Symmetry theory of the flexomagnetoelectric effect in the magnetic domain walls

A local flexomagnetoelectric (A.P. Pyatakov, A.K. Zvezdin, 2009) effect in the magnetic domain walls (DWs) of the cubic hexoctahedral crystal has been investigated on the basis of a symmetry analysis. The strong connection between magnetic symmetry of the DW and the type of the distribution of the electric polarization was shown. Results were systemized in the scope of the DW chirality. It was shown, that new type of the local flexomagnetoelectric coupling corresponds to the presence of the coupled electric charge in the DW. It was found that all time-noninvariant chiral DWs have identical type of spatial distribution of the magnetization and polarization. There are coincidence between the symmetry predictions and results obtaining from the known term of the flexomagnetoelectric coupling for transverse polarization components.     

Flexomagnetoelectric interaction in cubic,tetragonal and orthorhombic crystals

The phenomenological theory of the flexomagnetoelectric coupling in crystals of the cubic, tetragonal and orthorhombic crystal systems has been suggested. Secondary role of the crystal structure chirality was shown. Oppositely, significant role of the crystallographic point group type (symmetric, alternating, dihedral or cyclic) in the flexomagnetoelectric coupling has been derived. It was shown, that conceptually new features of the flexomagnetoelectric effects are expected in the crystals of the cyclic groups (crystal classes 4/m, 4? and 4). Proposed verification of the theory is investigation of the domain wall bend details (changes of the effect symmetry). Special case of such verification near the compensation point is suggested. First-principles mechanisms of the flexomagnetoelectric interaction were discussed.     

Effect of magnetoelectrical interactions on the multiferroic domain walls

The antiferromagnetic domain structure of a multiferroic has been investigated in the presence of a ferroelectric domain structure. It has been demonstrated that an inhomogeneous magnetoelectric (flexomagnetoelectric) interaction leads to pinning of antiferromagnetic domain walls at the walls of the ferroelectric domains and to a change in the structure of antiferromagnetic domain walls.     

Micromagnetic structure of the domain wall with Bloch lines in an electric field

The micromagnetic structure of the domain wall (DW) with periodically distributed horizontal Bloch lines in a ferromagnetic film in an external electric field has been studied. The effect of the electric field on the internal DW micromagnetic structure is caused by inhomogeneous magnetoelectric coupling. Possible scenarios of the DW internal structure transformations implemented with varying the electric fields strength have been analyzed in detail. For each scenario, static characteristics of the system, such as the energy, DW profile, DW effective thickness, and electric polarization have been calculated.     

On the effect of inhomogeneous magnetoelectric (flexomagnetoelectric) interaction on the spectrum and properties of magnons in multiferroics

The features of the magnon spectrum in easy-plane multiferroics (such as BiFeO₃), which allow inhomogeneous magnetoelectric (flexomagnetoelectric) interaction P[(L?)L ? LdivL], where L and P are the antiferromagnetic moment and electric polarization, respectively, have been theoretically analyzed. It has been shown that, in contrast to the magnon spectrum of a usual easy-plane antiferromagnet, a multiferroic with this magnetic structure is characterized by, first, the interaction between magnons of both branches propagating along the weak ferromagnetic moment and the appearance of a minimum (or zero) of the frequency of one of the branches, which reflects the instability of the system with respect to the transition to an inhomogeneous state with increasing flexomagnetoelectric interaction and, second, the nonequivalence (nonreciprocity) of the propagation of spin waves along and against the antiferromagnetism vector, which coincides with the toroidal moment in this system.     

Polarization reversal in multiferroic TbMnO3 with a rotating magnetic field direction

For the memory application of magnetoelectric multiferroics, not only bistability (i.e., ferroelectricity) but also the switching of the polarization direction with some noneverlasting stimulus is necessary. Here, we report a novel method for the electric polarization reversal in TbMnO3 without the application of an electric field or heat. The direction of the magnetic-field-induced polarization along the a axis (Pa) is memorized even in the zero field where Pa is absent. The polarization direction can be reversed by rotating the magnetic-field direction in the ab plane.     

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe_(12)O_(22)

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe_(12)O_(22),which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K.Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field,resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe_(12)O_(22)exhibits an anomalous magnetoelectric memory effect:the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K.We ascribe this memory effect to the pinning of multiferroic domain walls,where spin chirality and structure are preserved even in the nonpolar collinear spin state.     

Mikromagnetisch singuläre punkte in bubbles

The magnetization pattern of a vertical Bloch line in a bubble wall containing a singular point in its center is investigated by a variational method. Introducing a hypothetical intermediate structure, the energy of such a configuration can be separated into two parts: one which describes a local embedding energy for the singular point, and one which may be derived from the structure of the domain wall only. The results indicate that above a critical film thickness Bloch lines containing a singular point represent, in agreement with a prediction by Slonczewski, the energetically most favourable configuration.     

On the free energy of the flexomagnetoelectric interactions

It was shown that free energy density of the local flexomagnetoelectric effect is determined by the four phenomenological constants in case of the cubic (hexoctahedral) crystal. The well-known single-constant Lifshitz invariant term is correct only when fixed electric polarization induces the inhomogeneity of the magnetization. Proposed phenomenological theory was applied to the magnetic domain walls. The domain wall structure has been investigated in details. The four-constant phenomenological theory conforms to the symmetry based predictions (Bar'yakhtar et al., 1984, [12]). The proposed experimental verification of the four-constant flexomagnetoelectric phenomenology is a detection of the shift of the Néel domain walls under the strong homogeneous electric field.     

Effects of Bloch lines and Bloch points on resonances of magnetic bubbles

Resonant spectra of 180° domain-wall oscillations in isolated magnetic bubbles are obtained with swept rf sinusoidal excitation and magnetooptic detection. The resonant frequencies are found to depend on the domain wall state. A quantitative micromagnetic theory involving contributions to the wall mass from already-present vertical Bloch lines with and without Bloch points explains the resonant frequencies, including their field dependence, and relates them to the wall states. It is found that for small in-plane fields, Bloch lines provide most of the effective wall mass. Moreover, if a Bloch point is present on a Bloch line, it tends to decrease the Bloch-line mass contribution.     

基于磁电耦合效应的基本电路元件和非易失性存储器

磁电耦合效应是指磁场控制电极化或者电场控制磁性的物理现象,它们为开发新型电子器件提供了额外的物理状态自由度,具有巨大的应用潜力.磁电耦合系数作为磁电耦合材料的重要参量,体现了材料磁化和电极化的耦合性能,其随外加物理场的变化可以表现出非线性回滞行为,具备作为非易失存储的物理状态特征.本文讨论了基于磁电耦合效应如何建立起电荷-磁通之间的直接关联,继而实现了第四种基本电路元件并构建了完整的电路元件关系图.在此基础上,研究了多铁性异质结中的非线性磁电耦合效应,并利用其独特的电荷-磁通关联特性,开发了基于磁电耦合系数的电写-磁读型非易失性信息存储、逻辑计算与类神经突触记忆等一系列新型信息功能器件.     

Polarization singularities in optical lattices

Polarization singularities are shown to be unavoidable features of three-dimensional optical lattices. These singularities take the form of lines of circular polarization, C lines, and lines of linear polarization, L lines. The polarization figures surrounding a C line (L line) rotate about the line with winding number +/-1/2 (+/-1). C and L lines permeate the lattice, meander throughout the unit cell, and form closed loops. Surprisingly, every point in a linearly polarized optical lattice is found to be a singularity about which the surrounding polarization vectors rotate with an integer winding number.     

Electric polarization of magnetic textures: New horizons of micromagnetism

A common scenario of magnetoelectric coupling in multiferroics is the electric polarization induced by spatially modulated spin structures. It is shown in this paper that the same mechanism works in magnetic dielectrics with inhomogeneous magnetization distribution: the domain walls and magnetic vortexes can be the sources of electric polarization. The electric field driven magnetic domain wall motion is observed in iron garnet films. The electric field induced nucleation of vortex state of magnetic nanodots is theoretically predicted and numerically simulated. From the practical point of view the electric field control of micromagnetic structures suggests a low-power approach for spintronics and magnonics.     

铁电/铁磁1-3型结构复合材料磁电性能分析

磁致伸缩和压电复合材料通过机械力作用可获得较大的磁电效应，由Terfenol-D粉末与树脂黏接剂构成压磁相，和压电陶瓷PZT黏结在一起，形成1-3型柱状阵列结构，可获得很大的磁电转换系数.采用有限元方法，对此结构的复合材料进行静态分析.复合材料的介电常数和磁电系数的计算结果和实验数值一致，得到样品中应力、应变和电极化分布情况及其关系，并给出进一步提高磁电转换系数的途径，该种复合方式有望成为一种新型高性能的磁电结构. 关键词： 复合材料 磁电效应 有限元     

Ferroelectric control of the magnetocrystalline anisotropy of the Fe/BaTiO(3)(001) interface

Density-functional calculations are employed to investigate the effect of ferroelectric polarization of BaTiO(3) on the magnetocrystalline anisotropy of the Fe /BaTiO(3)(001) interface. It is found that the interface magnetocrystalline anisotropy energy changes from 1.33 to 1.02 erg cm (-2) when the ferroelectric polarization is reversed. This strong magnetoelectric coupling is explained in terms of the changing population of the Fe 3d orbitals at the Fe/BaTiO(3) interface driven by polarization reversal. Our results indicate that the electronically assisted magnetoelectric effects at the ferromagnetic/ferroelectric interfaces may be a viable alternative to the strain mediated coupling in related heterostructures and the electric field-induced effects on the interface magnetic anisotropy in ferromagnet/dielectric structures.     

Magnetoelectric effects in multiferroic Y-type hexaferrites Ba_(0.3)Sr_(1.7)Co_xMg_(2-x)Fe_(12)O_(22)

Y-type hexaferrites with tunable conical magnetic structures are promising single-phase multiferroics that exhibit large magnetoelectric effects. We have investigated the influence of Co substitution on the magnetoelectric properties in the Y-type hexaferrites Ba(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22)(x = 0.0, 0.4, 1.0, 1.6). The spin-induced electric polarization can be reversed by applying a low magnetic field for all the samples. The magnetoelectric phase diagrams of BaBa(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22) are obtained based on the measurements of magnetic field dependence of dielectric constant at selected temperatures. It is found that the substitution of Co ions can preserve the ferroelectric phase up to a higher temperature, and thus is beneficial for achieving single-phase multiferroics at room temperature.     

Phonon mechanism of the antiferromagnetic photogalvanic effect

The phonon mechanism of generation of photogalvanic current in centroantisymmetric tetragonal antiferromagnets with magnetic symmetry excluding the occurrence of a toroidal momentum is considered. It is demonstrated that photocurrent can be caused by scattering of charge carriers by phonons with a polarization other than longitudinal. The polarization of the phonons involved in scattering is determined by the long-range part of the electron-phonon interaction induced by polarization of the lattice due to the magnetoelectric effect. Conditions are given for the observation of photogalvanic current in easy-axis antiferromagnets.     

On the possible internal structure of the ferroelectric Ising lines in BaTiO3

ABSTRACT

This article is devoted to a detailed analysis of the inner structure of the Ising line, a topological defect separating two parts of a ferroelectric Bloch domain wall with opposite helicity, in the framework of the phenomenological Ginzburg–Landau–Devonshire model. Results performed for Ising lines in a ?211?-oriented 180-degree domain wall of rhombohedral BaTiO₃ suggest remarkably strong dependence of the polarization profiles on the form of gradient terms. Profiles of the skyrmion density, vorticity and divergence of the polarization field were calculated numerically in the few-nm vicinity of the Ising line defect.     

Increase in the electric polarization in a multiferroic induced by the flexoelectric effect

It has been shown that the electric polarization in multiferroic crystals exhibiting the magnetoelectric effect can be increased in the presence of the flexoelectric effect. The polarization in manganite-based perovskite-like multiferroics can be increased if the long axes of the rhombohedra formed upon the rhombohedral distortion of MnO₆ octahedra can undergo fan spatial rotations, leading to the flexoelectric effect.     

Néel lines in ferrimagnetic garnet epilayers with orthorhombic anisotropy and canted magnetization

Néel lines in (TbY)₃(FeAl)₅O₁₂ epitaxial layers with orthorhombic anisotropy and canted magnetization are optically observed by the presence of a kink of the wall plane. The intrinsically asymmetrical character of Bloch walls is demonstrated by wall widening experiments. It follows that wall kinking may either be a consequence of the anisotropy specific to the samples, and/or be the result of magnetostatic interactions. A relaxation type numerical computation of the Néel line configuration indicates that anisotropy is the main source of wall kinking, demagnetizing field effects contributing to an amplification of the kink. The structure of those Néel lines is shown to bear two characters, that of a half-circular line characteristic of samples with in-plane anisotropy and that of a splay line, well known in bubble materials. Further, the charge distribution appears basically dipolar and lines contract when the saturation magnetization increases. They are therefore magnetostatistically analogous to vertical Néel (most generally called Bloch) lines in garnet epilayers supporting bubbles.