The converse magnetoelectric coupling in asymmetric granule/matrix composite film with Ni/PZT component

In this work, a type of asymmetric granule/matrix composite film is designed, where the Ni granule is dispersed in PZT matrix, meanwhile the top and bottom electrode is constituted by Au and SRO respectively. Predicted through the electrostatic screening model and mean field approximation, considerable electrostatic charge is induced on Ni granule surface by ferroelectric PZT polarization. Predicted through the spin splitting model and spherical shell approximation, both the magnetization and magnetic anisotropy of Ni granule are modulated by ferroelectric PZT polarization. As the volume fraction of Ni granule is increased, the electric modulation of magnetization and magnetic anisotropy is reduced and enhanced respectively. As the dimension of granule/matrix composite is varied, such modulation is retained. Due to the large area–volume ratio of nano-granule, this work benefits to realize the converse magnetoelectric coupling in nanoscale.     

电场中三层铁电超晶格的物理性质研究

基于微分算符技术的相关有效场理论研究用伊辛模型描述的铁电三层超晶格的物理性质。讨论了电场对电极矩、电极化率、热电系数等物理性质的影响。     

Interaction of nuclear spins with a time-dependent electric field in ferroelectric KNbO3

The effect of transitions in the nuclear spin system on the motion of the electric polarization in ferroelectric potassium niobate is studied theoretically. The equations of motion of the polarization are derived with inclusion of the interaction of the polarization with the nuclear spins and employed to calculate the contribution from nuclear transitions to the dielectric susceptibility of the crystal. This contribution has resonance maxima at the frequencies of nuclear transitions and is proportional to the square of the background dielectric susceptibility of the crystal. For the ferroelectric, this contribution is far larger than for earlier studied nonferroelectric dielectrics. Numerical estimates show that, in potassium niobate, the contribution from the nuclei to the dielectric susceptibility is of the same order of magnitude as that to the magnetic susceptibility.     

Effect of electrode on dielectric susceptibility and pyroelectric properties of a ferroelectric thin film capacitor using landau-khalatnikov theory

The influence of electrodes on the dielectric susceptibility and pyroelectric properties of a ferroelectric thin film with surface transition layers has been investigated within the framework of Landau-Khalatnikov dynamic theory. The contribution of the electrodes is reflected by the depolarization field in the free-energy function. The large electrode effect implies the strong depolarization field in ferroelectric thin films. The results show that the electrode materials can greatly impact the dynamic dielectric and pyroelectric properties of a ferroelectric thin film.     

Study on the dynamic critical behavior of a ferroelectric heterostructure

A Landau–Devonshire theory added in Landau–Khalatnikov dynamic equation has been used firstly to explore the dynamic critical behavior of a ferroelectric heterostructure composed of two different ferroelectric films. Two identical surface transition layers within each film are assumed, and an antiferroelectric interfacial coupling between two materials is considered. One interfacial parameter β is introduced to describe the differences of physical characteristics between two constituent films, which can reflect more realistic dynamic mechanism. It is found that the ferroelectric heterostructure may exhibit multi-loop hysteresis loop and four peaks of dielectric susceptibility if the appropriate values of parameter β, antiferroelectric interfacial coupling and size of the system are selected. We obtain the critical behavior of the appearance in multi-loop hysteresis loops and four peaks of dielectric susceptibility by equilibrium action of parameter β and antiferroelectric interfacial coupling, which will provide theoretical guiding for designing the multi-state memory and miniaturized device in future.     

The polarization and dielectric susceptibility of a ferroelectric bilayer film

A ferroelectric bilayer film consisting of two different ferroelectric constituent films with a transition layer within each constituent film and interfacial coupling between two materials is investigated based on the Ginzburg-Landau-Devonshire phenomenological theory. A parameter α, which describing the differences between physical properties of two constituent films is first introduced in our paper, and reflects a more realistic situation compared to the previous treatments assuming the same two constituent films. We study the polarization and dielectric susceptibility properties of the ferroelectric bilayer film with two different constituent films. The results present some interesting phenomena due to the introduction of parameter α.     

Effect of electromechanical boundary conditions on the properties of epitaxial ferroelectric thin films

The effects of internal stresses and depolarization fields on the properties of epitaxial ferroelectric perovskite thin films are discussed by employing the dynamic Ginzburg-Landau equation (DGLE). The numerical solution for BaTiO₃ film shows that internal stress and the depolarization field have the most effects on ferroelectric properties such as polarization, Curie temperature and susceptibility. With the increase of the thickness of the film, the polarization of epitaxial ferroelectric thin film is enhanced rapidly under high internal compressively stress. With the thickness exceeding the critical thickness for dislocation formation, the polarization increases slowly and even weakens due to relaxed internal stresses and a weak electrical boundary condition. This indicates that the effects of mechanical and electrical boundary conditions both diminish for ferroelectric thick films. Consequently, our thermodynamic method is a full scale model that can predict the properties of ferroelectric perovskite films in a wide range of film thickness.     

Size Effects of the Critical Temperature in Ferroelectric Thin Films

The size effects of the critical behaviors for the systems of interacting spins are discussed extensively in literature. In this paper, the finite-size dependence of the critical temperature and susceptibility of the ferroelectric thin film are investigated numerically based on the four-state Potts model with the nearest-neighbor interactions between the dipole moments. The four orientations of the domains exist in the ferroelectric film and the movement of the domain walls determines the polarization switching process besides the boundary conditions of the film. The critical exponents are obtained and our investigations show that the boundary conditions play the important roles for the ferroelectric properties of the thin films and the critical behavior of the thin films strongly depends on the feature of the surface.     

Size Effects of the Critical Temperature in Ferroelectric Thin Films

The size effects of the critical behaviors for the systems of interacting spins are discussed extensively in literature. In this paper, the finite-size dependence of the critical temperature and susceptibility of the ferroelectric thin film are investigated numerically based on the four-state Potts model with the nearest-neighbor interactions between the dipole moments. The four orientations of the domains exist in the ferroelectric film and the movement of the domain walls determines the polarization switching process besides the boundary conditions of the film. The critical exponents are obtained and our investigations show that the boundary conditions play the important roles for the ferroelectric properties of the thin films and the critical behavior of the thin films strongly depends on the feature of the surface.     

Theory of the reciprocal susceptibility of the trigonal boracites and the experiment scheme

Landau theory is used to describe the ferroelectric trigonal phase of Boracites. The temperature dependence of the principal reciprocal susceptibility is obtained. The calculated result shows that at the Curie temperature T_c, the spontaneous polarization jumps. The component of the principal reciprocal susceptibility perpendicular to polarization jumps but the component parallel to polarization does not. A possible experimental scheme for the measurement of the principal reciprocal susceptibility components is presented.     

Dielectric anomalies in ferroelectric nanostructures

First-principles-based methods are used to determine the external dielectric susceptibility (i.e., the polarization response to the external electric field) and the internal susceptibility (i.e., the polarization response to the average internal field) in ferroelectric dots, wires, and films, as a function of the electrical boundary conditions. While the external susceptibility is obviously positive, we find that the internal one is negative over a wide range of boundary conditions for all kinds of nanostructures. A Landau-type phenomenological model provides a rationale for all of our findings.     

Influence of hydrostatic pressure on the polarization dynamics of order-disorder ferroelectrics: AgNa(NO2)2

The characteristic ferroelectric dispersion of the complex dielectric constant of the ferroelectric AgNa(NO₂)₂ is measured as function of temperature and hydrostatic pressure. For the first time the temperature and pressure dependences of the static order parameter susceptibility and the order parameter relaxation time could be reproducibly determined. The results are interpreted in terms of a kinetic Ising model, whose parameters show a characteristic pressure dependence. The results are discussed on the basis of thermodynamic relations and related to the dynamical behaviour of other order-disorder ferroelectrics.     

Influence of lateral size on dielectric properties of ferroelectric thin films with structure transition zones

By taking into account structural transition zones near the lateral and thickness direction edges, this paper uses a modified transverse Ising model to study dielectric properties of a finite size ferroelectric thin film in the framework of the mean-field approximation. The results indicate that the influence of the lateral size on the dielectric susceptibility cannot be neglected and lateral structural transition zones could be a crucial factor that improves the mean susceptibility of the fixed size film.     

Generation of second optical harmonic in a macroscopic array of parallel nanowires

The quadratic optical susceptibility tensor for a macroscopically ordered array of parallel nanowires is determined. An experimental investigation of the polarization properties of signals of the second harmonic from ferroelectric nanowires synthesized in channels of chrysotile asbestos demonstrate that its results can be useful in structural studies.     

Dynamical dielectric susceptibility of ferroelectric thin films and multilayers

The thickness dependence of the real and imaginary parts of the dynamical dielectric susceptibility is investigated phenomenologically for a multilayer structure consisting of alternating ferroelectric and paraelectric layers. It is shown that the frequency dependence of the linear dielectric response can be closely approximated by that of a damped harmonic oscillator, with the static susceptibility, relaxation time, and soft-mode frequency depending on the layer thickness and temperature. When the layer thickness and temperature are equal to their critical values corresponding to the onset of a size-driven ferroelectric phase transition, the static susceptibility and the relaxation time become anomalously large and then decrease with further increasing layer thickness. A spectrum of natural polarization oscillations is predicted to exist with thickness-dependent frequencies. This spectrum includes a soft-mode frequency which vanishes at the critical thickness and at the critical temperature. The frequency spectrum lies below the soft-mode frequency of a thick film (in which the gradient of polarization is negligible). The calculations are compared with experimentally measured dispersion of the dielectric response of a PbTiO₃-Pb_0.72La_0.28TiO₃ multilayer structure. The agreement between the theory and experiment is found to be good.     

Landau theory of ferroelectric transition in long cylindrical nanoparticles

Using a phenomenological Landau theory, the size dependence on ferroelectric transition for free-standing long cylindrical nanoparticles is discussed. We derive the size dependence of the transition temperature, polarization profile as well as the static susceptibility. The transition temperature vanishes below a critical size where the static susceptibility shows divergence obeying Curie-Weiss law. In order to make this result compared with experiments, the average polarization and the susceptibility is computed with a Gaussian particle size distribution. The average polarization in such a case shows smearing with respect to the particle size whereas the divergence in the susceptibilty gets rounded. This might correspond to a size dependent soft mode which can be observed in Raman measurement.     

Ferrielectric dielectric behaviour and the nature of phase transition of ammonium sulphate

Phenomenological treatments of ferrielectrics are made in terms of two non-equivalent sublattice polarizations in the paraelectric phase. Unlike the Curie-Weiss behaviour of usual proper ferro- and antiferro-electrics, the dielectric susceptibility does not depend linearly on temperature. Particularly in the case of a first order phase transition, the Curie-Weiss constant seems to be unusually small. As an example, the dielectric properties of (NH₄)₂SO₄, which was known to be a peculiar ferroelectric, is discussed from the ferrielectric point of view. The true Curie-Weiss constant is of a comparable order of magnitude to that of the order-disorder ferroelectric.     

Lattice mediated interactions and ferroelectric anomalies in the crystal (CH3NH3)5Bi2Cl11 (PMACB)

Abstract

A two-sublattice compressible pseudospin model has been constructed to describe the behaviour of the spontaneous polarisation of PMACB in the temperature region comprising the ferroelectric phase transition (T=307K) and the isomorphous anomaly (T= 180 K). A strong coupling of pseudospins with a “pancake” strain ?(2ε₃₃ ? ε₁₁ - ε₂₂) /3 resulting from the comparison of the model with the experimental spontaneous polarisation agrees with the existing data on the dielectric susceptibility, and on the specific heat. The ferroelectric phase transition then turns out to be close to a tricritical point. The numerical values of the parameters of the model show how the ferroelastic phase transition (T=260K) can occur in this crystal without any discernible effect on the specific heat and on the dielectric susceptibility.     

The effect of a strain induced field on the dielectric property of a ferroelectric bilayer system

The Transverse Ising Model (TIM) based on the effective-field theory has been developed to study the physical properties of the ferroelectric bilayer system BaTiO₃/SrTiO₃ (BTO/STO), based on the differential operator technique. The effect of strain on the interfacial layers between two different slabs (A and B) can be described by the effective built-in field E₂. The ferroelectric behavior of a bilayer system is strongly influenced by strain and associated with slab thickness. The phase transition temperature shifts toward a higher value on increasing the slab thickness. The susceptibility strongly depends on both the strength of strain and the slab thickness. The height of the peak from the plot of susceptibility against temperature decreases on increasing the slab thickness. The pyroelectric coefficient changes into a round peak at the transition temperature that is different from the sharp peak in the absence of external and strain-induced fields.