The first evidence for room‐temperature ferroelectric behavior in anatase‐phase titanium dioxide (a‐TiO 2 ) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra‐thin (20 nm to 80 nm) biaxially‐strained epitaxial films of a‐TiO 2 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‐TiO 2 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.
The five-layered (m = 5) Bi6Ti2.99Fe1.46Mn0.55O18 Aurivillius material is a rare example of a single-phase room temperature ferroelectric-ferrimagnetic multiferroic that shows promise for energy-efficient memory devices. Its ferrimagnetism is thought to derive from the natural partitioning of magnetic ions to the central perovskite layer, engendered by chemically-driven lattice strains, together with ferromagnetic coupling via superexchange mechanisms. Motivated by the expectation of an enhancement in magnetization with increased magnetic ion content, this study examines systematic B-site substitutions with the aim of increasing (from the current level of 40%) the proportion of magnetic ions within the structure. The solubility limits of magnetic cations in this structure and their influence on the superlattice layering are investigated. Studies of Aurivillius phase films on c-sapphire with composition Bi6TixFeyMnzO18 (B6TFMO; x = 2.3 to 3.2, y = 1.2 to 2.0, z = 0.3 to 0.9) demonstrated that above ca. 46% of B-site magnetic cations, the m = 5 structure first rearranges into a mixed-phase material based on m = 5 and six-layered (m = 6) structures and eventually evolves into an m = 6 phase with 54% magnetic cations at the B-site. It is postulated that increasing the number of perovskite layers by forming the m = 6 structure facilitates the accommodation of additional magnetic cations at a lower average manganese oxidation state (+3.3) compared with an equivalent m = 5 stoichiometry (+4.0). While the minor out-of-plane ferroelectric response decreases as expected with increasing structural reorganization towards the m = 6 phase, the predominant in-plane piezoresponse remains unaffected by magnetic cation substitution. This work shows that higher-layered Aurivillius homologues can be synthesized using aliovalent substitution, without requiring epitaxial growth or kinetically constrained methods.
The Aurivillius layer-structures, described by the general formula Bi2O2(Am-1BmO3m+1), are naturally 2-dimensionally nanostructured. They are very flexible frameworks for a wide variety of applications, given that different types of cations can beaccommodated both at the A- and B-sites. In this review article, we describe how the Aurivillius phases are a particularly attractive class of oxides for the design of prospective single phase multiferroic systems for multi-state data storage applications, as they offer the potential to include substantial amounts of magnetic cations within a strongly ferroelectric system. The ability to vary m yields differing numbers of symmetrically distinct B-site locations over which the magnetic cations can be distributed and generates driving forces for cation partitioning and magnetic ordering. We discuss how out-of-phase boundary and stacking fault defects can further influence local stoichiometry and the extent of cation partitioning in these intriguing material systems.
Abstract We describe the formation of RuO 2 thin films grown using atomic layer deposition (ALD) on (100) Si substrates from Ru(EtCp) 2 and O 2 , and the subsequent influence of annealing temperature and atmosphere on the surface morphology and structure of the deposited layers. The films are characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction (ED). The as‐deposited films consist of RuO 2 islands. No significant changes in composition or morphology are observed following annealing in N 2 for 4 h at either 500 or 700°C. Higher temperature annealing in N 2 (820°C, 4 h) results in some modifications to the morphology and structure where ED data indicate the formation of some Ru metal. However, complete transformation from as‐deposited RuO 2 to Ru metal is, obtained after annealing in forming gas (95% N 2 /5% H 2 ) at 420°C for 5 min.
A range of oxo-centred, carboxylate bridged tri-iron complexes of general formula [Fe3(mu3-O)(O2CR)6L3]ClO4(R=CH2CN, CH2F, CH2Cl, CH2Br, p-NO2C6H4; L=pyridine, 3-methylpyridine, 4-methylpyridine, 3,5-dimethylpyridine, 3-cyanopyridine and 3-fluoropyridine) have been prepared and characterised. The choice of R and L was dictated by the requirement that the complexes undergo a one-electron reduction when reacted with verdazyl radicals. All except the complexes where L=pyridine and R=CH2CN, CH2Cl and p-NO2C6H4 have not been previously reported. The redox behaviour of these compounds has been investigated using cyclic voltammetry in acetonitrile in the absence and in the presence of free L. In general, all complexes exhibited reversible one-electron reductions. Electrochemical behaviour improved in the presence of an excess of L. The kinetics of the electron transfer reaction observed when acetonitrile solutions of the complexes were reacted with a range of verdazyl radicals were monitored using stopped-flow spectrophotometry. Under the experimental conditions, the reactions were quite rapid and were monitored under second-order conditions. Marcus linear free energy plots indicated that the outer-sphere electron transfer reactions were non-adiabatic in nature. Nevertheless, application of the self-exchange rate constants of the verdazyl radicals, k11, and the tri-iron complexes, k22, to the Marcus cross-relation resulted in calculated values of the cross-reaction rate constant, k12, that were within a factor of five of the experimentally determined value.
Abstract Multiferroic domain walls are an emerging solution for future low-power nanoelectronics due to their combined tuneable functionality and mobility. Here we show that the magnetoelectric multiferroic Aurivillius phase Bi6TixFeyMnzO18 (B6TFMO) crystal is an ideal platform for domain wall-based nanoelectronic devices. The unit cell of B6TFMO is distinctive as it consists of a multiferroic layer between dielectric layers. We utilise atomic resolution scanning transmission electron microscopy and spectroscopy to map the sub-unit-cell polarisation in B6TFMO thin films. 180˚ charged head-to-head and tail-to-tail domain walls are found to pass through > 8 ferroelectric-dielectric layers of the film. They are structurally similar to BiFeO3 DWs but contain a large surface charge density (σ_s) = 1.09 |e|per perovskite cell, where |e| is elementary charge. Although polarisation is primarily in-plane, c-axis polarisation is identified at head-to-tail domain walls with an associated electromechanical coupling of strain and polarisation. Finally, we reveal that with controlled strain engineering during thin film growth, room-temperature vortexes are formed in the ferroelectric layer. These results confirm that sub-unit-cell topological features can play an important role in controlling the conduction properties and magnetisation state of Aurivillius phase films and other multiferroic heterostructures.
Here, we report the effect of A-site substitution of Tb at the expense of Bi on the ferroelectric and magnetic properties in m = 5 layered 2-D Aurivillius Bi6Ti3Fe2O18 thin films. The nominal stoichiometry of the prepared compound is Tb0.40Bi5.6Fe2Ti3O18, Tb0.90Bi5.1Fe2Ti3O18, and Bi6Ti3Fe2O18. Phase examination reveals that only 0.40 mol. % is successfully substituted forming Tb0.40Bi5.6Fe2Ti3O18 thin films. Lateral and vertical piezoresponse switching loops up to 200 °C reveal responses for Bi6Ti3Fe2O18, Tb substituted Tb0.40Bi5.6Fe2Ti3O18, and Tb0.90Bi5.1Fe2Ti3O18 thin films along the in-plane (±42.31 pm/V, 88 pm/V and ±134 pm/V, respectively) compared with the out-of-plane (±6.15 pm/V, 19.83 pm/V and ±37.52 pm/V, respectively). The macroscopic in-plane polarization loops reveal in-plane saturation (Ps) and remanence polarization (Pr) for Bi6Ti3Fe2O18 of ±26.16 μC/cm2 and ±22 μC/cm2, whereas, ±32.75 μC/cm2 and ±22.11 μC/cm2, ±40.30 μC/cm2 and ±28.5 μC/cm2 for Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 thin films, respectively. No ferromagnetic signatures were observed for Bi6Ti3Fe2O18 and Tb0.40Bi5.6Fe2Ti3O18. However, a weak response was observed for the Tb0.90Bi5.1Fe2Ti3O18 at 2 K. Microstructural analysis of Tb0.90Bi5.1Fe2Ti3O18 revealed that it contains 4 vol. % Fe:Tb rich areas forming FexTbyOz, which accounts for the observed magnetic moment. This study demonstrates the importance of thorough microstructural analysis when determining whether magnetic signatures can be reliably assigned to the single-phase system. We conclude that Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 samples are not multiferroic but demonstrate the potential for Fe-RAM applications.
C-axis oriented ferroelectric bismuth titanate (Bi 4 Ti 3 O 12 ) thin films were grown on (001) strontium titanate (SrTiO 3 ) substrates by atomic vapour deposition technique. Ferroelectric properties of thin films are greatly affected by the presence of various kinds of defects. Detailed x-ray diffraction data (XRD) and transmission electron microscopy (TEM) analysis showed presence of out-of-phase boundaries (OPBs). These OPBs originate at atomic steps on the SrTiO 3 substrate surface. It is found that the OPB density changes appreciably with the amount of titanium injected during growth of the thin films.
The sol-gel synthesis and characterization of n≥3 Aurivillius phase thin films deposited on Pt/Ti/SiO2–Si substrates is described. The number of perovskite layers, n, was increased by inserting BiFeO3 into three layered Aurivillius phase Bi4Ti3O12 to form compounds such as Bi5FeTi3O15 (n=4). 30% of the Fe3+ ions in Bi5FeTi3O15 were substituted with Mn3+ ions to form the structure Bi5Ti3Fe0.7Mn0.3O15. The electromechanical responses of the materials were investigated using piezoresponse force microscopy and the results are discussed in relation to the crystallinity of the films as measured by x-ray diffraction.
Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest. Layered bismuth titanates with a Aurivillius structure, (BiFeO 3 ) n Bi 4 Ti 3 O 12 , can potentially posses ferroelectric and ferromagnetic order paramaters simultaneously. It has recently been demonstrated that one such example, Bi 5 Fe 0.5 Co 0.5 Ti 3 O 15 where n = 1 with half the Fe 3+ sites substituted by Co 3+ ions exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report on the fabrication of well oriented polycrystalline ceramics of this material, via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is an extra secondary phase within the obtained ceramic which is rich in cobalt and iron. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. Further, the sintering temperature was varied and excess addition of bismuth oxide was employed in an attempt to reduce the secondary phase with limited success. The samples have been characterized by X-ray diffraction, polarization-field measurements and SQUID magnetometery as a function of sample orientation. It is inferred from the data that the resultant ferromagnetic response identified using SQUID measurements is due to the presence of the secondary phase.
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