Ferroelectric Subsystem Research Articles

Magnetoelectric fractals, Magnetoelectric parametric resonance and Hopf bifurcation

In the present work, we study the dynamics of the magnetic nanodisc coupled through a magnetoelectric coupling to a ferroelectric crystal. The model of our interest is nonlinear, and we explore the problem under different limits of weak and strong non-linearity. By applying two electric fields with different frequencies, we control the form of the confinement potential of a ferroelectric subsystem and realize different types of dynamics. We proved that the system is more sensitive to magnetoelectric coupling in the case of double-well potential. In particular, in the case of strong non-linearity, arbitrary small values of magnetoelectric coupling lead to chaotic dynamics. In essence, magnetoelectric coupling plays a role akin to the small perturbations destroying invariant tori according to the KAM theorem. We showed that bifurcations in the system are of Hopf’s type. We observed the formation of magnetoelectric fractals in the system. In the limit of weak non-linearity, we studied a problem of parametric nonlinear resonance and enhancement of magnetic oscillations through magnetoelectric coupling.

Anharmonic phonon properties in Eu0.5Ba0.5TiO3

Abstract Phonon properties have been studied using reduced sound velocity of Eu0.5Ba0.5TiO3 (EBTO). To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced sound velocity of multiferroic EBTO exhibits a kink at TN = 1.9 K. This anomalously reduced sound velocity can be interpreted as an effect of vanishing magnetic ordering above TN. What’s more, the ferroelectric subsystem cannot be influenced by the magnetic subsystem above TN for TN ≪TC in the EBTO. It was found that the reduced sound velocity decreases as T increases near ferroelectric transition TC. That is to say, the sound velocity softens near ferroelectric transition TC. It is also noteworthy that the reduced sound velocity softens when the RE (the coupling between the ferroelectric pseudo-spins and phonons), V(3) and |V(4)| (the third- and fourth-order atomic force constants of the anharmonic phonons, respectively) increase. These conclusions are all in good accordance with the experimental data and theoretical results.

Anomalous sound velocity in multiferroic BiFeO3

The sound velocity in multiferroic BiFeO3 (BFO) is studied with using Green's function technology on the basis of the magnetoelectric coupling, the spin-phonon interaction and the anharmonic phonon–phonon interaction. The Heisenberg-like model is employed to describe the magnetic subsystem, and the transverse Ising model is used to explain the ferroelectric subsystem. The reduced velocity is obtained in the limit of zero wave vectors. It is shown that the reduced velocity of sound in BiFeO3 exhibits a kink at the magnetic phase transition temperature TN. This anomaly in reduced velocity can be explained as an influence of vanishing magnetic ordering above TN and the ferroelectric subsystem can not be influenced by the magnetic subsystem above TN due to TN≪TC in the BFO. It is shown that the influence of the RM is only below TN in the phase where ferroelectric and magnetic properties exist together, whereas the RE influences the properties of the reduced velocity in the whole temperature region (T<TC). Above TC only the anharmonic phonon–phonon interactions remain. It can also be seen that the reduced velocities decrease with increasing temperature. The achieved conclusion is in accordance with the experimental results.

Effect of ferromagnetic and ferroelectric properties on the sound velocity anomaly in BiFeO3

The effect of ferromagnetic and ferroelectric properties on the sound velocity in multiferroic BiFeO3 (BFO) is studied with using Green’s function method on the basis of the magnetoelectric coupling, the spin-phonon interaction and the anharmonic phonon-phonon interaction. The Heisenberg-like model is employed to describe the magnetic subsystem, and the transverse Ising model is used to explain the ferroelectric subsystem. The reduced velocity is obtained in the limit of zero wave vectors. It is shown that the reduced velocity of sound of BiFeO3 exhibits a cusp-like at the magnetic phase transition temperature TN. This anomaly in reduced velocity can be explained as an influence of vanishing magnetic ordering above TN and the ferroelectric subsystem can not be influenced by the magnetic subsystem above TN due to TN ≪ TC in the BFO. The reduced velocity and TN increase with increasing of the J1, J2 and g, whereas reduced velocity decreases with I increases. The achieved conclusion is in accordance with the experimental results.

Electrooptic properties of one-dimensional photonic crystals based on organic ferroelectric and dye

We present the results of our study of the electrooptic properties of a one-dimensional photonic crystal created from alternating thin layers of two organic compounds, a ferroelectric and a dye. We show that the ferroelectric subsystem of molecular layers in such a heterostructure can be polarized by an external electric field, leading to a linear electrooptic effect and bistability of the electrooptic response in the spectral region of the photonic stop band. On the other hand, a quasi-linear electrooptic effect attributable to the quadratic Stark effect and the built-in spatially periodic static electric field is observed in the spectral region of the absorption of dye molecules. The emergence of this built-in field is explained in terms of the continuity of the electric displacement vector in a spatially periodic dielectric heterostructure with a macroscopically polarized subsystem of ferroelectric layers.

Microscopic approach to the magnetoelectric effect in R Mn2 O5

The multiferroic behavior of rare-earth manganites is studied within a microscopic model including a symmetry-allowed magnetoelectric coupling between polarization and magnetization. The magnetic subsystem is described by a frustrated Heisenberg spin model, whereas the ferroelectric subsystem is characterized by an Ising model in a transverse field. Using Green's function method we find analytically the temperature and wave vector dependent elementary excitation of the magnetoelectric system, the polarization and the magnetization for different magnetoelectric coupling strengths. The system undergoes a magnetic transition at and a further reduction of the temperature leads to a ferroelectric transition at depending on the coupling strength. That coupling is also manifested as a kink in the magnetization and the elementary excitation at . Due to the magnetoelectric coupling the excitation energy exhibits a gap at zero wave vector which increases with increasing coupling. We show that the macroscopic magnetization can be slightly enhanced by an external electric field nearby . An external magnetic field leads to an increase of the polarization.

Excitation spectrum of multiferroics at finite temperatures

A systematic microscopic theory of the magnetoelectric (ME) effect in multiferroic materials with well-separated phase-transition temperatures is presented. Whereas the ferroelectric subsystem is described by an Ising model in a transverse field, the magnetic one is characterized by the Heisenberg model with Dzyaloshinski-Moriya interaction (DMI). The symmetry-allowed quartic coupling between both subsystems, and the application of a Green's function technique in a dynamical mean-field approximation, exhibit the calculation of the elementary excitations analytically, which are mutually influenced by the respective other subsystem. The magnetic excitation is a Goldstone mode, while the ferroelectric dispersion relation shows a soft-mode-like behavior. We find the macroscopic polarization and the transverse magnetization in a broad temperature interval up to the corresponding phase-transition temperatures (type-I multiferroics). Due to the DMI, the system offers different spiral structures, which are incorporated into the model by using a transformation of the underlying spin operators into a representation without a fixed quantization axis. The polarization increases at the magnetic phase-transition temperature, and is also enhanced by increasing the ME coupling strength as well as the DMI. We demonstrate likewise the variation of the spin-wave dispersion relation with the ME coupling strength and the DMI. As a consequence, the macroscopic magnetization is enhanced with increasing coupling.

Ferroelectric-ferromagnetic correlations in BiMnO3 perovskite within Landau theory: comparison with experiment

We discuss a simple phenomenological Landau theory of phase transitions with two coupled single-component order parameters and compare the results with available experimental data. The model corresponds to the case of a ferroic system, in which ferromagnetic and ferroelectric transitions originally occur at temperatures $T_M$ and $T_f$, respectively. For $T_f>T_M$ the magnetoelectric coupling strongly renormalizes the magnetic transition temperature, $T_M\to T_{RM}$ (with $T_{RM}>>T_M$), as well as generates an additional anomaly in ferroelectric subsystem $T_{RM}$. Full susceptibility tensor has also been determined. The concept of \textit{Arrot plot} is replaced by the \textit{Arrot planes} which appear when both types of order coexist. The results are in good overall agreement with experimental data for the ferroelectromagnetic BiMnO$_3$. We also estimate the contribution of Gaussian fluctuations of both order parameters, that lead to corrections to the mean-field specific heat. Those corrections are still insufficient even though other quantities agree quite well with experiment. We calculate the temperature dependence of the coherence length for both types of order as well.

Study of magnetoelectric coupling effect in the hexagonal ferroelectromagnet

Soft-mode theory based on Diffour model for ferroelectric subsystem, and mean-field theory as well as Heisenberg model for antiferromagnetic subsystem are utilized to investigate the magnetoelectric coupling effect in a hexagonal ferroelectromagnet, in which the ferroelectric and antiferromagnetic orders spontaneously coexist below a certain temperature. An anomaly of polarization at the magnetic transition temperature is ascribed to the effect of magnetoelectric coupling. The magnetic excitation has also been studied by spin-wave theory over the three-sublattice model. It is demonstrated that role of magnetoelectric coupling effect is not only related with the strength of magnetoelectric coupling but also special spin lattice structure. Our results show the magnetic specific heat induced by magnetic excitation experiences a suppression by the magnetoelectric coupling.

Resonance excitation of spin waves in ferroelectric/ferromagnets by oscillations of ferroelectric domain walls in an alternating electric field

Abstract The problem of the excitation of spin waves by oscillations of ferroelectric domain walls (in an alternating electric field) in ferroelectric/ferromagnets is considered. First the situation is studied where the equilibrium state of the magnetic subsystem is homogeneous. It is shown that due to the influence of the magnetic subsystem on the dynamics of ferroelectric domain walls in the ferroelectric subsystem the amplitude of the generated spin waves can be considerably enhanced at some definite frequency. The second situation considered is one where the excitation of spin waves takes place in ferroelectric/ ferromagnets containing both ferroelectric and ferromagnetic domain walls which form a bound state in which they oscillate relative to one another with frequency ω0. It is shown that the existence of a bound state leads to the fact that the frequency dependence of ferroelectric domain wall oscillations in an alternating electric field has resonance shape. If the frequency ω0 lies in the spin w...

Dynamics of solitons in segnetomagnets with easy plane magnetic anisotropy

Nonlinear waves in systems with ferroelectric and ferromagnetic properties (segnetomagnets) are studied. The magnetic sub-system is assumed to have easy plane anisotropy. If the ferroelectric subsystem is homogeneous, then a magnetic soliton is described by the double sG equation. The interaction of ferroelectric and magnetic solitons is investigated. The parameters of coupled states of ferroelectric and magnetic solitons are calculated.

Thermodynamic formalism of phase transitions in solid solutions

A thermodynamic theory of ferroelectric and ferroelectric-semiconductor solid solutions is presented with account of the effect of hydrostatic pressure in the cases of sharp as well as diffuse transitions. A number of analytical relations are obtained which allow to estimate the effect of admixture concentration and hydrostatic pressure on dielectric permeability, transition temperature, Curie-Weiss constant, heat capacity, thermal expansion coefficient and other properties of crystals with a ferroelectric subsystem. The theoretical results are compared with the experimental data.