242 results found
Keeney L, Smith RJ, Palizdar M, et al., 2020, Ferroelectric behavior in exfoliated 2D aurivillius oxide flakes of sub-unit cell thickness, Advanced Electronic Materials, Vol: 6, Pages: 1-12, ISSN: 2199-160X
Ferroelectricity in ultrasonically exfoliated flakes of the layered Aurivillius oxide Bi5Ti3Fe0.5Co0.5O15 with a range of thicknesses is studied. These flakes have relatively large areas (linear dimensions many times the film thickness), thus classifying them as 2D materials. It is shown that ferroelectricity can exist in flakes with thicknesses of only 2.4 nm, which equals one‐half of the normal crystal unit cell. Piezoresponse force microscopy (PFM) demonstrates that these very thin flakes exhibit both piezoelectric effects and that the ferroelectric polarization can be reversibly switched. A new model is presented that permits the accurate modeling of the field‐on and field‐off PFM time domain and hysteresis loop responses from a ferroelectric during switching in the presence of charge injection, storage, and decay through a Schottky barrier at the electrode–oxide interface. The extracted values of spontaneous polarization, 0.04(±0.02) C m−2 and electrostrictive coefficient, 2(±0.1) × 10−2 m4 C−2 are in good agreement with other ferroelectric Aurivillius oxides. Coercive field scales with thickness, closely following the semi‐empirical scaling law expected for ferroelectric materials. This constitutes the first evidence for ferroelectricity in a 2D oxide material, and it offers the prospect of new devices that might use the useful properties associated with the switchable ferroelectric spontaneous polarization in a 2D materials format.
Crossley S, Nair B, Whatmore RW, et al., 2019, Electrocaloric cooling cycles in lead scandium tantalate with true regeneration via field variation, Physical Review X, Vol: 9, ISSN: 2160-3308
There is growing interest in heat pumps based on materials that show thermal changes when phase transitions are driven by changes of electric, magnetic, or stress field. Importantly, regeneration permits sinks and loads to be thermally separated by many times the changes of temperature that can arise in the materials themselves. However, performance and parameterization are compromised by net heat transfer between caloric working bodies and heat-transfer fluids. Here, we show that this net transfer can be avoided—resulting in true, balanced regeneration—if one varies the applied electric field while an electrocaloric (EC) working body dumps heat on traversing a passive fluid regenerator. Our EC working body is represented by bulk PbSc0.5Ta0.5O3 near its first-order ferroelectric phase transition, where we record directly measured adiabatic temperature changes of up to 2.2 K. Indirectly measured adiabatic temperature changes of similar magnitude are identified, unlike normal, from adiabatic measurements of polarization, at nearby measurement set temperatures, without assuming a constant heat capacity. The resulting high-resolution field-temperature-entropy maps of our material, and a small clamped companion sample, are used to construct cooling cycles that assume the use of an ideal passive regenerator in order to span ≤20 K. These cooling cycles possess well-defined coefficients of performance that are bounded by well-defined Carnot limits, resulting in large (>50%) well-defined efficiencies that are not unduly compromised by a small field hysteresis. Our approach permits the limiting performance of any caloric material in a passive regenerator to be established, optimized, and compared; provides a recipe for true regeneration in prototype cooling devices; and could be extended to balance active regeneration.
Peters JJP, Sanchez AM, Walker D, et al., 2019, Quantitative high-dynamic-range electron diffraction of polar nanodomains in Pb2ScTaO6, Advanced Materials, Vol: 31, ISSN: 0935-9648
Highly B‐site ordered Pb2ScTaO6 crystals are studied as a function of temperature via dielectric spectroscopy and in situ high‐dynamic‐range electron diffraction. The degree of ordering is examined on the local and macroscopic scale and is determined to be 76%. Novel analysis of the electron diffraction patterns provides structural information with two types of antiferroelectric displacements determined to be present in the polar structure. It is then found that a low‐temperature transition occurs on cooling at ≈210 K that is not present on heating. This phenomenon is discussed in terms of the freezing of dynamic polar nanodomains where a high density of domain walls creates a metastable state.
McQuaid R, Campbell M, Whatmore R, et al., 2017, Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite, Nature Communications, Vol: 8, ISSN: 2041-1723
Ferroelectric domain walls constitute a completely new class of sheet-like functional material. Moreover, since domain walls are generally writable, erasable, and mobile, they could be useful in functionally agile devices: for example, creating and moving conducting walls could make or break electrical connections in new forms of reconfigurable nanocircuitry. However, significant challenges exist: site-specific injection and annihilation of planar walls, which show robust conductivity, has not been easy to achieve. Here, we report the observation, mechanical writing and controlled movement of charged conducting domain walls in the improper ferroelectric Cu3B7O13Cl. Walls are straight, tens of microns long, and exist as a consequence of elastic compatibility conditions between specific domain pairs. We show that site-specific injection of conducting walls of up to hundreds of microns in length can be achieved through locally applied point-stress and, once created, that they can be moved and repositioned using applied electric fields.
Keeney L, Downing C, Schmidt M, et al., 2017, Direct atomic scale determination of magnetic ion partition in a room temperature multiferroic material, Scientific Reports, Vol: 7, ISSN: 2045-2322
The five-layer Aurivillius phase Bi6TixFeyMnzO18 system is a rare example of a single-phase room temperature multiferroic material. To optimise its properties and exploit it for future memory storage applications, it is necessary to understand the origin of the room temperature magnetisation. In this work we use high resolution scanning transmission electron microscopy, EDX and EELS to discover how closely-packed Ti/Mn/Fe cations of similar atomic number are arranged, both within the perfect structure and within defect regions. Direct evidence for partitioning of the magnetic cations (Mn and Fe) to the central three of the five perovskite (PK) layers is presented, which reveals a marked preference for Mn to partition to the central layer. We infer this is most probably due to elastic strain energy considerations. The observed increase (>8%) in magnetic cation content at the central PK layers engenders up to a 90% increase in potential ferromagnetic spin alignments in the central layer and this could be significant in terms of creating pathways to the long-range room temperature magnetic order observed in this distinct and intriguing material system.
Faraz A, Maity T, Schmidt M, et al., 2016, Direct visualization of magnetic-field-induced magnetoelectric switching in multiferroic aurivillius phase thin films, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 100, Pages: 975-987, ISSN: 0002-7820
Multiferroic materials displaying coupled ferroelectric and ferromagnetic order parameters could provide a means for data storage whereby bits could be written electrically and read magnetically, or vice versa. Thin films of Aurivillius phase Bi6Ti2.8Fe1.52Mn0.68O18, previously prepared by a chemical solution deposition (CSD) technique, are multiferroics demonstrating magnetoelectric coupling at room temperature. Here, we demonstrate the growth of a similar composition, Bi6Ti2.99Fe1.46Mn0.55O18, via the liquid injection chemical vapor deposition technique. High-resolution magnetic measurements reveal a considerably higher in-plane ferromagnetic signature than CSD grown films (MS=24.25 emu/g (215 emu/cm3), MR=9.916 emu/g (81.5 emu/cm3), HC=170 Oe). A statistical analysis of the results from a thorough microstructural examination of the samples, allows us to conclude that the ferromagnetic signature can be attributed to the Aurivillius phase, with a confidence level of 99.95%. In addition, we report the direct piezoresponse force microscopy visualization of ferroelectric switching while going through a full in-plane magnetic field cycle, where increased volumes (8.6% to 14% compared with 4% to 7% for the CSD-grown films) of the film engage in magnetoelectric coupling and demonstrate both irreversible and reversible magnetoelectric domain switching.
Keeney L, Schmidt M, Amann A, et al., 2016, 25. Novel approaches for genuine single phase room temperaturemagnetoelectric multiferroics, Nanoscale Ferroelectrics and Multiferroics: Key Processing and Characterization Issues, and Nanoscale Effects, Volume I & II, Publisher: Wiley, ISBN: 978-1-118-93575-0
This chapter reviews approaches currently under investigation for the fabrication of single-phase magnetoelectric multiferroics, from bulk ceramics to those in thin-film form. It presents an approach of inserting magnetic ions into the Aurivillius phase, layer-structured ferroelectric materials, whereby thin films of average composition Bi6Ti2.8Fe1.52Mn0.68O18 (B6TFMO) demonstrate room temperature ferroelectricity, ferromagnetism, and magnetoelectric coupling. The chapter also discusses the importance of careful microstructural analysis of the materials and the application of a statistical model to determine a confidence level that the observed effects are from genuine single-phase magnetoelectric multiferroics. It reviews how careful phase analysis and statistical treatment of the data confirmed that the B6TFMO phase is a single-phase multiferroic to a confidence level of 99.5%. Finally, it summarizes how direct evidence of magnetoelectric coupling in the B6TFMO thin films was obtained. This review demonstrates that with materials development and design, the development of room temperature multiferroic materials can be achieved.
Skiadopoulou S, Kamba S, Drahokoupil J, et al., 2015, Comment on "Interesting Evidence for Template-Induced Ferroelectric Behavior in Ultra-Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates", ADVANCED FUNCTIONAL MATERIALS, Vol: 26, Pages: 642-646, ISSN: 1616-301X
X-ray diffraction, second-harmonic generation and infrared reflectance investigations reveal no evidence for a polar phase or ferroelectric phase transition in 1.6% tensile strained anatase TiO2 thin films. This indicates that the previously-reported potential ferroelectric behaviour, observed using piezoelectric force microscopy, may have been defect related, or the polar distortion is too small to detect using these methods.
Douglas AM, Kumar A, Whatmore RW, et al., 2015, Local conductance: A means to extract polarization and depolarizing fields near domain walls in ferroelectrics, Applied Physics Letters, Vol: 107, ISSN: 0003-6951
Conducting atomic force microscopy images of bulk semiconducting BaTiO3 surfaces show clearstripe domain contrast. High local conductance correlates with strong out-of-plane polarization(mapped independently using piezoresponse force microscopy), and current-voltage characteristicsare consistent with dipole-induced alterations in Schottky barriers at the metallic tip-ferroelectricinterface. Indeed, analyzing current-voltage data in terms of established Schottky barrier modelsallows relative variations in the surface polarization, and hence the local domain structure, to bedetermined. Fitting also reveals the signature of surface-related depolarizing fields concentratednear domain walls. Domain information obtained from mapping local conductance appears to bemore surface-sensitive than that from piezoresponse force microscopy. In the right materials systems,local current mapping could therefore represent a useful complementary technique for evaluatingpolarization and local electric fields with nanoscale resolution
Deepak N, Carolan P, Keeney L, et al., 2015, Bismuth Self-Limiting Growth of Ultrathin BiFeO3 Films, CHEMISTRY OF MATERIALS, Vol: 27, Pages: 6508-6515, ISSN: 0897-4756
Bismuth ferrite (BiFeO3) is a widely studied material, because of its interesting multiferroic properties. Bismuth self-limiting growth of single-phase BiFeO3 (BFO) has previously been achieved using molecular beam epitaxy (MBE), but the growth of BFO by chemical vapor deposition (CVD) has proved to be very challenging, because of the volatile nature of bismuth. The growth window regarding temperature, pressure, and precursor flow rates that will give a pure perovskite BFO phase is normally very small. In this work, we have studied the metal–organic CVD (MOCVD) growth of epitaxial BFO thin films on SrTiO3 substrates and found that by carefully controlling the amount of the iron precursor, Fe(thd)3 (where thd = 2,2,6,6 tetra-methyl-3,5-heptanedionate), we were able to achieve bismuth self-liming growth, for the first time. The effect of the volume of the bismuth and iron precursors injected on the growth of BFO thin films is reported, and it has been found that the phase-pure films can be prepared when the Bi/Fe ratios are between 1.33 and 1.81 under temperature and pressure conditions of 650 °C and 10 mbar, respectively, and where the O2 gas flow was kept constant to 1000 sccm out of a total gas flow of 3000 sccm. Piezoresponse force microscopy (PFM) studies demonstrate the presence of bipolar switching in ultrathin BFO films.
Deepak N, Carolan P, Keeney L, et al., 2015, Tunable nanoscale structural disorder in Aurivillius phase, n=3 Bi4Ti3O12 thin films and their role in the transformation to n=4, Bi5Ti3FeO15 phase, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 5727-5732, ISSN: 2050-7526
Naturally super-latticed Aurivillius phase ferroelectrics can accommodate various magnetic ions, opening up the possibility of making new room temperature multiferroics. Here, we studied the growth of single-phase Aurivillius phase Bi5Ti3FeO15 (BTFO) thin films, grown onto single crystalline SrTiO3 (STO) substrates, by doping Bi4Ti3O12 (BTO) with iron by liquid injection metal–organic chemical vapour deposition. The crystalline properties of the resulting films were characterized by X-ray diffraction and transmission electron microscopy. It has been found that the structural properties of the films depend strongly on the relative iron and titanium precursor injection volumes. Nanoscale structural disorder starts to occur in BTO films on the onset of iron precursor flow. A small iron precursor flow causes the formation of half-unit cells of BTFO inside BTO lattice, which in turns causes disorder in BTO films. This disorder can be tuned by varying iron content in the film. Atomic force microscopy shows how the growth mode switches from island growth to layer-by-layer growth mode as the composition changes from BTO to BTFO.
Alpay SP, Mantese J, Trolier-McKinstry S, et al., 2014, Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion, MRS Bulletin, Vol: 39, Pages: 1099-1111, ISSN: 0883-7694
Thin-film electrocaloric and pyroelectric sources for electrothermal energy interconversion have recently emerged as viable means for primary and auxiliary solid-state cooling and power generation. Two significant advances have facilitated this development: (1) the formation of high-quality polymeric and ceramic thin films with figures of merit that project system-level performance as a large percentage of Carnot efficiency and (2) the ability of these newer materials to support larger electric fields, thereby permitting operation at higher voltages. This makes the power electronic architectures more favorable for thermal to electric energy interconversion. Current research targets to adequately address commercial device needs including reduction of parasitic losses, increases in mechanical robustness, and the ability to form nearly freestanding elements with thicknesses in the range of 1–10 μm. This article describes the current state-of-the-art materials, thermodynamic cycles, and device losses and points toward potential lines of research that would lead to substantially better figures of merit for electrothermal energy interconversion.
Dul'kin E, Salje EKH, Aktas O, et al., 2014, Ferroelectric precursor behavior of highly cation-ordered PbSc0.5Ta0.5O3 detected by acoustic emission: Tweed and polar nanoregions, Applied Physics Letters, Vol: 105, ISSN: 1077-3118
Highly ordered ferroelectric PbSc0.5Ta0.5O3 crystals were studied by acoustic emission over a widetemperature range. Acoustic emission was found at three characteristic temperatures: 330, 409, and600 K, which are close to those, known from the same disordered crystals, containing polarnanoregions. The microstructure in our crystals contains structural “tweed” rather thannanoregions. The coincidence of acoustic emission temperatures points towards a close structuralrelationship between nanoregions and “tweed.” Under electric field, these temperatures shiftsimilar to “critical end point” behavior. The obtained data prove that acoustic emission detectssignals in a wider parameter space than previously expected.
Schmidt M, Amann A, Keeney L, et al., 2014, Absence of Evidence not equal Evidence of Absence: Statistical Analysis of Inclusions in Multiferroic Thin Films, SCIENTIFIC REPORTS, Vol: 4, ISSN: 2045-2322
Whatmore R, 2014, Characterisation of Pyroelectric Materials, Characterisation of Ferroelectric Bulk Materials and Thin Films, Editors: Cain, Publisher: Springer, Pages: 65-86, ISBN: 9781402093104
This book presents a comprehensive review of the most important methods used in the characterisation of piezoelectric, ferroelectric and pyroelectric materials.
Deepak N, Caro MA, Keeney L, et al., 2014, Interesting evidence for template-induced ferroelectric behavior in ultra-thin titanium dioxide films grown on (110) neodymium gallium oxide substrates, Advanced Functional Materials, Vol: 24, Pages: 2844-2851, ISSN: 1616-301X
The first evidence for room-temperature ferroelectric behavior in anatase-phase titanium dioxide (a-TiO2) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra-thin (20 nm to 80 nm) biaxially-strained epitaxial films of a-TiO2 deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X-ray diffraction and high-resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 °C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 °C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a-TiO2 and opens up the possibility for a range of new applications. A model is presented for the effects of large in-plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.
Whatmore R, Watton R, 2013, Pyroelectric Materials and Devices, Infrared Detectors and Emitters: Materials and Devices, Editors: Capper, Elliott, Publisher: Springer Science & Business Media, Pages: 99-148, ISBN: 9781461516071
Kesim MT, Zhang J, Trolier-McKinstry S, et al., 2013, Pyroelectric response of lead zirconate titanate thin films on silicon: Effect of thermal stresses, JOURNAL OF APPLIED PHYSICS, Vol: 114, ISSN: 0021-8979
Ferroelectric lead zirconate titanate [Pb(ZrxTi1-xO)3, (PZT x:1-x)] has received considerable interest for applications related to uncooled infrared devices due to its large pyroelectric figures of merit near room temperature, and the fact that such devices are inherently ac coupled, allowing for simplified image post processing. For ferroelectric films made by industry-standard deposition techniques, stresses develop in the PZT layer upon cooling from the processing/growth temperature due to thermal mismatch between the film and the substrate. In this study, we use a non-linear thermodynamic model to investigate the pyroelectric properties of polycrystalline PZT thin films for five different compositions (PZT 40:60, PZT 30:70, PZT 20:80, PZT 10:90, PZT 0:100) on silicon as a function of processing temperature (25–800 °C). It is shown that the in-plane thermal stresses in PZT thin films alter the out-of-plane polarization and the ferroelectric phase transformation temperature, with profound effect on the pyroelectric properties. PZT 30:70 is found to have the largest pyroelectric coefficient (0.042 μC cm−2 °C−1, comparable to bulk values) at a growth temperature of 550 °C; typical to what is currently used for many deposition processes. Our results indicate that it is possible to optimize the pyroelectric response of PZT thin films by adjusting the Ti composition and the processing temperature, thereby, enabling the tailoring of material properties for optimization relative to a specific deposition process.
Aktas O, Salje EKH, Crossley S, et al., 2013, Ferroelectric precursor behavior in PbSc0.5Ta0.5O3 detected by field-induced resonant piezoelectric spectroscopy, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950
Keeney L, Maity T, Schmidt M, et al., 2013, Magnetic Field-Induced Ferroelectric Switching in Multiferroic Aurivillius Phase Thin Films at Room Temperature, Journal of the American Ceramic Society, Vol: 96, Pages: 2339-2357, ISSN: 0002-7820
Single-phase multiferroic materials are of considerable interest for future memory and sensing applications. Thin films of Aurivillius phase Bi7Ti3Fe3O21 and Bi6Ti2.8Fe1.52Mn0.68O18 (possessing six and five perovskite units per half-cell, respectively) have been prepared by chemical solution deposition on c-plane sapphire. Superconducting quantum interference device magnetometry reveal Bi7Ti3Fe3O21 to be antiferromagnetic (TN = 190 K) and weakly ferromagnetic below 35 K, however, Bi6Ti2.8Fe1.52Mn0.68O18 gives a distinct room-temperature in-plane ferromagnetic signature (Ms = 0.74 emu/g, μ0Hc =7 mT). Microstructural analysis, coupled with the use of a statistical analysis of the data, allows us to conclude that ferromagnetism does not originate from second phase inclusions, with a confidence level of 99.5%. Piezoresponse force microscopy (PFM) demonstrates room-temperature ferroelectricity in both films, whereas PFM observations on Bi6Ti2.8Fe1.52Mn0.68O18 show Aurivillius grains undergo ferroelectric domain polarization switching induced by an applied magnetic field. Here, we show for the first time that Bi6Ti2.8Fe1.52Mn0.68O18 thin films are both ferroelectric and ferromagnetic and, demonstrate magnetic field-induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature.
Deepak N, Zhang PF, Keeney L, et al., 2013, Atomic vapor deposition of bismuth titanate thin films, 21st IEEE ISAF, held Jointly with 11th ECAPD and 4th Conference on PFM and Nanoscale Phenomena in Polar Materials, Publisher: AMER INST PHYSICS, ISSN: 0021-8979
Varghese J, Ghoshal T, Deepak N, et al., 2013, Fabrication of Arrays of Lead Zirconate Titanate (PZT) Nanodots via Block Copolymer Self-Assembly, CHEMISTRY OF MATERIALS, Vol: 25, Pages: 1458-1463, ISSN: 0897-4756
Varghese J, Whatmore RW, Holmes JD, 2013, Ferroelectric nanoparticles, wires and tubes: synthesis, characterisation and applications, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 1, Pages: 2618-2638, ISSN: 2050-7526
Palizdar M, Comyn TP, Ward MB, et al., 2012, Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics, JOURNAL OF APPLIED PHYSICS, Vol: 112, ISSN: 0021-8979
Zhang PF, Deepak N, Keeney L, et al., 2012, The structural and piezoresponse properties of c-axis-oriented Aurivillius phase Bi5Ti3FeO15 thin films deposited by atomic vapor deposition, APPLIED PHYSICS LETTERS, Vol: 101, ISSN: 0003-6951
Keeney L, Kulkarni S, Deepak N, et al., 2012, Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 thin films, JOURNAL OF APPLIED PHYSICS, Vol: 112, ISSN: 0021-8979
Varghese J, O'Regan C, Deepak N, et al., 2012, Surface Roughness Assisted Growth of Vertically Oriented Ferroelectric SbSI Nanorods, CHEMISTRY OF MATERIALS, Vol: 24, Pages: 3279-3284, ISSN: 0897-4756
Keeney L, Groh C, Kulkarni S, et al., 2012, Room temperature electromechanical and magnetic investigations of ferroelectric Aurivillius phase Bi5Ti3(FexMn1-x)O-15 (x=1 and 0.7) chemical solution deposited thin films, JOURNAL OF APPLIED PHYSICS, Vol: 112, ISSN: 0021-8979
Varghese J, Barth S, Keeney L, et al., 2012, Nanoscale Ferroelectric and Piezoelectric Properties of Sb2S3 Nanowire Arrays, NANO LETTERS, Vol: 12, Pages: 868-872, ISSN: 1530-6984
Deepak N, Zhang P, Keeney L, et al., 2012, Atomic Vapor Deposition of Bismuth Titanate thin films, 21st IEEE ISAF, held Jointly with 11th ECAPD and 4th Conference on PFM and Nanoscale Phenomena in Polar Materials, Publisher: IEEE, ISSN: 1099-4734
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