180-degree Domain Wall Research Articles

Nonuniformity of magnetization processes of a cobalt-based amorphous alloys in the as-quenched state

The magnetization processes nonuniformity of an amorphous cobalt-based soft-magnetic alloy AMAG-172 (Co-Ni-Fe-Cr-Mn-Si-B) in the as-quenched state were studied. Studies have shown that the main reason of the magnetic characteristics and magnetization processes nonuniformity is the nonuniformity of internal stresses due to the alloy manufacture method. An indirect characteristic of internal stresses is the domains volume with orthogonal magnetization. The observed bimodal field dependence of the magnetic permeability and the field shift of the hysteresis loops in the region of 180-degree domain walls displacement are a consequence of the nonuniformity of the alloy over its thickness. The field shift and the asymmetry of hysteresis loops are explained in terms of the formation of a unidirectional exchange anisotropy at the interface between the ferromagnetic and antiferromagnetic phases of the planar magnetization components oriented along the ribbon axis. The field shift disappearance and the asymmetry of the hysteresis loops along the width of the tape can be interpreted as a decrease in the thickness of the CoO oxide antiferromagnetic layer due to the difference in the concentrations and depth of oxygen and hydrogen atoms penetration induced into the surface of the tape during the manufacture of the tape as a result of interaction with atmospheric vapor.

A puzzling insensitivity of magnon spin diffusion to the presence of 180-degree domain walls

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl0.5Fe1.5O4 (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180° domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion – nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180° domain wall.

The Features of the Flexomagnetoelectric Effect in an External Magnetic Field

The effect of a magnetic field on the behavior of 180-degree domain walls in a uniaxial ferromag-netic film with an inhomogeneous magnetoelectric interaction has been studied. It has been shown that, depending on the magnitude and direction of the field, it is possible to enhance or weaken the flexomagne-toelectric effect in the sample. In addition, it was found that in the reverse field the switching of the interac-tion between the electric field source and the domain wall from attraction to repulsion is possible.

Microstructural model of the behavior of a ferroalloy with shape memory in a magnetic field

In the article, within the framework of the theory of micromagnetism, a magneto-elastic microstructural model of the behavior of a ferromagnetic material with the shape memory (Heusler alloy) in a magnetic field is constructed. The dynamics of the magnetic process is described by the Landau-Lifshitz-Gilbert equation. Using the Galerkin procedure, variational equations corresponding to the differential relations of the magnetic problem are written out. A martensitic structure of the type “herringbone” (a twinned variant of martensite) with magnetic domains located at an angle of 180 ° is considered. 90-degree magnetic domain walls are the boundaries of the twins. The finite element method models the formation of these walls and the distribution of the magnetization vector in them. The evolution of this magnetic structure is investigated, namely the movement and interaction of 180-degree magnetic domain walls when an external magnetic field is applied in various directions. The problems of the formation of twins from the martensite variants (the twinning problem) and their disappearance (the detwinning problem) in an external magnetic field are considered. On the plane separating the two variants of martensite, the Hadamard compatibility condition (or twinning equation) is written out. The solution of this equation is given and the processes, that this equation describes, are considered in detail. The detwinning condition for a ferroalloy with the shape memory in a magnetic field is proposed and the processes related to the reorientation (detwinning) of the martensitic variants forming a twin are discussed. The problems considered earlier without taking into account the detwinning process are supplemented by this process, the magnetization curves are constructed and the components of the strain tensor are determined. The results obtained are in good agreement with the known experimental data.

Nonlinear Dynamics of a 180-Degree Domain Wall in an Antiferromagnet

The nonlinear dynamics of a 180-degree domain wall in a weak ferromagnet is studied. It is shown that different modes of magnetization reversal of a weak ferromagnet are possible, depending on a number of conditions (magnetic field, magnetic anisotropy constants, and the initial amplitude of nucleus).

Nontrivial magnetic field related phenomena in the singlelayer graphene on ferroelectric substrate (Review Article)

The review is focused on our predictions of nontrivial physical phenomena taking place in the nanostructure single-layer graphene on ferroelectric substrate, which are related with magnetic field. In particular we predicted that 180-degree domain walls in a strained ferroelectric film can induce p-n junctions in a graphene channel and lead to the unusual temperature and gate voltage dependences of the perpendicular modes v┴ of the integer quantum Hall effect. The non-integer numbers and their irregular sequence principally differ from the conventional sequence v┴ = 3/2, 5/3, … The unusual v┴-numbers originate from significantly different numbers of the edge modes, v1 and v2, corresponding to different concentration of carriers in the left (n1) and right (n2) ferroelectric domains of p-n junction boundary. The difference between n1 and n2 disappears with the vanishing of the film spontaneous polarization in a paraelectric phase, which can be varied in a wide temperature range by an appropriate choice of misfit strain originated from the film-substrate lattice mismatch. Next we studied the electric conductivity of the system ferromagnetic dielectric-graphene channel-ferroelectric substrate. The magnetic dielectric locally transforms the band spectrum of graphene by inducing an energy gap in it and making it spin-asymmetric with respect to the free electrons. It was demonstrated, that if the Fermi level in the graphene channel belongs to energy intervals, where the graphene band spectrum, modified by EuO, becomes sharply spin-asymmetric, such a device can be an ideal non-volatile spin filter. The practical application of the system under consideration would be restricted by a Curie temperature of a ferromagnet. Controlling of the Fermi level (e.g., by temperature that changes ferroelectric polarization) can convert a spin filter to a spin valve.

Extrinsic permittivity in domain engineered rhombohedral BaTiO3 monocrystal

Arrangement of 90-degree domain walls in the domain structure of the ⟨111⟩pc-oriented single crystal plates of BaTiO3 was intentionally modified by combinations of in-plane and out-of-plane steady state electric poling at ambient temperature. It is shown that the annealed, in-plane, and out-of-plane poled samples exhibit markedly different dielectric properties at low temperatures. The difference in low-temperature permittivity behavior is enhanced by the out-of-plane fast-decayed bipolar electric pulse treatment, but the efficiency of this enhancement is strongly dependent on the initial domain structure. The absence of a clear signature of the pulse treatment in optical observations of the domain structure suggests that it might involve mostly changes in the 180-degree domain wall subsystem.

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

ABSTRACTThis 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.

Identification of defect distribution at ferroelectric domain walls from evolution of nonlinear dielectric response during the aging process

Motion of ferroelectric domain walls greatly contributes to the macroscopic dielectric and piezoelectric response of ferroelectric materials. The domain wall motion through the ferroelectric material is however hindered by pinning on crystal defects which substantially reduces these contributions. Here, using thermodynamic models based on the Landau-Ginzburg-Devonshire theory, we find a relation between microscopic reversible motion of non-ferroelastic 180-degree domain walls interacting with a periodic array of pinning centers and the nonlinear macroscopic permittivity. We show, that the reversible motion of domain walls can be split into two basic modes. First, the bending of a domain wall between pinning centers, and, second, the uniform movement of domain wall plane. We show that their respective contributions may change when the distribution of pinning centers is rearranged during the material aging. We demonstrate that it is possible to indicate which mechanism of the domain wall motion is affected during material aging. This allows to judge whether the defects only homogeneously accumulate at domain walls or prefer to align in certain directions inside the domain wall plane. We suggest that this information can be obtained using simple macroscopic dielectric measurements and a proper analysis of the nonlinear response. Our results may therefore serve as a simple and useful tool to obtain details on domain wall pinning in an aging process.

Связанная динамика 180-градусных доменных стенок одинаковой полярности в двухосных ферромагнетиках

The bound dynamics of two same polarity 180° domain walls with the opposite topological charges in a biaxial ferromagnet with a large quality factor is theoretically investigated. The domain walls moving towards each other under the influence of an external magnetic field with regard to dissipation are considered. The case of a large size ferromagnet with small external magnetic fields and attenuations has been studied. An inhomogeneous exchange interaction, uniaxial and rhombic anisotropy are taken into account in the magnetic energy density. A one-dimensional in coordinates soliton model in which the interacting domain walls are regarded as a two-soliton solution is used. A system of ordinary differential equations describing the evolution of the soliton solution parameters under the influence of the magnetic field and attenuation is obtained. The numerical study of differential equations shows a strong dependence of walls interaction in the magnetic field on the attenuation. It was found that on the interaction of two 180-degree walls of the same polarity at a low attenuation and specific direction of the external magnetic field there occurs an annihilation of the walls, accompanied by multiple scattering process and the walls polarity change. With the increase of the attenuation parameter the annihilation of soliton-antisoliton pair happens as a result of a single scattering process. In the case of the field opposite direction same polarity 180-degree domain walls while moving come close at the minimum distance, and then the repulsion and subsequent divergence from each other occur.

The Structure of a 180-Degree Domain Wall near the Surface of Ferroics

We consider the influence of a built-in surface field on the 180-degree domain wall profile in primary ferroics within the Landau–Ginsburg–Devonshire phenomenological approach. The built-in surface field can be caused by various physical mechanisms for ferroelastic, ferroelectric, and antiferromagnetic media. For instance, it can be a component of the intrinsic surface stress tensor for ferroelastics with order parameter as a component of the strain tensor. We predict the effect of domain wall bending near the surface caused by the built-in field and derived the corresponding approximate analytical expressions.

Induced motion of domain walls in multiferroics with quadratic interaction

We theoretically study the dynamics of 180-degree domain wall of the ab-type in magnetic materials with quadratic magnetoelectric interaction in external alternating magnetic and electric fields. The features of the oscillatory and translational motions of the domain walls and stripe structures depending on the parameters of external fields and characteristics of the multiferroics are discussed. The possibility of the domain walls drift in a purely electric field is established.

Structural phase transitions and electronic phenomena at 180-degree domain walls in rhombohedral BaTiO3

Using Landau-Ginzburg-Devonshire theory we describe 180-degree domain wall structure, intrinsic energy and carrier accumulation in rhombohedral phase of BaTiO3 as a function of the wall orientation and flexoelectric coupling strength. Two types of domain wall structures (phases of the wall) exist depending on the wall orientation. The low-energy 'achiral' phase occurs in the vicinity of the {110} wall orientation and has odd polarization profile invariant with respect to inversion about the wall center. The second 'chiral' phase occurs around {211} wall orientations and corresponds to mixed parity domain walls that may be of left-handed or right-handed chirality. The transformation between the phases is abrupt, accompanied with 20-30% change of the domain wall thickness and can happen at fixed wall orientation with temperature change. We suggest that the phase transition may be detected through domain wall thickness change or by c-AFM. The structure of the domain wall is correlated to its conductivity through polarization component normal to the domain wall, which causes free carriers accumulation. Depending on the temperature and flexoelectric coupling strength relative conductivity of the wall becomes at least one order of magnitude higher than in the single-domain region, creating c-AFM contrast enhancement pronounced and detectable.

Domain relaxation dynamics in epitaxial BiFeO3 films: Role of surface charges

The thermodynamic parameters of domain relaxation process in the absence of external electric fields are related to the intrinsic electrostatic and stress/strain conditions inside the materials, such as the states at surface, states at interface with the electrode, and the atomic defects in the bulk. In order to perform systematical studies of these intrinsic effects, we investigated domain relaxation in a monodomain environment, which was obtained in strained epitaxial BiFeO3 (BFO)(111) films. Without as-grown domain walls and grain boundaries, the epitaxial BFO(111) film provided an ideal system for the dynamic observation of 180-degree domain wall motion. Nano-domains were initially created by writing voltage pulses under the tip of a scanning force microscope and then relaxed through time. The downward polarized domains exhibited much better retention behaviors than the upward domains. A two-step backswitching process was observed, and the behaviors varied with the initial domain sizes. Surface potential measurement showed the dissipation of surface screen charges with time, which was strongly coupled with the 1st step relaxation. The asymmetry behaviors for upward and downward backswitchings, and the two-stage relaxation processes can be explained by the mobile vacancies and the redistribution of surface charges. This study provides the basic understanding of the role of surface charges during the ferroelectric domain relaxation.

Anisotropic conductivity of uncharged domain walls in BiFeO3

Experimental observations suggest that nominally uncharged, as-grown domain walls in ferroelectric thin films can be conductive, yet comprehensive theoretical models to explain this behavior are lacking. Here, Landau theory is used to evolve an analytical treatment of the anisotropic carrier accumulation by nominally uncharged domain walls in multiferroic BiFeO3. Strong angular dependence of the carrier accumulation by 180-degree domain walls originates from local band bending via angle-dependent electrostriction and flexoelectric coupling mechanisms. Theoretical results are in qualitative agreement with experimental data, and provide a Landau-Ginzburg-Devonshire counterpart that is consistent with recent first principles calculations. These studies suggest that a significantly more diverse range of domain wall structures could possess novel electronic properties than previously believed. Similarly, emergent electronic behaviors at ferroic walls are typically underpinned by multiple mechanisms, necessitating first-principle studies of corresponding coupling parameters.

Impact of Strain Effects on the Stability of Ising Walls in Ferroelectrics

The stability of Ising 180-degree domain walls with respect to appearance of chirality is analytically studied in tetragonal perovskite-type ferroelectrics taking into account electro-mechanical coupling. It is shown that the widely used approximation neglecting the elastic effects may lead to qualitatively wrong results. Sufficient condition for the stability of Ising walls is formulated only in terms of elastic and electrostrictive properties, regardless the correlation energy. We demonstrate that elasticity stabilizes Ising walls in tetragonal Pb(Zr1-cTic)O3 and makes chiral walls unfavorable for any composition of the material (factor c), while nonelastic model predicts chirality of 180-degree walls near the morphotropic boundary.

Моделирование зарождения магнитных неоднородностей в ферромагнетиках с неоднородными материальными параметрами

Theoretically investigated the origin and evolution of the three types of dynamic magnetic inhomogeneities (a breather, breather transforming into a soliton, and a soliton), appearing in a flat layer of the parameters of magnetic anisotropy and exchange interaction, different from the parameters in the main area of the infinite in a ferromagnetic after the passage of the 180-degree domain wall. Found the values of parameters, which determine the possibility of the existence of each of the magnetic inhomogeneities. For the magnetic inhomogeneities of the types damped quiescent breather and soliton obtained dependence of the amplitude and frequency of oscillations of the parameters of the defect.

Dynamics of ab-type domain walls in magnets with quadratic magnetoelectric interaction

The dynamics of the ab-type 180-degree domain walls was studied in weak ferromagnets with quadratic magnetoelectric interaction in alternating magnetic and electric fields. The features of the oscillatory and drift motion of the domain walls are discussed. The drift velocity of the ab-type domain walls as a function of the frequency and phase shift of the external fields is obtained. The possibility of the drift of the domain walls in a purely electric field is established.

Head-to-head and tail-to-tail180°domain walls in an isolated ferroelectric

"Head-to-head" and "tail-to-tail" 180-degree domain-walls in a finite isolated ferroelectric sample are theoretically studied using Landau theory. The full set of equations, suitable for numerical calculations is developed. The explicit expressions for the polarization profile across the walls are derived for several limiting cases and wall-widths are estimated. It is shown analytically that different regimes of screening and different dependences for width of charged domain walls on the temperature and parameters of the system are possible, depending on spontaneous polarization and concentration of carriers in the material. It is shown that the half-width of charged domain walls in typical perovskites is about the nonlinear Thomas-Fermi screening-length and about one order of magnitude larger than the half-width of neutral domain-walls. The formation energies of "head-to-head" walls under different regimes of screening are obtained, neglecting the poling ability of the surface. It is shown that either "head-to-head" or "tail-to-tail" configuration can be energetically favorable in comparison with the monodomain state of the ferroelectric if the poling ability of the surface is large enough. If this is not the case, the existence of charged domain walls in bulk ferroelectrics is merely a result of the domain-growth kinetics. Size-effect corresponding to the competition between state with charged domain wall, single domain state, multidomain state, and the state with the zero polarization is considered. The results obtained for the case of an isolated ferroelectric sample were compared with the results for an electroded sample. It was shown that charged domain wall in electroded sample can be either metastable or stable, depends on the work function difference between electrodes and ferroelectric and the poling ability of the electrode/ferroelectric interface.

Domain Structure Reversible Reconstruction of Amorphous Ferromagnetic by Water Molecules Weak Adsorption

Influence of water molecules reversible adsorption on domain structure and domain wall structure in amorphous Fe-rich ferromagnet samples is investigated by magnetooptical method. Before investigation the samples were kept in the air. Domain structure of the samples significantly changes in wacuum: instead one 180-degree domain wall several domain walls appear which are oriented with angle 45o to the long side of the sample. This effect is completely reversible. Another new effect is found: domain wall width on the sample surface in vacuum is 35% bigger (13.5 µm) than in atmosphere of water vapor (10 µm).