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Iron oxide (α-Fe 2 O 3 ) nanoparticles were synthesized using the precipitation synthesis method focusing only on (FeCl 3 , 6H 2 O), NaOH, KOH and NH 4 OH as raw materials. The impact of varying the nature of the base on the crystalline phase, size and morphology of α-Fe 2 O 3 products was explored. XRD spectra revealed that samples crystallize in the rhombohedral (hexagonal) system at 800 °C.The Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were used to detect the morphology of synthesized nanoparticles and specify their sizes. However, the Fourier Transform Infra-Red (FT-IR) spectroscopy has permitted the observation of vibration band Fe-O. Raman spectroscopy was used not only to prove that we have synthesized hematite but also to identify their phonon modes. The Thermo Gravimetric Analysis (TGA) findings allow the thermal cycle determination of samples whereas Differential Thermal Analysis (DTA) findings allow the phase transition temperature identification. Besides, the optical investigation revealed that samples have an optical gap of about 2.1 eV. Findings highlight that the nature of the agent precipitant plays a significant role in the morphology of the products and the formation of the crystalline phase. Hematite synthesis with the base NH 4 OH brought about much stronger, sharper and wider diffraction peaks of α-Fe 2 O 3 . The morphology of samples are spherical with a size of about 61 nm while the size of the nanoparticles of hematite which we have synthesized with NaOH and KOH is respectively of the order of 82 and 79 nm. [ABSTRACT FROM AUTHOR]

The main limitations of current methods for synthesizing perovskite oxide (ABO3) nanoparticles (NPs), e.g. the high reagent costs and sophisticated equipment, the long time and high‐temperature processing, or multiple post‐processing and thermal treatment steps, hamper their full study and potential application. Here, we use a facile low temperature (50 °C) chemical bath synthesis and only one annealing step to successfully produce high phase purity and crystalline quality nano‐shaped rare‐earth‐based REMO3 NPs (RE=La, Nd, Sm, Gd; M=Fe, Mn, Al). We also show the versatility of this approach by fabricating La0.7Sr0.3MnO3 solid solution and non‐RE‐based BiFeO3 perovskite. To assess the potential of the as‐prepared REFeO3 and REMnO3 NPs, they are used for photocatalytic degradation of the norfloxacin antibiotic and show high efficiency. We believe this easy, robust, versatile, and general route for synthesizing ABO3‐based NPs can be further explored in the vast perovskite family and beyond. [ABSTRACT FROM AUTHOR]

BiFeO3 is the model multiferroic material widely considered as nanoparticles for its potential photocatalytic activity. In this work, we study the influence of annealing temperature on the structural, morphological and optical properties of BiFeO3 nanoparticles prepared by a facile chemical route and, the resulting photodegradation of Eosin B dye. When annealing temperature as low as 600 °C is used, some parasitic phases such as Bi2Fe4O9 appear in addition to the BiFeO3 phase. At the same time, the energy of the absorption onset and band gap shifts down and the photocatalytic activity toward Eosin B dye significantly increases by ∼27% when UV light is used while only a small if any, enhancement is observed in case of sunlight illumination. This photocatalytic enhancement could be therefore essentially attributed to the presence of secondary parasitic phases that could also explain previous reported observations. Furthermore, annealing at 500 °C to obtain pure parasitic-phase-free BiFeO3 nanopowder reveals excellent photodegradation of Eosin B reaching 95% efficiency after 40 min under sunlight illumination with good photostability after three consecutive degradation cycles. [ABSTRACT FROM AUTHOR]

Polycrystalline (Ba 0.70 Sr 0.30 )TiO 3 (BST) thin films were prepared by ion beam sputtering and postdeposition annealing method, and their crystallization kinetics was monitored in situ. It was found that 550 °C is a critical temperature for the BST׳s perovskite phase formation, below which a long annealing time is needed to obtain good perovskite structure because of a slow crystallization process, while generating much lower inhomogeneous and thermoelastic strains. As a consequence, the ferroelectric/paraelectric state formation in BST films, being strain sensitive, was purposefully tailored through simple kinetics instead of pure thermodynamics. A long time annealing at 530 °C induced a room temperature (RT) ferroelectric state, whereas the annealing at 550 °C and above promoted much faster perovskite crystallization and tailored the film at RT in its paraelectric state, thereby giving rise to two distinct dielectric behaviors. These findings provide important implications for simply controllable state growth of perovskite thin films with desired electrical properties so as to meet their respective requirements in different application fields. [ABSTRACT FROM AUTHOR]

The multiferroic Zn‐doped BiFeO3 materials with nominal formula BiFe1−xZnxO3 (0 ≤ x ≤ 0.08) are successfully synthesized using hydrothermal synthesis allowing to obtain powders with an average grain size of 8 nm. X‐ray diffraction and Raman spectroscopy analysis prove that nanopowders are pure and homogeneous and show an increase of the unit cell volume when the amount of Zn increases attesting of the substitution of Fe site. Using temperature‐dependent Raman spectroscopy, it is also found that the Néel antiferromagnetic critical temperature is weakly, if any, affected. In contrast, using UV–vis reflectance measurements, clear changes are observed in the features of the optical absorption with modifications of the crystal‐field when increasing the Zn amount. The photocatalytic activity of the pure and the most doped samples is evaluated by the degradation of the methylene blue dye under visible light irradiation. The photocatalytic activity of the Zn‐doped samples is found to be higher than that of BiFeO3. After 3 h of sunlight irradiation, the degradation efficiency increases from 61% to 83% for BiFeO3 and BiFe0.92Zn0.08O3, respectively. The crystal‐field modification after Zn substitution is considered as the principal cause of the obvious enhanced photocatalytic activity. [ABSTRACT FROM AUTHOR]

The field of multiferroics has experienced a rapid progress resulting in the discovery of many new physical phenomena. BiFeO 3 (BFO) compound, which is one of the few room-temperature single-phase multiferroics, has contributed subsequently to this progress. As a result, significant review articles have been devoted specifically to this famous system. This chapter is dedicated to the strain effects on the structure stability and property changes of BFO thin films. It is a short and non-exhaustive topical overview that may be seen as an invitation for interested readers to go beyond. There is a very active and prolific research in this field and we apologize to the authors whose relevant work is not cited here. After a short introduction, we will thus review the effect of strain on BFO films by describing the consequences on the structure and the phase transitions as well as on polar, magnetic and magnetoelectric properties. [ABSTRACT FROM AUTHOR]

The last few decades have witnessed the rapid development of electronic computers relying on von Neumann architecture. However, due to the spatial separation of the memory unit from the computing processor, continuous data movements between them result in intensive time and energy consumptions, which unfortunately hinder the further development of modern computers. Inspired by biological brain, the in situ computing of memristor architectures, which has long been considered to hold unprecedented potential to solve the von Neumann bottleneck, provides an alternative network paradigm for the next-generation electronics. Among the materials for designing memristors, i.e., nonvolatile memories with multistate tunable resistances, ferroelectric polymers have drawn much research interest due to intrinsic analog switching property and excellent flexibility. In this review, recent advances on artificial synapses based on solution-processed ferroelectric polymers are discussed. The relationship between materials' properties, structural design, switching mechanisms, and systematic applications is revealed. We first introduce the commonly used ferroelectric polymers. Afterward, device structures and the switching mechanisms underlying ferroelectric synapse are discussed. The current applications of organic ferroelectric synapses in advanced neuromorphic systems are also summarized. Eventually, the remaining challenges and some strategies to eliminate non-ideality of synaptic devices are analyzed. [ABSTRACT FROM AUTHOR]

Abstract: Combined X-Ray Diffraction (XRD) and Raman analyses were used to investigate the phase transitions of potassium sodium niobate (K0.5Na0.5)NbO3 thin films, prepared from the acetate–alkoxide based precursor solutions with different amounts of alkali acetate excess. Depending on the amount of sodium or potassium excess in the solutions, the microstructures of the about 240nm thick films on platinized silicon substrates consisted of columnar grains with the width of about 200nm or of equiaxed grains with the average size of about 50nm. Raman spectra and XRD patterns provided evidence of both monoclinic to tetragonal (TM–T ) and tetragonal to cubic (TT–C ) phase transitions in the films with different microstructures, similar as in the (K0.5Na0.5)NbO3 powder. In the films with columnar grains, the Curie temperature was decreased as compared to the value reported for the powder, which was connected to the presence of tensile stresses arising from the thermal expansion mismatch between the substrate and the film, while the TM–T in the films closely corresponded to the powder value which was related to the evolution of a pronounced b-axis orientation. Furthermore, the exact values of the TT–C depended on the chemical composition of the films which was also reflected in different sizes of respective unit cells. In the films with fine grains, the phase transitions had a diffuse character, but they were nevertheless also evidenced by dielectric spectroscopy. [Copyright &y& Elsevier]

We investigate the phase stability of Na½Bi½TiO3, a prototype lead-free relaxor material, under pressure. By means of total energy calculations within density functional theory, we study the pressure stability of several structures with polar and antipolar distortions, in-phase and out-of-phase tilts, and different chemically ordered configurations. Under positive (compressive) pressure an orthorhombic Pbnm-like phase is stabilized above 3 GPa. At negative (tensile) pressure a non-tilted polar P4mm-like phase is stable. At zero pressure two phases are coexisting. The local chemical configuration determines whether the high-pressure Pbnm-like or another tilted and polar R3c-like structure is favored. Thus, two different variants of pressure phase diagrams depending on the cation arrangement are obtained, which raises the question of the existence of a mixed phase ground state in the disordered system. We discuss the stability of the mixed phase state in terms of lattice and tilt misfits and possible shapes and ferroic properties of the coexisting regions with different average structures. Our results clearly support the view that there are chemically ordered nanoregions with their own local ferroic properties embedded in a chemically disordered ferroelectric matrix representing the ground state. [ABSTRACT FROM AUTHOR]

In cobaltite, the spin states transitions of Co3+/4+ ions govern the magnetic and electronic conduction properties. These transitions are strain-sensitive and can be varied using external parameters, including temperature, hydrostatic pressure, or chemical stresses through ionic substitutions. In this work, using temperature dependent Raman spectroscopy and X-ray diffraction, the epitaxial strain effects on both structural and vibrational properties of La0.7 Ba0.3 CoO3 (LBCO) cobaltite thin films are investigated. All Raman active phonon modes as well as the structure are found to be strongly affected. Both Raman modes and lattice parameter evolutions show temperature changes correlated with magnetic and electronic transitions properties. Combining Raman spectroscopy and X-ray diffraction appears as a powerful approach to probe the spin transition in thin film cobaltite. Our results provide insight into strong spin-charge-phonon coupling in LBCO thin film. This coupling manifests as vibrational transition with temperature in the Raman spectra near the ferromagnetic spin ordered transition at 220 K. [ABSTRACT FROM AUTHOR]

Temperature and frequency dependence of dielectric permittivity in nanotextured ceramics of PbMg1/3Nb2/3O3 has been performed in a wide range of grain sizes and shown to display strong variation. In particular, frequency relaxation follows the Vögel Fulcher law for the high grain sizes, whereas for intermediate sizes, two Debye processes are evidenced. This behavior as well as a diminution of the maximum of the real dielectric constant [variant_greek_epsilon]max could be explained within the mechanism of relaxation based on the existence of polar nanoregions with a core-shell structure for each grain. [ABSTRACT FROM AUTHOR]

We present neutron diffraction, dielectric permittivity, and photoconductivity measurements, evidencing that lead-magnesium niobate experiences a diffuse phase transformation between the spherical glass and quadrupole glass phases, in the temperature interval between 400 K and 500 K, with the quadrupole phase possessing extremely high magnitudes of dielectric permittivity. Our analysis shows that the integral diffuse scattering intensity may serve as an order parameter for this transformation. Our experimental dielectric permittivity data support this choice. These data are important for the applications desiring giant dielectric responses in a wide temperature intervals and not related to electron's excitations. [ABSTRACT FROM AUTHOR]

Macroscopic polarity and its dynamic response to external electric fields and temperature in the nominally ergodic relaxor phase of pristine lead magnesium niobate crystals and ceramics, Pb(Mg1/3Nb2/3)O3 (PMN), were investigated. Dynamic pyroelectric measurements provide evidence for persistent macroscopic polarity of the samples. Annealing experiments below and above Burns temperature of polarized samples relate this polarity to the presence of polar nano-entities and their dynamics. The dc electric field strength required for macroscopic polarization reversal is similar to the amplitude of the ac field where dynamic nonlinear dielectric permittivity reaches its maximum. Consequently, the aforementioned maximum is related to the reorientation of polar nano-entities. The results question the existence of an ergodic state in PMN below Burns temperature. [ABSTRACT FROM AUTHOR]

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO3) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO3 is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films. [ABSTRACT FROM AUTHOR]

Magnetoelectric multiferroics are provoking much research activity for their potential applications in novel multifunctional devices. Compared with single-phase multiferroics, magnetoelectric composites have competitive advantages. The magnetoelectric coupling of composites is heavily influenced by composition and grain size. We prepared BFO/PZT composites with separate phases via the solid-state sintering method. We show that equal molar ratio composition has a better magnetoelectric coupling response than other compositions. The composites with nano bismuth ferrite have better magnetoelectric coupling performance, and these with both nano grain sizes present the highest magnetoelectric voltage coefficients (α E = 16.9 mV cm−1 Oe−1). The strain-mediated mechanism was confirmed via a comparison between strain response and polarization of nano/micro type composites with the composition of 3:7. We found that the magnetoelectric voltage coefficient α E positively correlates with strain response instead of polarization in BFO/PZT composites, which provides experimental evidence for the strain-mediated ME composite theory. • Grain size effect on magnetoelectric coupling in perovskite-perovskite composites was investigated. • The highest magnetoelectric voltage coefficient was found in nano/nano type composites. • Great polarization and strain responses with magnetoelectric coupling were realized in nano/micro type composites. • Piezoelectricity is a more important factor of ME coupling than ferroelectricity. [ABSTRACT FROM AUTHOR]

Cold sintering of perovskite materials is still, despite years of research, challenging. The key objective when cold-sintering oxide materials is finding an appropriate liquid phase that triggers pressure-dissolution process and mechano-chemical compaction and densification of ceramics. In this study, cold sintering of the multiferroic BiFeO 3 perovskite is reported for the first time. When organic additives or solvents are used, these effectively sinter the compound, but cause precipitation of secondary phases that impede grain-to-grain contacts and the polarization coupling, and result in electrically conductive samples. We found that it is critical to carefully select the sintering additives based on their reactivity, decomposition temperature and products, while ensuring a significant level of wettability and matrix solubility. NaOH/KOH mixture was found to be the best sintering aid, resulting in remanent polarization and strain responses of cold-sintered BFO comparable to those reported for conventionally sintered ceramics. [ABSTRACT FROM AUTHOR]

In this work, we study the effect of Co-doping on the magnetoelectric (ME) response of multiferroic BiFeO 3 (BFO) ceramics made of powder synthesized using a stirred hydrothermal method known to facilitate chemical reactions and favor good chemical homogeneity and particles size distribution. An unprecedented ME coefficient up to 11.3 mV/(Oe.cm) i.e. 8 times higher than pure BFO ceramic, measured with a direct method is achieved for 3–5% Co-doping which makes bulk Co-doped-BFO among the largest single-phase ME materials. We show that Co-doping can be seen as a chemical pressure i.e. a proxy hydrostatic pressure resulting into an increase of Néel antiferromagnetic temperature and especially a destabilization of the cycloidal magnetic modulation because of magnetoelastic coupling. By suppressing the cycloidal arrangement that we evidenced unambiguously by using low energy Raman spectroscopy, the linear ME effect is no more inhibited and can then take place explaining the large ME response. We argue that the combination of mechanical and chemical means during the stirred hydrothermal approach we used might be at the origin of such response by favoring Co-distribution and avoiding parasitic phases and therefore such a chemical route might be further explored in the future. [ABSTRACT FROM AUTHOR]

In addition to the single polarization-versus-electric field hysteresis loop that is characteristic of ferroelectrics and the double hysteresis loop that is known to occur in antiferroelectrics, a third kind of polarization-versus-electric field function has been reported in several systems. This third kind is commonly termed the "pinched" loop due to its unusual shape, and is typically believed to originate from the pinning of domain walls interacting with defects. Here, using an atomistic effective Hamiltonian scheme, we demonstrate that such a belief has to be broadened since our simulations also yield pinched loops in defect-free ferroelectric materials, as a result of the occurrence of intermediate modulated phases exhibiting an inhomogeneous dipolar pattern leading to the coexistence of both ferroelectric and antiferroelectric orders. [ABSTRACT FROM AUTHOR]

Ceramics of pure phase Yttrium (Y) doped BiFeO 3 prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO 3 . Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO 3 to a Poole–Frenkel emission behavior in Bi 0.90 Y 0.10 FeO 3 . A Fowler-Nordheim tunneling mechanism in Bi 0.95 Y 0.05 FeO 3 ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi 0.90 Y 0.10 FeO 3 ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications. [ABSTRACT FROM AUTHOR]

Polarization fatigue in (Pb 0.97 La 0.02 )(Zr 0.95 Ti 0.05 )O 3 (PLZT) antiferroelectric thin films deposited onto silicon wafers is studied by investigating the effect of the peak/average/root-mean-square cycling voltage through varying the waveform of the electrical excitation. Interestingly, it is found that the fatigue behavior of the film is determined by the root-mean-square voltage of the external driving excitation rather than by the peak or average voltages. Our results can be well explained in the framework of the local phase decomposition model and indicate that the root-mean-square voltage should be considered as the effective driving voltage determining the polarization fatigue in PLZT antiferroelectric films. [ABSTRACT FROM AUTHOR]

The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in detail that there exists a critical thickness at which an unusual ferroelectric-"polar nonferroelectric" phase transition occurs in asymmetric ferroelectric tunnel junctions. In the "polar nonferroelectric" phase, there is only one nonswitchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closurelike domains are proposed to form so as to minimize the system energy. The transition temperature is found to decrease monotonically as the ferroelectric barrier thickness is decreased and the reduction becomes more significant for the thinner ferroelectric layers. As a matter of fact, the built-in electric field not only results in smearing of the phase transition, but also forces the transition to take place at a reduced temperature~ Such findings may impose a fundamental limit on the work temperature and thus should be further taken into account in the future ferroelectric-tunnel-junction-type or ferroelectric-capacitor-type devices. [ABSTRACT FROM AUTHOR]

Abstract: Driven by global environmental concerns, great efforts are currently made to develop novel, cheap and practical means for producing highly efficient electrocatalysts, specifically for the low-temperature fuel-cell applications. Employing an old but yet unexplored method based on electrochemical dispergation of platinum by the alternating current, we show that the freshly prepared Pt/C catalysts exhibit greatly enhanced catalytic activity (up to 350% for oxygen reduction and up to 200% for ethylene glycol oxidation) as compared with that of a commercially available counterpart (E-TEK). A key role in these reactions is attributed to a specific nanoparticle morphology. These findings should provide a revival of an ancient but simple and efficient electrochemical process and motivate further researches in the field. [Copyright &y& Elsevier]

We investigate the high-pressure phase transitions in BiFeO3 by single-crystal and powder x-ray diffraction. as well as single-crystal Raman spectroscopy. Six phase transitions are reported in the 0-0-GPa range. At low pressures, four transitions are evidenced at 4, 5, 7, and 11 GPa. In this range, the crystals display large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The nonpolar Prima phase remains stable over a large pressure range between I 1 and 38 GPa, where the distortion (tilt angle) changes only little with pressure. The two high-pressure phase transitions at 38 and 48 GPa are marked by the occurrence of larger unit cells and an increase of the distortion away from the cubic parent perovskite cell. We find no evidence for a cubic phase at high pressure, nor indications that the structure tends to become cubic. The previously reported insulator-to-metal transition at 50 GPa appears to be symmetry breaking. [ABSTRACT FROM AUTHOR]

Materials possessing multiple states are promising to emulate synaptic and neuronic behaviors. Their operation frequency, typically in or below the GHz range, however, limits the speed of neuromorphic computing. Ultrafast THz electric field excitation has been employed to induce nonequilibrium states of matter, called hidden phases in oxides. One may wonder if there are systems for which THz pulses can generate neuronic and synaptic behavior, via the creation of hidden phases. Using atomistic simulations, we discover that relaxor ferroelectrics can emulate all the key neuronic and memristive synaptic features. Their occurrence originates from the activation of many hidden phases of polarization order, resulting from the response of nanoregions to THz pulses. Such phases further possess different dielectric constants, which is also promising for memcapacitor devices. [ABSTRACT FROM AUTHOR]

For several decades, ferroelectrics with enhanced electrocaloric (EC) effect have served as competitive alternatives for solid-state cooling devices. In order to enhance the EC effect, there has been increasing research interest on exploring novel systems of EC materials and efficient cooling cycles. In contrast, optimizing intrinsic properties through doping effect has been relatively ignored. In this work, we introduce the binary compound CuO as an additive into the conventional lead-free BaSn 0·11 Ti 0·89 O 3 (BST) ferroelectric ceramic. This leads to a remarkably large adiabatic temperature change of Δ T = −0.33 °C under a relatively low electric field (10 kV/cm). This is higher than the ΔT of pure BST (Δ T = −0.15 °C, under 10 kV/cm). The largest of Δ T = −0.48 °C was achieved under 20 kV/cm. We assume that the large temperature change was achieved due to the addition of CuO additive, which improves the ferroelectric properties (e.g., a higher polarization and a lower coercive field). In addition, by introducing CuO additives, the breakdown electric field of the ceramics was also enhanced at high temperatures (above the Curie temperature, T C) and the working temperature range was greatly broadened (30 °C–60 °C) under a slightly high electric field. Our findings present a promising approach to enhance the EC effect by tuning the intrinsic properties of EC materials. We expect that our work emphasizes the importance of additives in enhancing the EC properties. • CuO was added into BaSn/TiO 3 as an additive for optimizing ferroelectric and electrocaloric effect (EC) properties. • EC temperature change was increased from −0.15°C to −0.33°C at the electric field of 10kV/cm after adding 2 mol%CuO. • A relatively large EC temperature change of −0.48°C at the electric field of 20kV/cm was achieved. • The relationship between the EC effect and intrinsic properties was explained based on Landau-Devonshire theory. [ABSTRACT FROM AUTHOR]

• Optical and structural investigation of In-rich InGaAs grown on (1 0 0) InP. • Effects of temperature and excitation density on the optical properties of InGaAs. • Good correlation between results provided by different characterization techniques. • Abnormal behavior of the luminescence keys has been observed. In-rich In x Ga 1−x As epitaxial layers were grown on InP (1 0 0) substrates by a metalorganic vapor phase epitaxy (MOVPE) technique. The effect of Indium (In) composition on the crystalline quality and optical properties are investigated. High resolution X-ray diffraction (HR-XRD) measurement and Raman scattering spectrum are used to evaluate the crystalline quality, the residual strain and dislocation density property. The number of dislocations in the epitaxial layers is found to increase by increasing the Indium content in order to release the stresses due to the epitaxial clamping. Photoluminescence (PL) measurement is used to characterize the optical properties. At 10 K, PL measurements show that the InGaAs band gap redshifts with the indium content. Moreover, the asymmetry at the low-energy side of the PL peak has been attributed to the presence of localized excitons. In all samples, a blue shift of PL peaks is evidenced by increasing the excitation power density, which is in line with the presence of carrier's localization and non-idealities in this system. Moreover, the temperature-dependence of the PL peak energy displays an unusual red-blue-red shift (S-shaped) behavior when raising the temperature. These observations can be related to the inhomogeneous distribution of indium which gives rise to the appearance of dislocations and other defects which serve as traps for charge carriers. Interestingly, those highly In-content In x Ga 1−x As epitaxial layers show PL emission located between 1637 and 1811 nm (depending on In content) and thus might be suitable for in the design of novel heterostructure devices such as short wave infrared (SWIR) detectors. [ABSTRACT FROM AUTHOR]

Multilayer capacitors (MLCs) represent promising electrocaloric (EC) cooling elements because they operate at low voltages, they possess large breakdown fields, they contain inner electrodes that facilitate heat transfer, and they exhibit a macroscopic volume of active EC material. However, almost ten years after they were first suggested as EC elements, the internal heat flow has not been well studied. In this paper, we use an infrared camera to investigate heat flow within representative MLCs of Pb(Mg,Nb)O3, and we achieve good agreement with the results of finite element modelling (FEM) by identifying for the inner electrodes an effective thermal conductivity that represents the limiting factor for heat flow. Increasing the inner electrode thickness by a factor of five would increase heat flow by a factor of 2.6. Our work therefore shows that FEM can play an important role in MLC design for EC applications. • In this paper, we model for the first time the heat flow of state of the art bespoke electrocaloric multilayer capacitors from an industrial source (MURATA) and compare it with experimental data. [ABSTRACT FROM AUTHOR]

Utilizing thermal changes in solid-state materials strategically offers caloric-based alternatives to replace current vapor-compression technology. To make full use of multiple forms of the entropy and achieve higher efficiency for designs of cooling devices, the multicaloric effect appears as a cutting-edge concept encouraging researchers to search for multicaloric materials with outstanding caloric properties. Here we report the multicaloric effect in BaTiO3 single crystals driven simultaneously by mechanical and electric fields and described via a thermodynamic phenomenological model. It is found that the multicaloric behavior is mainly dominated by the mechanical field rather than the electric field, since the paraelectric-to-ferroelectric transition is more sensitive to mechanical field than to electric field. The use of uniaxial stress competes favorably with pressure due to its much higher caloric strength and negligible elastic thermal change. It is revealed that multicaloric response can be significantly larger than just the sum of mechanocaloric and electrocaloric effects in temperature regions far above the Curie temperature but cannot exceed this limit near the Curie temperature. Our results also show the advantage of the multicaloric effect over the mechanically mediated electrocaloric effect or electrically mediated mechanocaloric effect. Our findings therefore highlight the importance of ferroelectric materials to develop multicaloric cooling. [ABSTRACT FROM AUTHOR]

In this study, a critical parameter termed as “effective electric strength” is developed to depict polarization fatigue in antiferroelectric thin films. Specifically, the effect of pulse width, frequency, and magnitude of driving voltage (or field) on polarization fatigue of antiferroelectric (Pb,La)(Zr,Ti)O 3 (PLZT) thin films was investigated in detail. By transforming all the waveforms with different pulse-widths/voltages/frequencies into their effective electric strength, a strong correlation between the effective strength and the degree of polarization fatigue in the PLZT films is revealed. For instance, it was found that appreciable fatigue usually occurs when the effective strength is larger than or equal to the voltage of phase transition from antiferroelectric phase to ferroelectric phase. In addition, it is shown that the films are more prone to fatigue for a lower frequency of the same effective strength. Our findings could be well explained in the framework of the local phase decomposition arising from switching induced charge injection. [ABSTRACT FROM AUTHOR]

Ferroelectric materials display exotic polarization textures at the nanoscale that could be used to improve the energetic efficiency of electronic components. The vast majority of studies were conducted in two dimensions on thin films that can be further nanostructured, but very few studies address the situation of individual isolated nanocrystals (NCs) synthesized in solution, while such structures could have other fields of applications. In this work, we experimentally and theoretically studied the polarization texture of ferroelectric barium titanate (BaTiO 3 , BTO) NCs attached to a conductive substrate and surrounded by air. We synthesized NCs of well-defined quasicubic shape and 160 nm average size that conserve the tetragonal structure of BTO at room temperature. We then investigated the inverse piezoelectric properties of such pristine individual NCs by vector piezoresponse force microscopy (PFM), taking particular care to suppress electrostatic artifacts. In all of the NCs studied, we could not detect any vertical PFM signal, and the maps of the lateral response all displayed larger displacement amplitude on the edges with deformations converging toward the center. Using field phase simulations dedicated to ferroelectric nanostructures, we were able to predict the equilibrium polarization texture. These simulations revealed that the NC core is composed of 180° up and down domains defining the polar axis that rotate by 90° in the two facets orthogonal to this axis, eventually lying within these planes forming a layer of about 10 nm thickness mainly composed of 180° domains along an edge. From this polarization distribution, we predicted the lateral PFM response, which was revealed to be in very good qualitative agreement with the experimental observations. This work positions PFM as a relevant tool to evaluate the potential of complex ferroelectric nanostructures to be used as sensors.

Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO 3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Among today's nonvolatile memories, ferroelectric-based capacitors, tunnel junctions and field-effect transistors (FET) are already industrially integrated and/or intensively investigated to improve their performances. Concurrently, because of the tremendous development of artificial intelligence and big-data issues, there is an urgent need to realize high-density crossbar arrays, a prerequisite for the future of memories and emerging computing algorithms. Here, a two-terminal ferroelectric fin diode (FFD) in which a ferroelectric capacitor and a fin-like semiconductor channel are combined to share both top and bottom electrodes is designed. Such a device not only shows both digital and analog memory functionalities but is also robust and universal as it works using two very different ferroelectric materials. When compared to all current nonvolatile memories, it cumulatively demonstrates an endurance up to 10 10 cycles, an ON/OFF ratio of ~10 2 , a feature size of 30 nm, an operating energy of ~20 fJ and an operation speed of 100 ns. Beyond these superior performances, the simple two-terminal structure and their self-rectifying ratio of ~ 10 4 permit to consider them as new electronic building blocks for designing passive crossbar arrays which are crucial for the future in-memory computing., (© 2024. The Author(s).)

Competing Interests: Conflict of interest The authors declare that they have no conflict of interest.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO 3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO 3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

Recently, piezoelectric-based catalysis has been demonstrated to be an efficient means and promising alternative to sunlight-driven photocatalysis, where mechanical vibrations trigger redox reactions. Here, 60 nm-size BiFeO 3 nanoparticles are shown to be very effective for piezo-degrading Rhodamine B (RhB) model dye with record degradation rate reaching 13 810 L mol -1  min -1 , and even 41 750 L mol -1  min -1 (i.e., 100% RhB degradation within 5 min) when piezocatalysis is synergistically combined with sunlight photocatalysis. These BiFeO 3 piezocatalytic nanoparticles are also demonstrated to be versatile toward several dyes and pharmaceutical pollutants, with over 80% piezo-decomposition within 120 min. The maintained high piezoelectric coefficient combined with low dielectric constant, high-elastic modulus, and the nanosized shape make these BiFeO 3  nanoparticles extremely efficient piezocatalysts. To avoid subsequent secondary pollution and enable their reusability, the BiFeO 3 nanoparticles are further embedded in a polymer P(VDF-TrFE) matrix. The as-designed flexible, chemically stable, and recyclable nanocomposites still keep remarkable piezocatalytic and piezo-photocatalytic performances (i.e., 92% and 100% RhB degradation, respectively, within 20 min). This work opens a new research avenue for BiFeO 3 that is the model multiferroic and offers a new platform for water cleaning, as well as other applications such as water splitting, CO 2 reduction, or surface purification., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Analog storage through synaptic weights using conductance in resistive neuromorphic systems and devices inevitably generates harmful heat dissipation. This thermal issue not only limits the energy efficiency but also hampers the very-large-scale and highly complicated hardware integration as in the human brain. Here we demonstrate that the synaptic weights can be simulated by reconfigurable non-volatile capacitances of a ferroelectric-based memcapacitor with ultralow-power consumption. The as-designed metal/ferroelectric/metal/insulator/semiconductor memcapacitor shows distinct 3-bit capacitance states controlled by the ferroelectric domain dynamics. These robust memcapacitive states exhibit uniform maintenance of more than 10 4 s and well endurance of 10 9 cycles. In a wired memcapacitor crossbar network hardware, analog vector-matrix multiplication is successfully implemented to classify 9-pixel images by collecting the sum of displacement currents ( I  =  C  × d V /d t ) in each column, which intrinsically consumes zero energy in memcapacitors themselves. Our work sheds light on an ultralow-power neural hardware based on ferroelectric memcapacitors., Competing Interests: The authors declare no conflicts of interest., (© 2023 The Authors. Exploration published by Henan University and John Wiley & Sons Australia, Ltd.)

The atomic-level response of zigzag ferroelectric domain walls (DWs) was investigated with in situ bias scanning transmission electron microscopy (STEM) in a subcoercive-field regime. Atomic-level movement of a single DW was observed. Unexpectedly, the change in the position of the DW, determined from the atomic displacement, did not follow the position of the strain field when the electric field was applied. This can be explained as low mobility defect segregation at the initial DW position, such as ordered clusters of oxygen vacancies. Further, the triangular apex of the zigzag wall is pinned, but it changes its shape and becomes asymmetric under electrical stimuli. This phenomenon is accompanied by strain and bound charge redistribution. We report on unique atomic-scale phenomena at the DW level and show that in situ STEM studies with atomic resolution are very relevant as they complement, and sometimes challenge, the knowledge gained from lower resolution studies.

Many material properties such as superconductivity, magnetoresistance or magnetoelectricity emerge from the non-linear interactions of spins and lattice/phonons. Hence, an in-depth understanding of spin-phonon coupling is at the heart of these properties. While most examples deal with one magnetic lattice only, the simultaneous presence of multiple magnetic orderings yield potentially unknown properties. We demonstrate a strong spin-phonon coupling in SmFeO 3 that emerges from the interaction of both, iron and samarium spins. We probe this coupling as a remarkably large shift of phonon frequencies and the appearance of new phonons. The spin-phonon coupling is absent for the magnetic ordering of iron alone but emerges with the additional ordering of the samarium spins. Intriguingly, this ordering is not spontaneous but induced by the iron magnetism. Our findings show an emergent phenomenon from the non-linear interaction by multiple orders, which do not need to occur spontaneously. This allows for a conceptually different approach in the search for yet unknown properties., (© 2022. The Author(s).)

Since the emergence of memristors (or memristive devices), how to integrate them into arrays has been widely investigated. After years of research, memristor crossbar arrays have been proposed and realized with potential applications in nonvolatile memory, logic and neuromorphic computing systems. Despite the promising prospects of memristor crossbar arrays, one of the main obstacles for their development is the so-called sneak-path current causing cross-talk interference between adjacent memory cells and thus may result in misinterpretation which greatly influences the operation of memristor crossbar arrays. Solving the sneak-path current issue, the power consumption of the array will immensely decrease, and the reliability and stability will simultaneously increase. In order to suppress the sneak-path current, various solutions have been provided. So far, some reviews have considered some of these solutions and established a sophisticated classification, including 1D1M, 1T1M, 1S1M (D: diode, M: memristor, T: transistor, S: selector), self-selective and self-rectifying memristors. Recently, a mass of studies have been additionally reported. This review thus attempts to provide a survey on these new findings, by highlighting the latest research progress realized for relieving the sneak-path issue. Here, we first present the concept of the sneak-path current issue and solutions proposed to solve it. Consequently, we select some typical and promising devices, and present their structures and properties in detail. Then, the latest research activities focusing on single-device structures are introduced taking into account the mechanisms underlying these devices. Finally, we summarize the properties and perspectives of these solutions., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Ultrathin Hf 1- x Zr x O 2 films have attracted tremendous interest since they show ferroelectric behavior at the nanoscale, where other ferroelectrics fail to stabilize the polar state. Their promise to revolutionize the electronics landscape comes from the well-known Si compatibility of HfO 2 and ZrO 2 , which (in amorphous form) are already used as gate oxides in MOSFETs. However, the recently discovered crystalline ferroelectric phases of hafnia-based films have been grown on Si only in polycrystalline form. Better ferroelectric properties and improved quality of the interfaces have been achieved in epitaxially grown films, but these are only obtained on non-Si and buffered Si(100) substrates. Here, we report direct epitaxy of polar Hf 1- x Zr x O 2 phases on Si, enabled via in situ scavenging of the native a-SiO x layer by Zr (Hf), using pulsed laser deposition under ballistic deposition conditions. We investigate the effect of substrate orientation and film composition to provide fundamental insights into the conditions that lead to the preferential stabilization of polar phases, namely, the rhombohedral (r-) and the orthorhombic (o-) phases, against the nonpolar monoclinic (m-), on Si., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

Hafnia-based thin films are a favoured candidate for the integration of robust ferroelectricity at the nanoscale into next-generation memory and logic devices. This is because their ferroelectric polarization becomes more robust as the size is reduced, exposing a type of ferroelectricity whose mechanism still remains to be understood. Thin films with increased crystal quality are therefore needed. We report the epitaxial growth of Hf 0.5 Zr 0.5 O 2 thin films on (001)-oriented La 0.7 Sr 0.3 MnO 3 /SrTiO 3 substrates. The films, which are under epitaxial compressive strain and predominantly (111)-oriented, display large ferroelectric polarization values up to 34 μC cm -2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This finding, in conjunction with density functional theory calculations, allows us to propose a compelling model for the formation of the ferroelectric phase. In addition, these results point towards thin films of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two-terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Li x CoO 2 layer. These observations are very well correlated with the observed insulator-to-metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling - much further than the present cycling life of usual lithium-ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Piezoelectric actuators transform electrical energy into mechanical energy, and because of their compactness, quick response time and accurate displacement, they are sought after in many applications. Polycrystalline piezoelectric ceramics are technologically more appealing than single crystals due to their simpler and less expensive processing, but have yet to display electrostrain values that exceed 1%. Here we report a material design strategy wherein the efficient switching of ferroelectric-ferroelastic domains by an electric field is exploited to achieve a high electrostrain value of 1.3% in a pseudo-ternary ferroelectric alloy system, BiFeO 3 -PbTiO 3 -LaFeO 3 . Detailed structural investigations reveal that this electrostrain is associated with a combination of several factors: a large spontaneous lattice strain of the piezoelectric phase, domain miniaturization, a low-symmetry ferroelectric phase and a very large reverse switching of the non-180° domains. This insight for the design of a new class of polycrystalline piezoceramics with high electrostrains may be useful to develop alternatives to costly single-crystal actuators.

Domain walls (DWs) in ferroic materials exhibit a plethora of unexpected properties that are different from the adjacent ferroic domains. Still, the intrinsic/extrinsic origin of these properties remains an open question. Here, density functional theory calculations are used to investigate the interaction between vacancies and 180° DWs in the prototypical ferroelectric PbTiO 3 , with a special emphasis on cationic vacancies and released holes. All vacancies are more easily formed within the DW than in the domains. This is interpreted, using a phenomenological model, as the partial compensation of an extra-tensile stress when the defect is created inside the DW. Oxygen vacancies are found to be always fully ionized, independently of the thermodynamic conditions, while cationic vacancies can be either neutral or partially ionized (oxygen-rich conditions), or fully ionized (oxygen-poor conditions). Therefore, in oxidizing conditions, holes are induced by neutral and partially ionized Pb vacancies. In the bulk PbTiO 3 , these holes are more stable as delocalized rather than small polarons, but at DWs, the two forms are found to be possible.

We report the structural, magnetoelectric (ME), magnetic and electric control of magnetic properties in Co 4 Nb 2 O 9 (CNO) single crystal. A detailed ME measurement reveals a nonlinear ME effect instead of a linear ME effect in CNO single crystal. By fitting the magnetization-electric field (M-E) curve, it can be found that the linear ([Formula: see text]) and quadratic (γ) coefficients equal to ~8.27 ps/m and ~-6.46 ps/MV for upper branch, as well as ~8.38 ps/m and ~6.75 ps/MV for the lower branch. More importantly, a pronounced response was observed under a small cooling magnetic field, which cannot even cause the spin flop. This suggests a magnetoelectric effect can occur at paraelectric state for CNO single crystal. Furthermore, we also found that the magnetization of every axis responds to electric field applied along a-axis, but fails to do so when the electric field is applied c-axis. Such findings supply a direct evidence to the magnetic structure and ME coupling mechanism indirectly reflected by our neutron experiment.

Antiferroelectric thin films are demonstrated as a new class of giant electrocaloric materials that exhibit a negative electrocaloric response of about -5 K near room temperature. The giant negative electrocaloric effect may open up a new paradigm for light, compact, reliable, and high-efficiency refrigeration devices., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Pb(Zn(1/3)Nb(2/3))O3-xPbTiO(3) (x=4.5%-12%) relaxor ferroelectric crystals have been studied by means of acoustic emission (AE) in the 400-540 K temperature range. An anomalous AE activity independent of the ground state relaxor/morphotropic/ferroelectric crossover has been revealed at around 500 K, and it is associated with the "waterfall" feature related to the existence of polar nanoregions (PNRs). The 500 K AE anomaly is attributed to local martensitelike cubic-to-tetragonal ferroelectric transitions within the PNRs imbedded in a nonpolar (cubic) matrix.