ ion size causes an increase in sheet resistance and an increase in the onset temperatures for increased scattering.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 100
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Perovskite manganites are complex systems in which the structural, electronic, and magnetic properties are strongly coupled - in many cases, non-linearly. A small change in one property (e.g., structure) can produce a large change in other properties (e.g., the electronic state). Conceptually, creating a surface by cleaving a single crystal or growing a thin film provides a controlled way to disturb the coupled system by breaking the symmetry without changing the stoichiometry, which may lead to completely new physical properties. The emerging surface electronic phases can be further enriched by surface reconstructions which often occur due to bond breaking at the surface and also by extrinsic carrier doping from adsorbates as well. While the cleaving method is limited to a small amount of cleavable layered materials, the thin-film growth method, such as laser molecular beam epitaxy (Laser MBE), becomes the most desired technique for the study of surfaces in general. As an example, La1-[subscript x]Ca[subscript x]MnO3, which is one of the most investigated cubic perovskite manganite systems, has a surface that cannot be created by cleaving the bulk but can be prepared by laser MBE. In this dissertation, La1-[subscript x]Ca[subscript x]MnO2 (001) (x = 3/8) thin films grown by laser MBE were studied with a combination of in-situ techniques such as low-energy electron diffraction (LEED) and high-resolution scanning probe microscopy (SPM). Two different electronic conductivities were observed using scanning tunneling spectroscopy (STS) on the thin-film surface. The "atomic" resolution scanning tunneling microscopy (STM) images revealed that the two different conductivities come from two reconstructed surfaces, (1 x 1) and ([square root]2 x [square root]2)R45°, respectively. The (1 x 1) and ([square root]2 x [square root]2)R45° reconstructions were found to be reversible by oxygen adsorption/desorption, and as a result, the conductivity of the surface can be tuned from metallic to insulating by controlling the oxygen adsorption. Further investigations revealed the existence of a surface structure transition driven by the film thickness. The (1 x 1) surface without oxygen adsorption actually has specific superstructures; it changes from c(2 x 4) to (3[square root]2 x 4[square root]2)R45° with a critical thickness of 14 ML associated with an extensive strain induced by the substrate. The discoveries of the unexpected surface structural and electronic transitions revealed in this dissertation open up a new direction for exploring the functionality relationship at the surfaces of complex transition-metal compound thin films.
Author: Joseph A. Cianfrone
Publisher:
ISBN:
Category :
Languages : en
Pages :
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ABSTRACT: Oxide electronics offer a wide array of interesting properties, including ferromagnetism, ferroelectricity and superconductivity. This dissertation investigates the transport, magnetic, and structural properties of several perovskite and spinel material systems, with particular interest in their manipulation at the nanoscale through the growth of functional heterostructures. Spinel phase ZnCo2O4 is investigated as a ferromagnetic semiconductor whose carrier type changes with oxygen concentration. The perovskite system of K(Ta, Nb)O3 is investigated for its interesting properties as a solid solution which exhibits a ferroelectric transition dependent on composition. Its thin film epitaxial growth modes are investigated via reflection high energy electron diffraction, with particular attention paid to the volatile potassium ion. Superlattices of K(Ta, Nb)O3 and SrTiO3 are also investigated for the possibility of dielectric enhancement and conductive interfaces. The multiferroic properties of BaFeO3 K(Ta, Nb)O3 superlattices are investigated, with particular attention paid to the role of magnetoelectric coupling and strain as stabilizing mechanisms.
Author: Roger Herger
Publisher: Sudwestdeutscher Verlag Fur Hochschulschriften AG
ISBN: 9783838104461
Category :
Languages : de
Pages : 132
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The unique combination of (a) highly brilliant x-rays produced in a modern third generation synchrotron source, (b) the availability of a fast, single-photon counting area pixel detector and (c) a pulsed laser deposition equipment for in-situ growth enables one to study both the structure and kinetics of the thin film growth of perovskites. The surface structure of titanium-dioxideterminated strontium titanate (STO) was analyzed by surface x-ray diffraction (SXRD) for two different environments: One (cold) at room temperature and in ultra-high vacuum, and the other (hot) at elevated temperatures and in an oxygen background, i.e., under conditions typical for perovskite thin film growth. SXRD was used to determine the atomic structures of lanthanum strontium manganate thin films, grown monolayer-by-monolayer on STO by pulsed laser deposition. Structures were solved using the COBRA phase-retrieval method and subsequent structural refinement. These results allowed concluding on the onset of colossal magnetoresistance. In-situ kinetic studies of the thin film growth led to the proposition of a novel energetic smoothing mechanism for the growth of complex metal-oxide thin films.
Author: Andrei Akbasheu
Publisher:
ISBN:
Category : Ferroelectric thin films
Languages : en
Pages : 264
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The growth of oxide thin films has long been of great interested for materials scientists for the application in advanced technologies such as nanoelectronics, photovoltaics and sensors. Atomic layer deposition (ALD) is known for being a non-expensive, low-vacuum and low-temperature deposition method able to conformally coat even high-aspect-ratio materials and structures. ALD has been mainly applied to prepare binary oxides such as ZnO and TiO2, and the synthesis of functional complex oxides such as BiFeO3 and CoFe2O4 has little been studied. In the first part of this work, the application of ALD to the preparation of complex oxide thin films based on the Bi-Fe-O system is explored. The important aspects of the ALD approach such as (a) the growth of amorphous Bi-Fe-O by ALD, (b) the epitaxial crystallization of the multiferroic perovskite BiFeO3 films on different substrates, (c) appearance of impurity phases in the films are addressed. Ultimately, the dissertation aims at dispelling the widely held notion that atomic layer deposition is not appropriate for attaining high-quality chemically complex oxide films in the epitaxial form, demonstrating the applicability as an inexpensive, facile, and highly scalable route. The second part of this work is devoted to the preparation and characterization of new ferroelectric thin films as photovoltaic materials. Large optical band gaps of ferroelectric oxides make the optical absorption impractical for solar cell applications. Recently a new strategy for band gap lowering by doping the perovskite KNbO3 with Ba on the A-site and Ni on the B-site resulting in the generation of Ni2+-O vacancy pairs has been shown to significantly increase the optical absorption without loss of ferroelectricity. Using pulsed laser deposition, the synthesis approach to thin films of these new oxides was developed despite significant challenges for the stoichiometry control. The chemical doping approach to the reduction of the band gap was extended to produce new visible-light-absorbing ferroelectric thin films in another perovskite oxide. These materials showed a switchable photovoltaic effect under the visible light illumination along with the retention of the ferroelectric order. This dissertation demonstrates the feasibility of thin film deposition of aforementioned doped ferroelectric oxide perovskites and their ferroelectric photovoltaic properties, which can be extended to other ferroelectric oxides, and provide important information on the synthesis of such materials as thin films.
Author: Anuj Chopra
Publisher:
ISBN:
Category :
Languages : en
Pages : 110
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PbSc0.5Ta0.5O3 (PST) is an example of relaxor ferroelectrics having two B-site cations in the perovskite structure, which gives rise to B-site cation-ordering and disordering. In addition, PST is a candidate for uncooled infrared detector applications. Epitaxial PST films were deposited on different single-crystal substrates and epitaxial electrode layers, respectively, by pulsed laser deposition. Films grown in optimised conditions were phase-pure perovskites. The crystallographic structure and epitaxial quality of the films were investigated by X-ray diffraction and transmission electron microscopy. Further, it was found that the degree of cation-ordering and the size of the ordered regions can be varied by changing the thickness of the films and by substrate-induced strain. Cation-ordered epitaxial PST thin films with good ferroelectric properties were also successfully deposited on buffered Si(100) substrates. Piezoelectric and dielectric non-linearities as well as phase transitions of different order were observed near room temperature.
