Ferroelectricity in spiral magnets

It was recently observed that the ferroelectrics showing the strongest sensitivity to an applied magnetic field are spiral magnets. We present a phenomenological theory of inhomogeneous ferroelectric magnets, which describes their thermodynamics and magnetic field behavior, e.g., dielectric susceptibility anomalies at magnetic transitions and sudden flops of electric polarization in an applied magnetic field. We show that electric polarization can also be induced at domain walls and that magnetic vortices carry electric charge.     

Lattice-scale domain wall dynamics in ferroelectrics

Ferroelectric domain walls are atomically thin, and consequently their dynamics are sensitive to the periodic potential of the underlying lattice. Despite their central role in domain dynamics, lattice-scale effects have never been directly observed. We investigate local domain dynamics in thin film ferroelectrics using atomic-force microscopy. Upon combined dc and ac electric driving, fluctuations in the local piezoresponse are observed. Fourier analysis of the fluctuations reveals the presence of narrow band and broad band noise, and Barkhausen jumps. The narrow band noise is attributed to dynamics associated with lattice-scale pinning and is reproduced by a simple physical model.     

Theory of periodic ferromagnetic multilayers

We investigate the statics and dynamics of periodic multilayers consisting of two ferromagnetic materials. Our theory is based on a general Ginzburg-Landau functional for inhomogeneous systems and the appropriate equations of motion. We compute the transition temperature of the composite material, the temperature dependence of the magnetization and the magnon dispersion relation. The statics apply also to other multilayer systems such as ferroelectrics.     

Polarization anomalies in a relaxor ferroelectric

With a strontium barium niobate crystal used as an example, it is experimentally demonstrated that relaxor ferroelectrics exhibit pronounced polarization anomalies, which manifest themselves in the difference between the trajectories of the dielectric hysteresis observed in several repeated cycles of a varying external electric field. These anomalies originate from the nonuniformity of the relaxor composition, local symmetry lowering, local internal electric field, and a wide distribution of potential barriers in energy for the polarized regions. The anomalies can be observed only in slowly varying (quasistatic) or constant electric fields.     

Solitons and critical breakup fields in lithium niobate type uniaxial ferroelectrics

Ferroelectric materials, such as lithium niobate, show interesting non-linear hysteresis behavior that can be explained by a dynamical system analysis. By using variational principle, a non-linear Klein-Gordon (K-G) equation is derived for lithium niobate type of uniaxial ferroelectrics involving various types of energy, which was not considered previously to construct the Hamiltonian. This leads to soliton solutions under different conditions of soliton velocity. The critical value of the (dimensional) effective electric field has been estimated to be 54–58 kV/cm for lithium niobate depending on the impurity content in these type inhomogeneous ferroelectrics. Beyond this critical field, there is no existence of solitons. This critical field is related to a break-up mechanism of Landau-Ginzburg two-well potential to a single well, as the driving force is increased.     

Kinetic approach for describing the fatigue effect in ferroelectrics

A new kinetic approach is proposed for explaining the fatigue effect in ferroelectrics. A self-consistent variation in the area and geometry of the switching region of a sample upon a cyclic switching accompanied by the formation and growth of kinetically frozen domains is considered. It is assumed that fatigue is due to self-organized formation of a spatially inhomogeneous internal bias field due to retardation of bulk screening of the depolarization field. Variations in the switching charge and in the amplitude of switching current, which are calculated with the help of computer simulation of domain kinetics upon cyclic switching, are in good agreement with experimental data obtained for thin lead zirconate-titanate (PZT) thin films.     

铁电存储技术

简要说明了铁电随机存储器的工作原理及特点，详细阐述了阻碍铁电存储技术发展的技术难点，重点讨论了铁电薄膜材料的疲劳机理，并对铁电存储器的发展作了展望。     

Relaxor ferroelectric behavior in high-energy electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers

This paper presents experimental results showing that in a certain composition range and under a proper electron-irradiation treatment, a normal ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer can be converted into a material exhibiting many typical features of relaxor ferroelectrics, suggesting that this is a new class of relaxor ferroelectric material. Furthermore, the irradiated copolymer can generate giant electrostriction (ƽ%) with a high elastic energy density. The X-ray diffraction results obtained from the irradiated copolymer under electric field, indicate that the observed polarization and strain responses are mainly due to the local phase transformation from a non-polar phase to a polar phase.     

A model of the size effect in a ferroelectric thin film under conditions of the phase transition

The size effects observed in thin films of displacive ferroelectrics are considered. A model of the dependence of the spontaneous polarization and phase transition temperature on the film thickness is proposed.     

Dynamics of the domain structure in a ferroelectric triglycinesulfate crystal

The dynamics of the domain structure in ferroelectrics of order-disorder type like triglycinesulfate is studied. The polarization properties of ferroelectrics with relaxation-type absorption are described in terms of the pseudospin formalism. A kinetic equation describing the polarization dynamics of ferroelectrics is derived using the Hamiltonian of the problem. The domain structure dynamics is obtained for different problem parameters and initial conditions. Analytical results are compared with experimental data.     

Temperature dependence of flexoelectricity in BaTiO3 and SrTiO3 perovskite nanostructures

Piezoelectricity is a property of non-centrosymmetric crystals. In most typically used ferroelectrics, this property is lost as the temperature is increased beyond the Curie point thus strongly reducing the availability of efficient materials that can be used for high temperature energy harvesting. Flexoelectricity, as can be shown from simple symmetry arguments, is a universal and linear electromechanical coupling that dictates the development of polarization upon application of inhomogeneous strains. The implications of this phenomenon become amplified at the nanoscale. In this communication, we develop a molecular dynamics approach predicated on a specially tailored interatomic force-field to extract the temperature dependence of flexoelectricity. Surprisingly, we find that it, at least for Barium Titanate and Strontium Titanate nano structures, increases with temperature. Apart from cataloging this interesting observation for the future use in high temperature energy harvesting, we also examine the physical mechanisms that lead to the observed temperature dependence.     

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T_c and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above T_c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.     

Temperature-dependent Raman scattering study of the defect pyrochlores RbNbWO6 and CsTaWO6

Lattice dynamics calculations and temperature-dependent Raman scattering experiments were performed on RbNbWO(6) and CsTaWO(6) pyrochlore oxides. The observed bands were assigned to the respective motions of atoms in the unit cell. The spectra showed the presence of additional Raman bands not allowed for by the [Formula: see text] cubic structure. We have shown that these bands appear due to both substitutional disorder in the 16c sites and displacive disorder of the A ions. Raman studies also revealed the presence of an additional 80?cm(-1) band at room temperature for RbNbWO(6), not observed for CsTaWO(6). The presence of this band has been attributed to off-center displacement of the Nb and W ions due to structural phase transition into a tetragonal ferroelectric phase. The temperature evolution of the 80?cm(-1) band intensity revealed that it disappeared at a much higher temperature (about 650?K) than the reported phase transition temperature (about 360?K). This behavior is reminiscent of chemically disordered perovskite ferroelectrics, including relaxor ferroelectrics, and was attributed to the presence of small polar regions with local tetragonal distortion embedded in the paraelectric matrix of the [Formula: see text] structure.     

Investigation into the domain structure dynamics in ferroelectrics with an incommensurate phase

The domain structure dynamics in ferroelectrics with an incommensurate phase is studied. Sodium nitrite is considered to be a prominent representative of the above ferroelectrics. The polarization properties of these ferroelectrics are described using pseudospin formalism. A kinetic equation for describing polarization of ferroelectrics is derived using the Hamiltonian. This equation is numerically solved simultaneously with the sound-vibration equation. The results are analyzed for various crystal parameters and initial conditions. The crystal domain structure is shown to be localized for reasonably long times. This circumstance makes it possible to conclude that these domain-structure states are long-lived states of a soliton type. The importance of finding and describing these states is emphasized. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 66–70, August, 2006.     

Unusual domain evolution in semiconducting ferroelectrics: A phase field study

The effect of electrical conductivity on the domain evolution of semiconducting ferroelectrics is investigated using a phase field model which includes the drift of space charges. Phase field simulations show that the tail-to-tail 90° charged domain wall appears during the domain formation in the semiconducting ferroelectrics at zero field, which is prohibited in common insulating ferroelectrics. Due to the screening of polarization charges, the domain switching takes place through the motion of head-to-head 180° charged domain wall in the semiconducting single-domain ferroelectrics subjected to an electric field. Comparing to the insulating ferroelectrics, the semiconducting ferroelectrics have a lower speed of domain evolution due to the decrease of mobility of charged domain walls. The response of semiconducting ferroelectrics to a mechanical load is also found different from that of insulating ferroelectrics.     

局部不均匀性对时空系统振荡频率的影响

以复金兹堡一朗道方程为模型,利用数值实验方法观察了时空系统中螺旋波斑图的演化行为,发现在局域非均匀参数条件下,系统的螺旋波可以受到该杂质区域的影响而演化成为稳定的靶波.研究表明,内传的螺旋波转换为稳定靶波的必要条件是非杂质系统和杂质系统的振荡频率相等且小于系统的固有频率,并在参数一频率空间形成一个特殊的V形区域,进一步分析表明,该V形区域具有左右对称、两侧靶波传播方向相反以及随杂质区域参数α_2的增大而向参数β_2减小方向平移等性质.     

Potts-Ising model for simulation of polarization switching in polycrystalline ferroelectrics

This paper proposes a scheme based on the Potts and Ising models for simulating polarization switching of polycrystalline ferroelectrics using the Monte Carlo method. The polycrystalline texture with different average grain size is produced from the Potts model. Then Ising model is implemented in the polycrystalline texture to produce the domain pattern and hysteresis loop. The domain patterns and hysteresis loops have been obtained for polycrystalline texture with different average grain size. From the results of domain pattern evolution process under an applied electric field using this scheme, an extended domain, which covers more than one grain with polarization aligned roughly in the same direction, has been observed during the polarization reversal. This scheme can well reproduce the basic properties of polycrystalline ferroelectrics and is a valuable tool for exploring the physical properties of polycrystalline ferroelectrics.     

Surface-stimulated nucleation of reverse domains in ferroelectrics

We present a model for reverse domain nucleation in ferroelectrics, which takes into account ferroelectric-electrode coupling in both homogeneous and random cases. The model provides a solution to the coercivity paradox--i.e., the large discrepancy between the observed and predicted coercive fields, common to many systems. We demonstrate the possibility of not thermally activated nucleation of reverse domains. We find that small inhomogeneities in the ferroelectric-electrode interface may lead to an exponentially wide spectrum of waiting times for switching. The model predicts that switching is facilitated near morphotropic phase boundaries in perovskite-type ferroelectrics.     

Influence of crystallite-boundary potential barriers on the thermopower and peltier effect in polycrystalline ferroelectric semiconductors

The influence of local charged states at crystallite boundaries on the thermoelectric effects in polycrystalline ferroelectrics is considered. It is shown that the differential thermopower and the Peltier coefficient depend on the height of the crystallite-boundary potential barriers. The possible manifestation of an anomalous behavior of the thermopower in the vicinity of the Curie ferroelectric point is demonstrated.