Ferroelectric Domain Switching Research Articles

The enhanced multiferroic properties of BiFeO3 composite film by doping ions in the magnetic layer

Bi0.88Gd0.09Sr0.03Fe0.94Mn0.04Co0.02O3/Co1−xNixFe2O4 (BGSFMC/CNxFO, x = 0.1–0.5) composite films were successfully prepared by the sol–gel method. The structure and performance changes of BGSFMC/CNxFO composite film were studied. The results show that after the Ni2+-doped CNxFO magnetic bottom layer is combined with the upper BGSFMC, stress is generated in the film interface, which makes the upper BGSFMC layer phase structure change from a single R3c:H phase to the coexistence of R3c:H and R3m:R phases. The change of the structure causes the Fe–O bond length and Fe–O–Fe bond angle of the upper layer of the BGSFMC to change, the internal oxygen vacancy concentration decreases and the Fe3+ concentration increases. As a result, the inclination of the upper BGSFMC layer octahedron changes, limiting spatial modulation and releasing magnetism. At the same time, the change of structure also inhibits the conversion of Fe3+ to Fe2+ and strengthens the spin tilt and Fe–O–Fe super exchange interaction to enhance ferromagnetism. The ferromagnetic properties of the BGSFMC/CNxFO composite film are significantly enhanced, and the residual magnetization of 48.29–56.94 emu/cm3 is obtained. The reduction of defect complexes makes the ferroelectric domains of the BGSFMC/CNxFO composite film easier to flip under an external electric field, the symmetry and peak sharpness of the C–V characteristic curve increase, and the intrinsic polarization increases. The BGSFMC/CN0.1FO composite film obtained an increased Pr = 112 μC/cm2, a polarization switching current Is = 0.081 mA, a ferroelectric domain switching capacitance peak CS = 24.9 μF/cm2, and a reduced leakage conductivity CL = 4.73 μF/cm2. Through the ion doping of the magnetic layer, the ferromagnetic and ferroelectric properties of the BFO film can be adjusted and improved.

Shear-strain-mediated large nonvolatile tuning of ferromagnetic resonance by an electric field in multiferroic heterostructures

Controlling magnetism by an electric field is of critical importance for the future development of ultralow-power electronic and spintronic devices. Progress has been made in electrically driven nonvolatile tuning of magnetic states in multiferroic heterostructures for the information storage industry, which is exclusively attributed to the ferroelectric-polarization-switching-induced interfacial charge effect or nonlinear lattice strain effect. Here, we demonstrate that a hitherto unappreciated shear strain in the ferroelectric 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 substrate triggered by an electric field can be adopted to obtain robust nonvolatile control of the ferromagnetic resonance in an elastically coupled epitaxial Fe70Rh30 thin film. The disappearance of the resonance peak in a low-field-sweeping mode and the large resonance field shift of 111 Oe upon polarization switching demonstrate a strong shear-strain-mediated magnetoelectric coupling effect. In particular, in situ Kerr measurement identifies that the nonvolatile magnetic switching purely originates from electric-field-induced 109° ferroelastic domain switching rather than from 71°/180° ferroelectric domain switching even without the assistance of a magnetic field. This discovery illustrates the role of shear strain in achieving electrically tunable nonvolatile modulation of dynamic magnetic properties, and favors the design of future energy-efficient magnetoelectric microwave devices.

Ultrafast switching and linear conductance modulation in ferroelectric tunnel junctions via P(VDF-TrFE) morphology control.

Neuromorphic computing architectures demand the development of analog, non-volatile memory components operating at femto-Joule/bit operation energy. Electronic components working in this energy range require devices operating at ultrafast timescales. Among different non-volatile, analog memories, ferroelectric tunnel junctions (FTJs) have emerged as an important contender due to their voltage-driven operation leading to extreme energy-efficiency. Here, we report a study on the switching timescale and linear conductance modulation of organic FTJs comprising a metal/ferroelectric/semiconductor (MFS) stack with different morphologies of ferroelectric copolymer P(VDF-TrFE) ultrathin films. The results show that due to different annealing temperatures and protocols, the spin-coated copolymer films are modified significantly, which can have a large effect on the switching timescales and threshold fields of the FTJs with the best quality devices having a projected switching timescale of sub-nanosecond range. An improvement in switching speed by 7 orders of magnitude can be obtained with an increase of the programming voltage by less than a factor of 2 in these devices. This ultrafast switching of ferroelectric domains in our FTJs leads to pico to femto joule range of operation energy per bit opening the pathways for energy efficient and fast operating non-volatile memories while devices with higher domain pinning sites show a route for tuning analog conductivity for bio-realistic neuromorphic architectures.

Domain and homogeneous switching in ferroelectrics

The polarization switching kinetics of ferroelectric crystals and the transition from domain switching to homogeneous in nanoscale films are considered. Homogeneous (non-domain) switching according to the Ginzburg–Landau theory is possible only in two-dimensional ferroelectrics. The analysis of the results of calculations from the first principles and experiments is given.

Nanoscale Tailoring of Ferroelectricity in a Thin Dielectric Film.

New opportunities in the development and commercialization of novel photonic and electronic devices can be opened following the development of technology-compatible arbitrary-shaped ferroelectrics encapsulated in a passive environment. Here, we report and experimentally demonstrate nanoscale tailoring of ferroelectricity by an arbitrary pattern within the nonferroelectric thin film. For inducing the ferroelectric nanoregions in the nonferroelectric surrounding, we developed a technology-compatible approach of local doping of a thin (10 nm) HfO2 film by Ga ions right in the thin-film capacitor device via focused ion beam implantation. Local crystallization of the doped regions to the ferroelectric structural phase occurs during subsequent annealing. The remnant polarization of the HfO2:Ga regions reached 13 μC/cm2 at a Ga concentration of 0.6 at. %. Piezoresponse force microscopy over the capacitor device revealed an asymmetrical switching of ferroelectric domains within written HfO2:Ga patterns after capacitor switching, which was attributed to the mechanical stress across the doped film. The lateral spatial resolution of ferroelectricity tailoring is found to be ∼200 nm, which enables diverse applications in switchable photonics and microelectronic memories.

Periodic Wrinkle-Patterned Single-Crystalline Ferroelectric Oxide Membranes with Enhanced Piezoelectricity.

Self-assembled membranes with periodic wrinkled patterns are the critical building blocks of various flexible electronics, where the wrinkles are usually designed and fabricated to provide distinct functionalities. These membranes are typically metallic and organic materials with good ductility that are tolerant of complex deformation. However, the preparation of oxide membranes, especially those with intricate wrinkle patterns, is challenging due to their inherently strong covalent or ionic bonding, which usually leads to material crazing and brittle fracture. Here, wrinkle-patterned BaTiO3 (BTO)/poly(dimethylsiloxane) membranes with finely controlled parallel, zigzag, and mosaic patterns are prepared. The BTO layers show excellent flexibility and can form well-ordered and periodic wrinkles under compressive in-plane stress. Enhanced piezoelectricity is observed at the sites of peaks and valleys of the wrinkles where the largest strain gradient is generated. Atomistic simulations further reveal that the excellent elasticity and the correlated coupling between polarization and strain/strain gradient are strongly associated with ferroelectric domain switching and continuous dipole rotation. The out-of-plane polarization is primarily generated at compressive regions, while the in-plane polarization dominates at the tensile regions. The wrinkled ferroelectric oxides with differently strained regions and correlated polarization distributions would pave a way toward novel flexible electronics.

Controllable electric field tuning of anisotropic magnetic response of Ni/PMN-PT heterostructures

Epitaxial Ni films with various thicknesses were grown on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) substrates. Electric field tuning (E-tuning) of ferromagnetic resonance field (Hr) was utilized to investigate the magnetic response of Ni films along different film directions. Butterfly-like Hr vs E curves are shown when magnetic field is applied along film edge. While the combination of both loop-like and butterfly-like Hr vs E curves are observed for Ni film with thicknesses of 180 nm and 240 nm, and merely loop-like curves can be seen for film thicknesses of 330 nm and 510 nm, as magnetic field is applied along film diagonal. These interesting phenomena are assumed to be related to ferroelectric and ferroelastic domain switching in PMN-PT, film strain status, as well as magnetic anisotropy in Ni film. This work demonstrates the feasibility of control of anisotropic magnetic response via electric field in multiferroic heterostructure.

Impact of stress and doping effects on the polarization behavior and electrical characteristics of hafnium–zirconium oxides

In this study, we investigated the stress and doping effects of ferroelectric hafnium zirconium oxide (HfZrO) based on material analysis and electrical characteristics. The experimental results revealed that the ferroelectric crystalline phase is effectively transferred by mechanical stress of tantalum nitride (TaN) electrodes, and the origin of stress is mainly the thermal residual stress formed near the TaN and HfZrO interface. The ferroelectric polarization characteristics of HfZrO can be further modified by additional Al doping, which not only improves the leakage current of HfZrO due to the large bandgap of Al2O3 but also simultaneously stabilizes the ferroelectric domain switching under a large coercive field.

Shear-strain-induced over 90° rotation of local magnetization in FeCoSiB/PMN-PT (011) multiferroic heterostructures

Strain-mediated magnetoelectric effect can be utilized as an energy-efficient approach for spin manipulation. However, over 90° magnetization rotation is still challenging in un-patterned magnetic films, as the piezo-strain driven by ferroelectric domain switching is generally uniaxial rather than unidirectional, which limits the developments of non-volatile magnetic memory and logic devices. Here we demonstrate the rotation of local magnetization with a large angle of 136° by applying strains with a shear component at a fixed magnetic field of 45 Oe in FeCoSiB/PMN-PT (011) multiferroic heterostructures, revealed by a vector-resolved quantitative magneto-optical Kerr effect (MOKE) microscopy. Phase-field simulations confirm that the approximate 140° rotation of magnetization vectors is a consequence of the shear strain associated with ferroelectric/ferroelastic switching of PMN-PT (011) substrates. The visualization of over 90° magnetization rotation induced by the strain with a shear component paves the way for deterministic magnetization switching that has important implications in the energy-efficient spintronic devices.

Distinct ferroelectric domain switching dynamics in epitaxial BiFeO3 (001) capacitors depending on the bias polarity

Understanding ferroelectric domain switching dynamics at the nanoscale is a great of importance in the viewpoints of fundamental physics and technological applications. Here, we investigated the intriguing polarity-dependent switching dynamics of ferroelectric domains in epitaxial BiFeO3 (001) capacitors using transient switching current measurement and piezoresponse force microscopy. We observed the distinct behavior of nucleation and domain wall motion depending on the polarity of external electric bias. When applying the negative bias to the top electrode, the sideways domain wall motion initiated by only few nuclei was dominant to polarization switching. However, when applying the positive bias, most of domains started to grow from the pre-existed pinned domains and their growth velocity was much smaller. We suggest that the observed two distinct domain switching behavior is ascribed to the interfacial defect layer.

Morphotropic phase boundary-like effect in hybrid electrically poled, mechanically depolarized ferroelectric ceramics

The morphotropic phase boundary (MPB) plays an important role in ferroelectric materials. Typically, two phases coexist in materials near the MPB. Such materials usually exhibit large piezoelectricities, dielectricities, and actuation strains. In this work, we produce an MPB-like effect in hybrid electrically poled, mechanically depolarized (HEPMD) BaTiO3 and lead zirconate titanate ceramics where depolarized region A and vertical electrically poled region C intersect each other with a period of 400 μm in both in-plane directions. The polarization and strain of both HEPMD samples are over 3 times those of conventionally poled samples under unipolar electric loading. The large polarization and strain decrease steadily as the frequency increases and stabilize at approximately twice the values from the conventionally poled samples. Furthermore, the large polarizations and actuation strains of the HEPMD samples are fairly stable and change little after 30 000 cycles of operation. Under bipolar electric loading, the tendencies are similar and the coercive fields of the HEPMD samples are considerably smaller, which is similar to the MPB effect in traditional ferroelectric ceramics. Enhanced polarization and strain occur in HEPMD samples due to reversible ferroelectric domain switching during loading and unloading under large electric fields. In comparison, the small-signal properties, i.e., the d33 and dielectric properties, are slightly larger in HEPMD samples than in conventionally poled ones. The HEPMD method may be applied to all types of multiaxial ferroelectric ceramics to enhance actuation strain and polarization.

Room-temperature multiferroic behavior in layer-structured Aurivillius phase ceramics

Multiferroics that simultaneously exhibit ferroelectricity and ferromagnetism have recently attracted great attention due to their potential application in next generation electronic devices. However, only a few single-phase multiferroic materials exhibit ferroelectric and ferromagnetic orders at room temperature. Recently, some bismuth layer-structured Aurivillius compounds were reported as multiferroics at room temperature, but the origin of their magnetic property is still under debate because the net magnetization may originate from the presence of secondary phases that are not easily detected by laboratory XRD diffractometers. Here, textured Aurivillius phase Bi5.25La0.75FeCoTi3O18 ceramics were prepared by Spark Plasma Sintering. The ferromagnetic character of the ceramics was indicated by the magnetic field-induced reversible intensity changes of a certain set of crystalline planes belonging to the Aurivillius phase, as measured by in situ neutron diffraction under the applied magnetic field. The first principles calculations indicate that the ferromagnetism originates from double exchange interactions Fe3+–O–Fe3+, Co3+–O–Co3+, and Fe3+–O–Co3+ in the ferro-toroidal main phase. The magnetic-controlled ferroelectric domain switching was observed by piezoelectric force microscopy at room temperature. The prepared Aurivillius phase ceramics, with Co/Fe contributing to magnetization and polarization at the same time, can be considered an intrinsic room-temperature multiferroic.

Voltage-driven displacement of magnetic vortex cores

Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.

Experimental Investigation of Thermal Annealing and Ferroelectric Capacitor Area Effects for Hafnium-Zirconium Oxide Devices

In this study, we reveal that the thermal budget of post-metal annealing not only determines the formation of the ferroelectric phase and dipole domain but also the film quality of the gate stack in a metal-ferroelectric-metal capacitor. The higher leakage current caused by defect traps or grain boundaries within a gate stack would influence the stability of the ferroelectric domain switching. Furthermore, the ferroelectric domain switching and polarization current also depend on the ferroelectric capacitor area. We observe that a HfAlO ferroelectric capacitor can dominate the transfer characteristics of a p-type SnO thin-film transistor through the modulation of series capacitance in the gate stack based on a one-transistor one-capacitor series configuration. According to experimental results, the memory hysteresis window can be improved significantly with the area scaling due to the improvement of capacitance matching accuracy.

Integration of Dielectric and Ferroelectric Hafnium Aluminum Oxides for Thin‐Film Transistor Applications

Herein, the integration of dielectric and ferroelectric HfAlOx within a one‐transistor and one‐capacitor thin‐film transistor (TFT)‐based device structure is successfully demonstrated. It is found that the origin of gate stress for enhancing ferroelectricity is mainly ascribed to nonlinear thermal stress distribution, and the ferroelectricity of HfAlOx is further improved by nitrogen plasma treatment without sacrificing leakage current during post metal annealing. Notably, the p‐type SnO TFT in series with a ferroelectric HfAlOx capacitor with nitrogen plasma treatment achieves an almost 43% improvement on threshold voltage shift (5.45 V) at a low operating voltage of 3 V. Therefore, the ferroelectric nitridation treatment not only strengthens the ferroelectric domain switching of HfAlOx but also improves capacitance matching with the HfAlOx dielectric layer of p‐type SnO TFT.

Ferroelectric domain wall memory with embedded selector realized in LiNbO3 single crystals integrated on Si wafers.

Interfacial 'dead' layers between metals and ferroelectric thin films generally induce detrimental effects in nanocapacitors, yet their peculiar properties can prove advantageous in other electronic devices. Here, we show that dead layers with low Li concentration located at the surface of LiNbO3 ferroelectric materials can function as unipolar selectors. LiNbO3 mesa cells were etched from a single-crystal LiNbO3 substrate, and Pt metal contacts were deposited on their sides. Poling induced non-volatile switching of ferroelectric domains in the cell, and volatile switching in the domains in the interfacial (dead) layers, with the domain walls created within the substrate being electrically conductive. These features were also confirmed using single-crystal LiNbO3 thin films bonded to SiO2/Si wafers. The fabricated nanoscale mesa-structured memory cell with an embedded interfacial-layer selector shows a high on-to-off ratio (>106) and high switching endurance (~1010 cycles), showing potential for the fabrication of crossbar arrays of ferroelectric domain wall memories.

Ferroelastic‐Domain‐Assisted Mechanical Switching of Ferroelectric Domains in Pb(Zr,Ti)O3 Thin Films

AbstractA recent breakthrough in mechanical polarization switching provides a valuable handle to achieve nanoscale ferroelectric domain control. This flexoelectric switching is usually observed in ultrathin films (≈10 nm or less in thickness), where a large strain gradient is possible. However, from the point of view of device applications, it will be more attractive to achieve mechanical domain switching in thicker films. Here, it is experimentally demonstrated that by introducing ferroelastic a‐domains in PbZr0.1Ti0.9O3 films the potential barrier against 180° c‐domain switching can be greatly decreased, enabling mechanical ferroelectric domain switching in 50 nm thick films by applying a loading force from an atomic force microscope tip. Moreover, these a‐domains are stable in the c‐domain matrix without further mechanical stressing. This makes it possible to create nanoscale domain wall circuitry. These results shed light on the mechanism of domain switching in ferroelectric thin films and may facilitate the design of mechanically controlled novel ferroelectric devices.

Nanoscale imaging of ferroelectric domain and resistance switching in hybrid improper ferroelectric Ca3Ti2O7 thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions such as the rotation and tilting of Ti-O octahedral in Ca3Ti2O7 (CTO) family. In this work, we prepared different thickness CTO thin films on Pt substrates by pulsed laser deposition, and investigated their ferroelectric polarization reversal and the current transport properties by using the piezoresponse force microscopy and conducting atomic force microscopy, respectively. It is found that the CTO films exhibit clear ferroelectric domain switching and ferroelectric resistance switching behaviors, and the maximum resistive ratios of CTO film reaches ∼1750. These results demonstrate that hybrid improper ferroelectrics CTO films are promising materials for being employed in non-volatile memory and logic devices.

A semi-analytical scale bridging approach towards polycrystalline ferroelectrics with mutual nonlinear caloric-electromechanical couplings

Various caloric aspects of polycrystalline ferroelecrics are investigated, based on an efficient modelling approach. In this context, both linear reversible effects, leading to heating or cooling of the material, and nonlinear dissipation heating, being associated with ferroelectric domain switching, are considered. The semi-analytical approach focusses on the constitutive behavior of a polycrystalline representative volume element (RVE) under the impact of combined electromechanical loadings, incorporating mutual interactions of caloric and electromechanical fields within a framework of non-equilibrium thermodynamics. Temperature dependence of material coefficients is further taken into account. The so-called condensed method (CM) is employed to bridge the scales of domain wall motion within grains and grain interaction in the RVE. Results are compared to those of finite element simulations and experiments taken from literature, illustrating hysteresis loops and dissipation heating due to cyclic electrical loading. Additional findings illuminate the role of mechanical compression during electric cycling, behaviors of single vs. polycrystals, and separate the impacts of linear and nonlinear caloric effects.

Electric field dependence of ferroelectric stability in BiFeO3 thin films co-doped with Er and Mn

Bi0.9Er0.1Fe1−xMnxO3 (BEFMxO, x = 0.00–0.03) films are synthesized by a sol–gel technique. The BEFO film exhibits a conduction mechanism based on electron tunneling. The high applied electric field causes dissociation of the defect complex, and the resulting oxygen vacancies contribute to fake polarization. Consequently, the BEFO film has poor polarization stability at high applied electric fields. Coexistence of two phases (with space groups R3c:H and R3m:R) and reduced concentrations of oxygen vacancies and Fe2+ in BEFMxO are achieved by co-doping with Er and Mn. The presence of bulk-based conduction in the BEFMxO films then leads to ferroelectric domain switching contributing to the real polarization and to excellent ferroelectric stability. In addition, the BEFM0.02O film shows a typical symmetrical butterfly curve, the highest remnant polarization of ~109 μC/cm2, and the highest switching current of ~1.66 mA. It also has the smallest oxygen vacancy concentration and thus the smallest amount of defect complex, which means that there are fewer pinning effects on ferroelectric domains and therefore excellent ferroelectric stability. This excellent ferroelectric stability makes the BEFMxO films obtain good stability and reliability in the application of ferroelectric memory devices.