Polarization Switching Mechanism Research Articles

Ferrimagnetism and Ferroelectricity in Cr-Substituted GaFeO3 Epitaxial Films

GaFeO3-type iron oxides are promising multiferroic materials due to the coexistence of a large spontaneous magnetization and polarization near room temperature. However, magnetic substitution, which is a general method to control multiferroic properties, is difficult due to instability of the substituted GaFeO3. In this study, Ga0.5Cr0.5FeO3 epitaxial thin films are successfully fabricated through epitaxial stabilization. These films exhibit in-plane ferrimagnetism and out-of-plane ferroelectricity simultaneously. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements of the Ga0.5Cr0.5FeO3 film reveal that the oxidation states of the Fe and Cr ions are trivalent. In addition, some Fe ions are located at tetrahedral Ga1 sites. Compared to the GaFeO3 film, the Ga0.5Cr0.5FeO3 film shows a higher magnetic phase transition temperature (240 K), weaker saturation magnetization at 5 K, and a unique temperature dependence of the magnetization behavior. The effects of Cr substitution on the magnetic properties are strongly affected by the sites of the Fe3+ (3d5) and Cr3+ (3d3) ions. Furthermore, room-temperature ferroelectricity in the GaFeO3 and Ga0.5Cr0.5FeO3 films was demonstrated. Interestingly, the change in the ferroelectric parameters via Cr substitution is very small, which disagrees with the previously proposed polarization switching mechanism. Our findings are key to understanding the genuine polarization switching mechanism of the multiferroic GaFeO3 system.

Polarization switching mechanism in surface-emitting semiconductor lasers

Within the scope of the earlier proposed approach to the description of polarization dependences in VCSEL based on analysis of the experimental and theoretical data, an explicit expression for the function describing the relationship between the amplification anisotropy and the current density is suggested. The conducted numerical calculations support the peculiarities of the polarization switching (PS) process and enable one to explain the effect of the PS point anomalous shift when the current variation rate is high. A special attention is given to a fairly clear physical interpretation of the formation of polarization radiation in VCSEL.

Local polarization switching in stressed ferroelectric polymers

Ferroelectric polymers are used in flexible organic ferroelectric memories, ferroelectric polarization enhanced organic solar cells, and organic multiferroics. Therefore, understanding their polarization switching mechanism under bending is important for the operation of such devices. Here, we study locally by piezoresponse force microscopy (PFM) polarization switching in bent thin films of the ferroelectric polymer poly(vinylidene fluoride-ran-trifluoroethylene). In bent samples, higher probability of domain nucleation, faster domain wall propagation, and lower coercive field are consistently observed by PFM. We ascribe these observations to a decrease of the domain wall pinning energy, resulting from the mechanical energy stored in the sample due to bending in the presence of the compression gradient generated below the PFM tip.

Intrinsic polarization switching mechanisms inBiFeO3

A first-principles-based effective Hamiltonian technique is used to investigate the polarization switching mechanisms in two polymorphic phases of ${\mathrm{BiFeO}}_{3}$ having no defects. The switching mechanism is homogeneous for any switching field in the rhombohedral phase, while in the supertetragonal phase it changes from the classical nucleation and domain-wall motion to nucleation-limited switching with virtually no propagation, and then to homogeneous switching with increasing electric field. The first two inhomogeneous switching mechanisms of the supertetragonal phase of ${\mathrm{BiFeO}}_{3}$ are thus intrinsic in nature, and can be well described by the classical and nucleation-limited switching models, respectively. The reason behind their absence in the rhombohedral phase is also indicated. Moreover, the field-induced changes of switching mechanism within the supertetragonal phase are further elucidated from an energetic point of view.

Polarization Switching Induced by Slowing the Dynamic Swinglike Motion in a Flexible Organic Dielectric

Molecular motions with large amplitude in close-packed crystals accompany large distortions of the molecular configuration, which generally generate orientational structural transitions between diverse states and enable the tuning of their bulk physical properties. We present a flexible organic dielectric, di-n-butylammonium chlorodifluoroacetate (1), which exhibits a reversible temperature-induced spontaneous polarization switching at 243 K (Tc). Ferroelectric hysteresis loop measurements and second harmonic generation experiments reveal its excellent polarization switching capacity with spontaneous polarization of 3.9 μC cm–2. Temperature-dependent solid-state nuclear magnetic resonance measurements clearly elucidate the dynamical mechanism of polarization switching of 1. Above Tc, an active swinglike motion in long-chain di-n-butylammonium (DBA) cation is confirmed, resulting in complete obliteration of the dipole moments. When the temperature decreases below Tc, the swinglike motions are frozen and th...

Optoelectronic Composite Logic Gates Controlled by the Logic Operation in a VCSEL with External Optical Injection

Based on the polarization switching mechanism in an optically injected vertical-cavity surface-emitting laser (VCSEL), and relying on the new electro-optic modulation theory, we propose a novel approach to implement optoelectronic logic gates. The two linear-polarization (LP) lights from the output of the laser are considered as two logic outputs. Under the electro-optic modulation, one of the logic outputs is the NOT operation with the other one. With the same logic input signal, we perform various digital signal processing (AND, OR, XNOR, NAND, NOR and XOR) in the optical domain, controlling the logic operations of the applied electric field between the two logic input signals. On this basis, the logic operation of half-adder is further implemented.

Directional Electron Transfer in Crystals of [CrCo] Dinuclear Complexes Achieved by Chirality-Assisted Preparative Method.

The polarization switching mechanism is used in various devices such as pyroelectric sensors and memory devices. The change in polarization mostly occurs by ion displacement. The development of materials whose polarization switches via electron transfer in order to enhance operation speed is a challenge. We devised a synthetic and crystal engineering strategy that enables the selective synthesis of a [CrCo] heterometallic dinuclear complex with a polar crystal structure, wherein polarization changes stem from intramolecular charge transfer between Co and the ligand. Polarization can be modulated both by visible-light irradiation and temperature change. The introduction of chiral ligands was paramount to the successful polarization switching in the valence tautomeric compound. Mixing Cr and Co complexes with enantiopure chiral ligands resulted in the selective formation of only pseudosymmetric [CrCo] heterometallic complexes. Furthermore, the left-handed chiral ligands preferentially interacted with their right-handed counterparts, enabling molecules to form a polar crystal structure.

Mechanism of polarization switching in wurtzite-structured zinc oxide thin films

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P63mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P63/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (Ec) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in lowering Ec during polarization, with a 5% biaxial expansion resulting in a decrease of Ec to 3.5 MV/cm. Comparison with calculated values for conventional ferroelectric materials suggests that the ferroelectric polarization switching of wurtzite-structured ZnO may be achievable by preparing high-quality ZnO thin films with suitable strain levels and low defect concentrations.

Induced instability in local structure and ferroelectric polarization of rare earth modified BZT relaxor ceramics

Chemically induced instability in local symmetry of rare earth modified barium zirconate titanate relaxor ceramics Ba1-xLn2x/3Zr0.3Ti0.7O3 (Ln = Sc, La, Sm, Nd, Eu and Gd), where x = 0.02–0.10 and its effect on structural, microstructural, interband transition along with ferroelectric switching polarization are studied experimentally. Rare earth substitution induced A-site vacancies and distortion in three dimensional cation-oxygen networks leads to random local strain with short range ordering. In addition, these cations control the formation of grain, grain boundaries and band gap values. Substitution of rare earth ions induced intermediate energy levels in between conduction and valence band and guided the emission spectra towards blue-green region. At room temperature polarization hysteresis loop and normalized capacitance versus electric field loop indicated the existence of relaxation and polarization switching mechanism in these materials. However, at room temperature feeble polarization switching is observed in case of scandium, europium and gadolinium based BZT compositions indicated a weak relaxor phase whereas in lanthanum, samarium, neodymium substituted BZT materials shows a dominating relaxor phases induced by static and dynamic of nano size polar regions separated by non-polar region.

Mechanisms of polarization switching in graphene oxides and poly(vinylidene fluoride)–graphene oxide films

Polarization switching in graphene oxides (GOs) and poly(vinylidene fluoride) (PVDF)–GO nanocomposite is investigated by piezoelectric force microscopy (PFM). The dynamical switching results reveal that GO films exhibit ferroelectric and piezoelectric properties with two-dimensional characteristics. Abnormal polarization switching is observed in PVDF–GO films, which is promising for electronic applications.

Controllable optoelectric composite logic gates based on the polarization switching in an optically injected VCSEL.

Based on the polarization switching mechanism in an optically injected vertical cavity surface emitting laser (VCSEL), and the new electro-optic modulation theory, we propose a novel approach to implement optoelectric logic gates. Here, the two linearly polarized lights from the output of the laser are considered as two logic outputs. Under the electro-optic modulation, one of the logic outputs is the NOT operation with the other one. With the same logic input signal, we perform various digital signal processing (AND, OR, XNOR, NAND, NOR and XOR) in the optical domain, controlling the logic operation of the applied electric field between the two logic input signals. On this basis, the logic operation of half-adder is further implemented.

Analysis of The Computational and Experimental Studies of the Polarization Switching in the PVDF and P(VDF-TrFE) Ferroelectric Films at the Nanoscale

In this paper, molecular models are used to investigate and analyze the polarization switching in the polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) Langmuir-Blodgett (LB) nanofilms, in comparison with the experimental data at the nanoscale. Quantum-mechanical calculations and modeling, as well as molecular dynamics (MD) simulations based on semi-empirical quantum-chemical methods (such as PM3), show that the energy of the studied PVDF and P (VDF-TrFE) molecular structures, and their polarization switching proceed by the intrinsic homogeneous switching mechanism in the framework of the phenomenological theory of Landau-Ginzburg-Devonshire (LGD) in the linear approximation at low values of the electric field. The magnitude of the resulting critical coercive field is within the EC ~ 0.5 ... 2.5 GV/m, which is consistent with experimental data. It is also found that the uniform polarization switching mechanism of the polymer chains PVDF and P (VDF-TrFE) is due to the quantum properties of the molecular orbitals of the electron subsystem. This is clearly seen in both the polarization hysteresis loops, and the total energy changes. In this case, the turnover chain time, obtained by molecular dynamics within semi-empirical quantum-chemical PM3 approach in a limited Hartree-Fock approximation, when approaching this critical point, increases sharply, tending to infinity, which corresponds to the theory of LGD. Otherwise, at the high values of the applied electric field the polarization switching correspond to the extrinsic domain mechanism in the frame of the microscopic Kolmogorov–Avrami–Ishibashi (KAI) theory, describing bulk ferroelectric crystals and thick films. The performed analysis of computational and experimental data allows us to estimate the critical sizes of the possible transition region approximately on the order of 10 nm between intrinsic homogeneous and extrinsic domain switching mechanisms.

Компьютерное моделирование и молекулярная динамика переключения поляризации в сегнетоэлектрических пленках ПВДФ и П(ВДФ-ТрФЭ) на наноуровне

In this paper, molecular models are used to investigate and analyze the structure and polarization of polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) Langmuir-Blodgett (LB) nanofilms, depending on the structure and composition of the monomers of their polymer and copolymer chains. Quantum-mechanical calculations and modeling, as well as molecular dynamics (MD) simulations based on semi-empirical quantum-chemical methods (such as PM3), show that the energy of the studied PVDF and P (VDF-TrFE) molecular structures, and their polarization switching proceed by homogeneous switching mechanism in the framework of the phenomenological theory of Landau-Ginzburg-Devonshire (LGD) in the linear approximation of low values of the electric field. The magnitude of the resulting critical coercive field is within the EC ~ 0.5 ... 2.0 GV/m, which is consistent with experimental data. It is also found that the uniform polarization switching mechanism of the polymer chains PVDF and P (VDF-TrFE) is due to the quantum properties of the molecular orbitals of the electron subsystem: the applied electric field induces a gradual shift of the electron "clouds" density (electron polarizability), which in turn causes a gradual shift of the nuclear cores, in accordance with the principle of minimum total energy of the system, and this leads eventually, when it reaches a critical point (bifurcation) - to overturn of the entire chain and a sharp decrease in the total energy of the total system to its energetically more favorable state. This is clearly seen in both the polarization hysteresis loops, and the total energy changes. In this case, the turnover chain time, obtained by molecular dynamics within semi-empirical quantum-chemical PM3 approach in a limited Hartree-Fock approximation, when approaching this critical point, increases sharply, tending to infinity, which corresponds to the theory of LGD.

Symmetry breaking and electrical frustration during tip-induced polarization switching in the nonpolar cut of lithium niobate single crystals.

Polarization switching in ferroelectric materials is governed by a delicate interplay between bulk polarization dynamics and screening processes at surfaces and domain walls. Here we explore the mechanism of tip-induced polarization switching at nonpolar cuts of uniaxial ferroelectrics. In this case, the in-plane component of the polarization vector switches, allowing for detailed observations of the resultant domain morphologies. We observe a surprising variability of resultant domain morphologies stemming from a fundamental instability of the formed charged domain wall and associated electric frustration. In particular, we demonstrate that controlling the vertical tip position allows the polarity of the switching to be controlled. This represents a very unusual form of symmetry breaking where mechanical motion in the vertical direction controls the lateral domain growth. The implication of these studies for ferroelectric devices and domain wall electronics are discussed.

Enhanced dielectric, pyroelectric and ferroelectric properties of Mn-doped 0.15Pb(In1/2Nb1/2)O3–0.55Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 single crystals

Mn-doped 0.15Pb(In1/2Nb1/2)O3–0.55Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 single crystals were grown by modified Bridgman technique. High dielectric peak temperature of 161°C, low dielectric loss of 0.03% and enhanced pyroelectric coefficient of 10.1×10−4C/m2K are observed. Effects of oxygen vacancies related to annealing atmosphere have been investigated to explain enhanced dielectric and pyroelectric properties. Detectivity figure of merit of 34.1×10−5Pa−1/2 is achieved for oxygen-annealed crystals, which is the highest value among reported PIMNT crystals. Two different linear relationships between the coercive field and frequency are observed to obey Merz’s law, which reveals the domain and polarization switching mechanism.

Statistical electric field and switching time distributions in PZT 1Nb2Sr ceramics: Crystal- and microstructure effects

Dispersive polarization response of ferroelectric PZT ceramics is analyzed assuming the inhomogeneous field mechanism of polarization switching. In terms of this model, the local polarization switching proceeds according to the Kolmogorov-Avrami-Ishibashi scenario with the switching time determined by the local electric field. As a result, the total polarization reversal is dominated by the statistical distribution of the local field magnitudes. Microscopic parameters of this model (the high-field switching time and the activation field) as well as the statistical field and consequent switching time distributions due to disorder at a mesoscopic scale can be directly determined from a set of experiments measuring the time dependence of the total polarization switching, when applying electric fields of different magnitudes. PZT 1Nb2Sr ceramics with Zr/Ti ratios 51.5/48.5, 52.25/47.75, and 60/40 with four different grain sizes each were analyzed following this approach. Pronounced differences of field and switching time distributions were found depending on the Zr/Ti ratios. Varying grain size also affects polarization reversal parameters, but in another way. The field distributions remain almost constant with grain size whereas switching times and activation field tend to decrease with increasing grain size. The quantitative changes of the latter parameters with grain size are very different depending on composition. The origin of the effects on the field and switching time distributions are related to differences in structural and microstructural characteristics of the materials and are discussed with respect to the hysteresis loops observed under bipolar electrical cycling.

Effect of dislocation walls on the polarization switching of a ferroelectric single crystal

Phase field simulations were conducted to study the influence of dislocation walls on the domain configuration and polarization switching behavior of a ferroelectric single crystal. The simulation results show that the domain configuration and polarization switching behavior depend highly on the dislocation spacing in the dislocation wall. The ferroelectric properties can be greatly improved by reducing the coercive field and meanwhile enhancing the remanent polarization if an appropriate density of dislocations is introduced at high temperature. The phase field simulations also put insights into the mechanism of polarization switching.

Domain evolution processes during poling of a near-morphotropic Pb(Zr, Ti)O3 ceramic

Domain wall motion during the poling of near-morphotropic Pb(Zr,Ti)O3 PZT was observed using Piezoresponse Force Microscopy (PFM). Poling was conducted on bulk polycrystalline PZT in a series of steps, interrupted by vertical PFM scans, which were used to identify the domain evolution processes. The mechanisms of evolution in complex domain patterns such as herringbone and checkerboard structures are revealed. Of interest, in the case of a herringbone pattern consisting of two sets of lamellae angled to each other, one set of lamellae expands and is observed to overwrite the other, transforming the herringbone structure into a single lamination. Also, lengthening without broadening, and simultaneous lengthening and broadening of lamellar domain bands in checkerboard structures are observed. The observations show that 180° and non-180° domain switching can occur simultaneously in complex domain patterns. Methods are developed for identifying the polarization directions of the individual domains in near-morphotropic PZT. The methods combine a knowledge of the compatible domain configurations with crystallographic data from electron backscatter diffraction and PFM data. The resulting map of polarization directions enables clear identification of the polarization switching mechanisms.

On ferroelectric domain polarization switching mechanism subject to an external electric field by simulations with the phase-field method

The ferroelectric domain formation (FDF) and polarization switching (FDPS) subjected to an external electric field are simulated using the phase-field (PF) method, and the FDPS mechanism under different external electric fields is discussed. The results show that the FDF is a process of nucleation and growth in ferroelectric without applying any external stress. Four kinds of parallelogram shaped ferroelectric domains are formed at the steady state, in which the 180° anti-phase domains regularly align along the −45° direction and the 90° anti-phase domains regularly distribute like a stepladder. Steady electric fields can rotate domain polarization by 90° and 180°, and force the orientation-favorite domains and the average polarization to grow into larger ones. The greater the steady electric field, the larger the average polarization at the steady state. In ferroelectrics subject to an alternating electric field, domain polarization switches to cause a hysteresis loop and an associated butterfly loop with the alternating electric field. The coercive field and remnant field are enhanced with the increase of the electric field frequency or strength, or with the decrease of temperature.

Ferroelectric switching in Bi4Ti3O12nanorods

We report the piezoelectric and ferroelectric properties of individual one-dimensional objects made of Bi(4)Ti(3)O(12) (BiT). The nanorods and nanowires investigated in this study were fabricated by a two-step process: 1) preparation of reactive templates using hydrothermal-like synthesis and colloidal chemistry and 2) transformation of the reactive templates in Bi(4)Ti(3)O(12) by solid-state reaction, overcoming the morphological instability problem of 1-D templates. Using piezoresponse force microscopy (PFM) with both out-of-plane and in-plane detection capability, we show that both types of objects exhibit strong piezoelectric activity and good switching ferroelectric behavior. Analysis of the PFM hysteresis loops obtained revealed that the coercive voltage of the in-plane PFM signal can be either equal to or different from that of the out-of-plane response. We associate these situations with two types of polarization switching mechanisms: direct 180° switching, and via rotation of polarization, resulting from the independent switching of the components along the a- and ccrystallographic axes. In a few instances, we observe a negative piezoelectric coefficient, which we explain by the specific shape of the piezoelectric surface of Bi(4)Ti(3)O(12).