Macroscopic Switching Research Articles

Robust resistance switching performance in a Ca3Ti2O7/La0.67Sr0.33MnO3 heterostructure induced by the interface

Ruddlesden–Popper phase oxides, such as Ca3Ti2O7, have been established as hybrid improper ferroelectrics. However, investigations into Ca3Ti2O7 have primarily concentrated on their structural and ferroelectric properties. In this study, we prepared epitaxial Ca3Ti2O7 thin films via magnetron sputtering. Conducting atomic force microscopy was employed to characterize local current variations under an applied bias voltage. Electron paramagnetic resonance measurements of the Ca3Ti2O7/La0.67Sr0.33MnO3 film were conducted to assess its defect characteristics. Interestingly, the Ca3Ti2O7/La0.67Sr0.33MnO3 stacks exhibited remarkable macroscopic resistance switching, with a resistance on/off ratio reaching 100, alongside robust retention (∼2500 s) and endurance (∼2000 cycles) features. Additionally, density functional theory calculations suggest that the resistance switching is attributable to the interface barrier of the Ca3Ti2O7/La0.67Sr0.33MnO3 interface and the efficacy of space charge limitation. This work proposes an avenue for the utilization of Ruddlesden–Popper phase Ca3Ti2O7 in various applications.

Nanoscale Investigation of the Effect of Annealing Temperature on the Polarization Switching Dynamics of Hf0.5Zr0.5O2 Thin Films

AbstractRecently, HfO2‐based ferroelectric thin films have attracted widespread interest in developing next‐generation nonvolatile memories. To form a metastable ferroelectric orthorhombic phase in HfO2, a post‐annealing process is typically necessary. However, the microscopic mechanism underlying the effect of annealing temperature on ferroelectric domain nucleation and growth is still obscure, despite its importance in optimizing the operation speed of HfO2‐based devices. In this study, the ferroelectric properties and polarization switching of Hf0.5Zr0.5O2 thin films annealed at different temperatures (550–700 °C) are systematically investigated. Evidently, the crystal structure, remnant polarization, and dielectric constant monotonically change with annealing temperature. However, microscopic piezoresponse force microscopy images as well as macroscopic switching current measurements reveal non‐monotonic changes in the polarization switching speed with annealing temperature. This intriguing behavior is ascribed to the difference in the ferroelectric‐domain nucleation process induced by the amount of oxygen vacancies in the Hf0.5Zr0.5O2 thin films annealed at different temperatures. This work demonstrates that controlling the defect concentration of ferroelectric HfO2 by tuning the post‐annealing process is critical for optimizing device performance, particularly polarization switching speed.

A Solvent Free Neutral Cobalt Complex Exhibiting Macroscopic Polarization Switching Induced by Directional Charge Transfer

Materials with polarization switching induced by directional charge transfer under the external stimuli are of great interest due to their fast swicthing rate and potential applications. Nonetheless, most of these...

Ferroelectric Al1−xBxN thin films integrated on Si

Ferroelectric Al0.93B0.07N thin films are prepared (100) Si substrates. The necessary c-axis out-of-plane orientation to observe macroscopic ferroelectric switching was achieved by implementing an initial Ar/N2 plasma treatment, followed by a thin layer of AlN to initiate the desired texture and a 150 nm W layer. The plasma treatment facilitates crystallinity enhancement of the AlN template layer, allowing for subsequent growth of highly oriented W and Al0.93B0.07N layers. The W layer exhibits random in-plane orientation and exclusive (110) out-of-plane orientation with a rocking curve width of 1.4°. When grown on these W surfaces, 175 nm thick Al0.93B0.07N films exhibit random in-plane orientation and exclusive (001) texture with rocking curve full-width-half-max values of 1.6° and RMS roughness values less than 1 nm. Polarization hysteresis measurements show robust hysteresis with coercive field values of 5.4 MV/cm and remanent polarization values of 136 μC/cm2. XPS depth profile analysis suggests that the plasma treatment converts the existing native oxide to a nitrogen rich oxynitride with approximate composition Si3O0.5N3.67. Cross-sectional TEM reveals that the oxynitride interlayer is amorphous and ∼3.4 nm thick, more than double the native oxide thickness measured by multiwavelength ellipsometry, implying that (oxy)nitride growth continues after conversion of the native oxide. This new family of ferroelectric wurtzites is interesting from an integration perspective given their chemical compatibility with mainstream semiconductors. Developing synthesis routes that promote needed texture while preserving compatible processing windows is an important step toward practical integration.

Energy conversion and storage via photoinduced polarization change in non-ferroelectric molecular [CoGa] crystals

To alleviate the energy and environmental crisis, in the last decades, energy harvesting by utilizing optical control has emerged as a promising solution. Here we report a polar crystal that exhibits photoenergy conversion and energy storage upon light irradiation. The polar crystal consists of dinuclear [CoGa] molecules, which are oriented in a uniform direction inside the crystal lattice. Irradiation with green light induces a directional intramolecular electron transfer from the ligand to a low-spin CoIII centre, and the resultant light-induced high-spin CoII excited state is trapped at low temperature, realizing energy storage. Additionally, electric current release is observed during relaxation from the trapped light-induced metastable state to the ground state, because the intramolecular electron transfer in the relaxation process is accompanied with macroscopic polarization switching at the single-crystal level. It demonstrates that energy storage and conversion to electrical energy is realized in the [CoGa] crystals, which is different from typical polar pyroelectric compounds that exhibit the conversion of thermal energy into electricity.

Mesoscopic-scale grain formation in HfO2-based ferroelectric thin films and its impact on electrical characteristics

Ferroelectric memory devices are expected for low-power and high-speed memory applications. HfO2-based ferroelectric is attracting attention for its CMOS-compatibility and high scalability. Mesoscopic-scale grains, of which size is almost comparable to device size, are formed in HfO2-based ferroelectric poly-crystalline thin films, which largely influences electrical characteristics in memory devices. It is important to study the impact of mesoscopic-scale grain formation on the electrical characteristics. In this work, first, we have studied the thickness dependence of the polarization switching kinetics in HfO2-based ferroelectric. While static low-frequency polarization is comparable for different thickness, dynamic polarization switching speed is slower in thin Hf0.5Zr0.5O2 (HZO) capacitors. Based on the analysis using the NLS model and physical characterization, thinner HZO contains smaller grains with orientation non-uniformity and more grain boundaries than thicker HZO, which can impede macroscopic polarization switching. We have also theoretically and experimentally studied the polar-axis alignment of a HfO2-based ferroelectric thin film. While in-plane polar orientation is stable in as-grown HZO, out-of-plane polarization can be dominant by applying electric field, which indicates the transition from in-plane polar to out-of-plane polar orientation in the ferroelectric phase grains. This is confirmed by calculating kinetic pathway using ab-initio calculation.

Dynamical limits for the molecular switching in a photoexcited material revealed by X-ray diffraction

Time-resolved studies with temporal resolution that separate molecular level dynamics from macroscopic changes, allow clear distinction between the time scales of the different degrees of freedom involved. Cooperative molecular switching in the solid state is exemplified by spin crossover phenomenon in crystals of transition metal complexes. Here we show the existence of a delay between the crystalline volume increase, and the cooperative macroscopic switching of molecular state. Using 100 ps X-ray diffraction, we track the molecular spin state and the structure of the lattice during the photoinduced low spin to high spin transition in microcrystals of [FeIII(3-MeO-SalEen)2]PF6. Model simulations explain the phenomenon with thermally activated kinetics governed by local energy barriers separating the molecular states. Such behaviour is different from that encountered in materials with no local energy barriers, where phase transformation can occur simultaneously with propagation of strain. Broadly, this motivates an optimised material design, scalable with size and intrinsic energetics.

Sequential Activation of Molecular and Macroscopic Spin‐State Switching within the Hysteretic Region Following Pulsed Light Excitation (Adv. Mater. 6/2022)

Spin-State Switching In article number 2105468, Karl Ridier, Gábor Molnár, Azzedine Bousseksou, and co-workers dissect the spatiotemporal dynamics of the spin transition in crystals of [Fe(HB(1,2,4-triazol-1-yl)3)2], using pump–probe optical microscopy. On the sub-millisecond timescale, the response of the crystal to the laser pulse involves a homogeneous and gradual molecular conversion process, as if there were no hysteresis loop, whereas the macroscopic switching is delayed to the millisecond timescale. These results provide guidance for achieving efficient photocontrol of bistable materials.

Sequential Activation of Molecular and Macroscopic Spin-State Switching within the Hysteretic Region Following Pulsed Light Excitation.

Molecular spin-crossover (SCO) compounds constitute a promising class of photoactive materials exhibiting efficient photoinduced phase transitions (PIPTs). Taking advantage of the unique, picture-perfect reproducibility of the spin-transition properties in the compound [Fe(HB(1,2,4-triazol-1-yl)3 )2 ], the spatiotemporal dynamics of the PIPT within the thermodynamic metastability (hysteretic) region of a single crystal is dissected, using pump-probe optical microscopy. Beyond a threshold laser-excitation density, complete PIPTs are evidenced, with conversion rates up to 200 switched molecules per absorbed photon. It is shown that the PIPT takes place through the sequential activation of two (molecular and macroscopic) switching mechanisms, occurring on sub-microsecond and millisecond timescales, governed by the intramolecular and free energy barriers of the system, respectively. The main finding here is that the thermodynamic metastability has strictly no influence on the sub-millisecond switching dynamics. Indeed, before this millisecond timescale, the response of the crystal to the laser excitation involves a gradual, molecular conversion process, as if there wereno hysteresis loop. Consequently, in this regime, even a 100% photoinduced conversion may not give rise to a PIPT. These results provide new insight on the intrinsic dynamical limits of the PIPT, which is an important issue from a technological perspective.

Microscopic model for switching kinetics in organic ferroelectrics following the Merz law

From an application perspective, one of the most important parameters of a ferroelectric is its switching time, and understanding its limiting factors is key to improve device performance. While there is a variety of competing models for switching kinetics in realistic (disordered) ferroelectrics, they are often merely descriptive and provide little insight into the underlying microscopic mechanisms. This holds in particular for the classical Merz law, which describes the commonly observed exponential field dependence of the switching time. Here, we investigate the switching kinetics in the archetypical molecular ferroelectric trialkylbenzene-1,3,5- tricarboxamide using an electrostatic kinetic Monte Carlo model. The simulated field dependence follows the Merz law, which shows that a simple system of interacting dipoles is sufficient to obtain this behavior, even without explicitly considering domain walls or defects that are commonly thought to be involved in the emergence of the Merz law. Through a detailed analysis of the nucleation process, we can relate the macroscopic switching time to the microscopic nucleation energy barrier, which in turn is related to a field-dependent nucleus size. Finally, we use the acquired insight into the nucleation process to derive the Merz law from the theory of thermally activated nucleation-limited switching. This analytical model provides a physically transparent description of the switching kinetics in both experiments and simulations.

Macroscopic Change in Luminescent Color by Thermally Driven Sliding Motion in [3]Rotaxanes.

Systematic investigation of rotaxane structures has revealed a rational design for thermally driven switching of their macroscopic properties. At low temperature, the luminophore is insulated by the macrocycles and displays monomer emission, whereas at high temperature, the luminophore is exposed owing to a change in the macrocyclic location distribution and interacts with external molecules, affording a thermally driven luminescent color change with high reversibility and responsiveness. This macroscopic switching through efficient thermal sliding was made possible by appropriate tuning of both the macrocycle-luminophore interactions within the rotaxane and the coupling between the excited luminophore and external molecules in an exciplex. The ability to switch properties by a simple and clean thermal stimuli should expand the utilization of rotaxanes as components of thermally driven molecular systems.

Single step densification of high permittivity BaTiO3 ceramics at 300 ºC

Dense nanocrystalline BaTiO3 ceramics are prepared in a single step by the Cold Sintering Process at 300 °C, under a uniaxial pressure of 520 MPa for 12 h using a molten hydroxide flux. Transmission electron microscopy reveals a dense microstructure with sharp grain boundaries. The average grain sizes are 75−150 nm depending on the flux amount. The dielectric permittivity is 700–1800 at room temperature at 106 Hz, with a dielectric loss, tan δ ∼ 0.04. The difference in permittivity and phase transition behavior are explained in terms of the intrinsic size effect of the BaTiO3. The nanocrystalline BaTiO3 ceramics still shows a macroscopic ferroelectric switching via a hysteresis loop. This work demonstrates that cold-sintering process could enable the densification of ferroelectric oxides in a single step. Futhermore, comparable dielectric properties to reported values for nanocrystalline grains are obtained, but at this time, with the lowest processing temperatures ever used.

Chromatin state switching in a polymer model with mark-conformation coupling.

We investigate the phase transition properties of the polymer-Potts model, a chain composed of monomers with magnetic degrees of freedom, with the motivation to study the conformation and mark switching dynamics of chromatin. By the mean-field approximation, we find that the phase transition between the swollen-disordered state and the compact-ordered state is discrete; it is first order as in the long-range Potts model, but with a significantly larger jump in magnetization (i.e., mark coherence) upon the ordering transition. The results imply how small changes in epigenetic writer concentrations can lead to a macroscopic switching of the chromatin state, suggesting a simple mechanism of discrete switching observed, for instance, in cell differentiation.

Fluid Imprint and Inertial Switching in Ferroelectric La:HfO2 Capacitors.

Ferroelectric (FE) HfO2-based thin films, which are considered as one of the most promising material systems for memory device applications, exhibit an adverse tendency for strong imprint. Manifestation of imprint is a shift of the polarization-voltage (P-V) loops along the voltage axis due to the development of an internal electric bias, which can lead to the failure of the writing and retention functions. Here, to gain insight into the mechanism of the imprint effect in La-doped HfO2 (La:HfO2) capacitors, we combine the pulse switching technique with high-resolution domain imaging by means of piezoresponse force microscopy. This approach allows us to establish a correlation between the macroscopic switching characteristics and domain time-voltage-dependent behavior. It has been shown that the La:HfO2 capacitors exhibit a much more pronounced imprint compared to Pb(Zr,Ti)O3-based FE capacitors. Also, in addition to conventional imprint, which evolves with time in the poled capacitors, an easily changeable imprint, termed as "fluid imprint", with a strong dependence on the switching prehistory and measurement conditions, has been observed. Visualization of the domain structure reveals a specific signature of fluid imprint-continuous switching of polarization in the same direction as the previously applied field that continues a long time after the field was turned off. This effect, termed as "inertial switching", is attributed to charge injection and subsequent trapping at defect sites at the film-electrode interface.

Energetic electron assisted synthesis of highly tunable temperature-responsive collagen/elastin gels for cyclic actuation: macroscopic switching and molecular origins

Thermoresponsive bio-only gels that yield sufficiently large strokes reversibly and without large hysteresis at a well-defined temperature in the physiological range, promise to be of value in biomedical application. Within the present work we demonstrate that electron beam modification of a blend of natural collagen and elastin gels is a route to achieve this goal, viz. to synthesize a bioresorbable gel with largely reversible volume contractions as large as 90% upon traversing a transition temperature that can be preadjusted between 36 °C and 43 °C by the applied electron dose. Employing circular dichroism and temperature depending confocal laser scanning microscopy measurements, we furthermore unravel the mechanisms underlying this macroscopic behavior on a molecular and network level, respectively and suggest a stringent picture to account for the experimental observations.

Understanding, Predicting, and Designing Ferroelectric Domain Structures and Switching Guided by the Phase-Field Method

Understanding mesoscale ferroelectric domain structures and their switching behavior under external fields is critical to applications of ferroelectrics. The phase-field method has been established as a powerful tool for probing, predicting, and designing the formation of domain structures under different electromechanical boundary conditions and their switching behavior under electric and/or mechanical stimuli. Here we review the basic framework of the phase-field model of ferroelectrics and its applications to simulating domain formation in bulk crystals, thin films, superlattices, and nanostructured ferroelectrics and to understanding macroscopic and local domain switching under electrical and/or mechanical fields. We discuss the possibility of utilizing the structure-property relationship learned from phase-field simulations to design high-performance relaxor piezoelectrics and electrically tunable thermal conductivity. The review ends with a summary of and an outlook on the potential new applications of the phase-field method of ferroelectrics.

Analysis and Design of LLC Converter Considering Output Voltage Regulation Under No-Load Condition

In the no-load condition, LLC converter usually fails to regulate its output voltage although it operates at a high switching frequency. Till now, it is hard to obtain the exact relationship between design parameters and the maximum switching frequency for no-load regulation capability. In this paper, a specific criterion for no-load regulation of LLC converter is provided, without using active components or other modulation schemes. By analyzing the macroscopic switching period and microscopic switching transition in the no-load condition, it is shown that not only the peaking resonant current during the switching transition, but also the resonant tank design affect the no-load regulation of the LLC converter, which affects the no-load regulation capability. Furthermore, the relationship among design parameters is analyzed and the design guideline is also provided to achieve no-load regulation at the specified maximum switching frequency. To verify the effectiveness of the proposed design, 400 V input and 50 V/200 W output prototype is built and tested.

Photochromism-Induced Macroscopic Polarization Switching in (K,Na)NbO 3-Based Piezoceramics

The intriguing coupling interactions between photochromism and ferroelectric polarization were firstly observed in Sm doped (K0.5Na0.5)NbO3 piezoceramics. After 407 nm irradiation, the maximum polarization (Pmax) and remanent polarization (Pr) intensities were dramatically decreased, whereas the coercive field, leakage current and photocurrent showed the remarkable increase. The variations of the polarization and photocurrent can be reversibly converted by alternating light irradiation and thermal stimulus, suggesting good polarization switching performance. This work provides a new route to effectively modulate ferro-/piezoelectric and photocurrent properties of ferroelectric materials by the non-contact remote control of light.

Soft-mode driven dynamics in ferroelectrics -new insight from ultrafast terahertz and x-ray experiments

The coupled lattice and charge dynamics connected with soft-mode excitations are mapped by nonlinear terahertz spectroscopy and ultrafast x-ray diffraction. Sub-picometer lattice displacements induce electron relocations over 100 pm in polycrystaline aspirin and ferroelectric ammonium sulfate (AS). In AS, the electron motions induce a macroscopic polarization switching.

Physical reality of the Preisach model for organic ferroelectrics

The Preisach model has been a cornerstone in the fields of ferromagnetism and ferroelectricity since its inception. It describes a real, non-ideal, ferroic material as the sum of a distribution of ideal ‘hysterons’. However, the physical reality of the model in ferroelectrics has been hard to establish. Here, we experimentally determine the Preisach (hysteron) distribution for two ferroelectric systems and show how its broadening directly relates to the materials’ morphology. We connect the Preisach distribution to measured microscopic switching kinetics that underlay the macroscopic dispersive switching kinetics as commonly observed for practical ferroelectrics. The presented results reveal that the in principle mathematical construct of the Preisach model has a strong physical basis and is a powerful tool to explain polarization switching at all time scales in different types of ferroelectrics. These insights lead to guidelines for further advancement of the ferroelectric materials both for conventional and multi-bit data storage applications.