Magnetoelectric coupling of domains, domain walls and vortices in a multiferroic with independent magnetic and electric order

Magnetically induced ferroelectrics exhibit rigidly coupled magnetic and electric order. The ordering temperatures and spontaneous polarization of these multiferroics are notoriously low, however. Both properties can be much larger if magnetic and ferroelectric order occur independently, but the cost of this independence is that pronounced magnetoelectric interaction is no longer obvious. Using spatially resolved images of domains and density-functional theory, we show that in multiferroics with separately emerging magnetic and ferroelectric order, the microscopic magnetoelectric coupling can be intrinsically strong even though the macroscopic leading-order magnetoelectric effect is forbidden by symmetry. We show, taking hexagonal ErMnO3 as an example, that a strong bulk coupling between the ferroelectric and antiferromagnetic order is realized because the structural distortions that lead to the ferroelectric polarization also break the balance of the competing superexchange contributions. We observe the manifestation of this coupling in uncommon types of topological defects like magnetoelectric domain walls and vortex-like singularities.

STABILITY OF SKYRMIONS IN THE FRUSTRATED ANTIFERROMAGNETIC/FERROELECTRIC BILAYERS WITH THE TRIANGULAR LATTICE

The formation and conditions of stability of a skyrmions at the interface between a ferroelectric layer and antiferromagnetic layer with triangilar lattice and its phase transition are studied. All interactions between spins and polarizations are limited to nearest neighbors (NN). The antiferromagnetic exchange interaction among the spins inside antiferromagnetic layer will compete with the perpendicular interface interaction between adjacent layers. The ground state spin configuration at zero temperature is calculated by using the numerical high performance steepest descent method. The resulting configuration is non-collinear. Small values of external field yields small values of angles between spins in the plane so that the ground state configurations have antiferromagnetic and non collinear domains. We observe the creation of single spin vortices. We noted that for zero applied magnetic field the skyrmions in the antiferromagnetic/ferroelectric bilayers with triangular lattice can be created in the region of interface magnetoelectric interaction value between 0.85 and 1.95. The strong external magnetic field applied perpendicular to the interface with non-collinear Dzyaloshinskiy-Morya-like magnetoelectric interaction at the interface leads to remove the skyrmion phase and magnetic phase transitions. With increasing the interface magnetoelectric coupling, the skyrmion lattice disappear. We found the formation perfect skyrmion structure at non-zero external magnetic field and moderate values of magnetoelectric interaction. The skyrmions structure is stable in a large region of the interface magnetoelectric interaction between antiferromagnetic and ferroelectric films. The results of Monte Carlo simulations that we carried out confirm that observed skyrmions are stable up to a finite temperature.

FLUXGATE MAGNETOMETERS WITH A NEW EXCITATION METHOD BASED ON MAGNETOELECTRIC INTERACTION

The analysis of the functional peculiarities of the known flux gates has shown that the constructive and technological methods used to increase their operational characteristics within the framework of traditional so-lutions have basically exhausted themselves. The paper proposes a method for exciting flux gates based on a new physical principle relying on the effect of magnetoelectric interaction. The possibility of obtaining a magnetoelectric effect in local inhomogeneities of solid-state structures of ferrite elements of flux-gates when exposed to an alternating electric field is analyzed and substantiated. In this case, in the entire volume of the fer-romagnetic core at the same frequencies of electromechanical and magnetic resonances, a modulating physical process is excited in the form of a standing magnetoelectric wave. This creates a corresponding magnetic modu-lation structure in the entire volume of the ferromagnetic system of the flux gate. The considered design of a fluxgate modulator implements a new method of exciting flux gates, containing a ferromagnetic rod system in the form of two ferrite rod half-elements, located coaxially and joined together by a ferrite permanent magnet with magnetization along the axis of the rods. The authors have experimentally studies a new method for exci¬ting flux gates. The proposed method for exciting flux gates opens up wide opportunities for research in a new direction in science – spintronics, in particular, the applied use of magnetoelectric interaction, which can be effectively used to create various options for flux gates based on new physical principles of operation.

Spin dynamics, antiferrodistortion and magnetoelectric interaction in multiferroics. The case of BiFeO3

We present a theoretical study of the spin dynamics in perovskite-like multiferroics with homogeneous magnetic order in the presence of external magnetic and electric fields. A particular example of such material is BeFeO3 in which the spin cycloid can be suppressed by application of external magnetic field, doping or by epitaxial strain. Understanding the effect of the external electric field on the spin-wave spectrum of these systems is required for devices based on spin wave interference and other innovative advances of magnonics and spintronics. Thus, we propose a model for BiFeO3 in which the thermodynamic potential is expressed in terms of polarization \boldsymbol{P}, antiferrodistortion \boldsymbol{\Omega}, antiferromagnetic moment \boldsymbol{L} and magnetization \boldsymbol{M}. Based on this model, we derive the corresponding equations of motion and demonstrate the existence of electromagnons, that is, magnons that can be excited by electric fields. These excitations are closely related to the magnetoelectric effect and the dynamics of the antiferrodistortion \boldsymbol{\Omega}. Specifically, the influence of the external electric field on the magnon spectra is due to reorientation of both polarization \boldsymbol{P} and antiferrodistortion \boldsymbol{\Omega} under the influence of the electric field and is linked to emergence of a field-induced anisotropy.

Микромагнитные структуры, индуцированные неоднородным электрическим полем, в магнитодноосных пленках с флексомагнитоэлектрическим эффектом

The features of the manifestation of the flexo-magnetoelectric effect in magnetically uniaxial films under the local influence of an electric field on their surface are investigated. It is shown that with its increasing influence, there is a gradual transformation of the structure of the 180° domain boundary from the Bloch to the quasi-Bloch, and at a certain field value to the Néel boundary. It was revealed that in large fields it is possible to nucleate a 0° domain boundary with a non-Bloch structure, the laws of which have analogies with the formation of magnetic inhomogeneities on defects of the “potential well” type. The contribution of the partial parts of the inhomogeneous magnetoelectric interaction due to the presence of div m and curl m in the phenomena under consideration is also determined.