Stability of magnetic domains in an amorphous metal wire

Dimensions and energy of domains in the core of an amorphous metal wire are estimated in the work. Influence of the external magnetic field strength and coercive force on the dimensions of a stable domain is investigated. It is demonstrated that the highest stability to an external magnetic field and the least energy have domains comprising cylinder and cone-shaped domain tops and domains with zigzag domain tops. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 83–91, January, 2009.     

Magnetostatic energy and stripe domain structure in a ferromagnetic plate of finite width with in-plane anisotropy

The magnetostatic energy and domain structure (DS) in a long ferromagnetic plate of a finite width with in-plane anisotropy are calculated for the case of the domain magnetization vectors lying in the plane of the plate. The situation where the DS period is much shorter than the width but is considerably larger than the thickness of the plate is analyzed in detail. The equilibrium DS period and the width ratio of two adjacent domains are determined as functions of an external magnetic field parallel to the plane of the plate by minimizing the energy. The DS period is found to be proportional to the plate width and the domain wall energy and inversely proportional to the squared saturation magnetization. While the width of the favorable domains (with the magnetization parallel to the field) grows with increasing field, the unfavorable domains, rather than disappearing completely, form relatively narrow transition regions between the favorable domains, i.e., 360° domain walls.     

Influence of magnetostriction on the domain structure of cobalt

The paper presents the calculation of the energy of elastic stresses in the domain structure of uniaxial massive ferromagnetics. The inclusion of this energy permits the formation of a structure with walls perpendicular to the external field to be elucidated and the critical thickness of the sample on which this structure is realized to be determined.     

紫外激光诱导近化学计量比钽酸锂晶体铁电畴反转

对紫外激光诱导近化学计量比钽酸锂晶体铁电畴反转进行了实验研究。波长为351 nm的连续紫外激光被聚焦在近化学计量比钽酸锂晶体的-z表面,同时沿与晶体自发极化相反的方向施加均匀外电场。实验证实紫外激光辐照可以有效地降低晶体畴反转所需的矫顽电场,采用数字全息干涉测量技术检测证实在激光辐照区域实现局域畴反转。研究表明采用紫外激光诱导可以实现对近化学计量比钽酸锂晶体铁电畴反转的局域控制。提出了物理机理的理论分析,认为外电场和激光辐照场的共同作用在晶体内部产生高浓度、大尺寸的缺陷结构,缺陷一定程度上降低畴体成核和畴壁运动所需要克服的退极化能和畴壁能,实现激光诱导畴反转。     

Ferroelectric domain breakdown

We observe a stringlike domain penetration from a ferroelectric surface deep into the crystal bulk induced by a high voltage atomic force microscope tip. The domains, which resemble channels of an electrical breakdown, nucleate under an electric field of around 10(7) V/cm at the ferroelectric surface, and grow throughout the crystal bulk where the external electric field is practically zero. A theory explaining the shape of the formed domains is presented. It shows that the driving force for the domain breakdown is the decrease of the total free energy of the system with increasing domain length.     

Mathematical Modeling of the Magnetization of a Two-Dimensional System of Magnetic Moments

Using a system that reaches its minimum energy of interaction at equilibrium, the magnetization of a discrete two-dimensional system of interacting magnetic dipoles by an external magnetic field is modeled mathematically. Magnetization curves for rectangular two-dimensional clusters of dipoles and the region of the magnetic domain are calculated.     

消磁场对纳米铁磁线磁畴壁动力学行为的影响

为了实现基于磁畴壁运动的自旋电子学装置, 掌握磁畴壁动力学行为是重要争论之一.研究了在外磁场驱动下L-型纳米铁磁线磁畴壁的动力学行为. 通过微磁学模拟,在各种外磁场的驱动下考察了纳米铁磁线磁畴壁的动力学特性; 在较强外磁场的驱动下, 在不同厚度纳米线上考察了纳米线表面消磁场对磁畴壁动力学行为的影响. 为了进一步证实消磁场对磁畴壁动力学的影响, 在垂直于纳米线表面的外磁场辅助下分析了磁畴壁的动力学行为变化. 结果表明, 随着纳米线厚度和外驱动磁场强度的增加, 增强了纳米线表面的消磁场的形成, 使得磁畴壁内部自旋结构发生周期性变化, 导致磁畴壁在纳米线上传播时出现Walker崩溃现象. 在垂直于纳米线表面的外磁场辅助下, 发现辅助磁场可以调节消磁场的强度和方向. 这意味着利用辅助磁场可以有效地控制纳米铁磁线磁畴壁的动力学行为.     

Globally optimal superconducting magnets part II: symmetric MSE coil arrangement

A globally optimal superconducting magnet coil design procedure based on the Minimum Stored Energy (MSE) current density map is outlined. The method has the ability to arrange coils in a manner that generates a strong and homogeneous axial magnetic field over a predefined region, and ensures the stray field external to the assembly and peak magnetic field at the wires are in acceptable ranges. The outlined strategy of allocating coils within a given domain suggests that coils should be placed around the perimeter of the domain with adjacent coils possessing alternating winding directions for optimum performance. The underlying current density maps from which the coils themselves are derived are unique, and optimized to possess minimal stored energy. Therefore, the method produces magnet designs with the lowest possible overall stored energy. Optimal coil layouts are provided for unshielded and shielded short bore symmetric superconducting magnets.     

Dynamics of domain walls in magnetically multiaxial films in the thickness range corresponding to the Bloch-Néel transformations of the structure

The numerical minimization of the total energy functional and the solution of the nonlinear Landau-Lifshitz equation have been performed exactly taking into account the fundamental (including dipole-dipole) interactions in terms of the two-dimensional magnetization distribution. The equilibrium structure, energy, mobility, and scenario of the dynamic transformation of the domain walls (in their non- steady-state motion) have been determined as a function of the film thickness b and external magnetic field H for two different ((010) and (110)) orientations of the surfaces of magnetically triaxial films. The range of film thicknesses, including the thickness b = b _N, for which the Néel domain walls can be transformed into the Bloch domain walls, has been investigated. The phenomena of anisotropy of the domain-wall energy, the domain-wall mobility, and the period of dynamic transformations of the domain walls have been analyzed as a function of the film thickness b and external magnetic field H. The range of film thicknesses has been determined, in which the non-steady-state motion of the Néel domain walls is accompanied by the creation and annihilation of vortex-like structures despite the one-dimensional character of the magnetization distribution in these walls.     

Dynamic stability of a spiral domain in an ac magnetic field

The dependence of the stability of a magnetic spiral domain in a film on the parameters of the film and its domain structure and on an external magnetic field is considered within a phenomenological model. The model allows one to explain a number of experimentally observed properties of dynamic spiral domains resulting from the process of self-organization of domains and domain walls in an iron-garnet film placed in an external ac magnetic field.     

Structure and dynamic properties of the twisted magnetic domain wall in an electric field

The structure of the domain wall in a magnetically uniaxial ferromagnetic film placed in an external electric field has been studied. It has been shown that the domain wall has a complex twisted structure whose characteristics (thickness, profile, and limit velocity of steady motion) depend on the film thickness, quality factor, and external electric field. The effect of the electric field on the domain wall is caused by inhomogeneous magnetoelectric coupling taking place in domain walls with a twisted structure.     

Dynamics of ferroelectric domain formation in an atomic force microscope

The dynamics of ferroelectric domain formation in a non-homogeneous electric field of an atomic force microscope (AFM) tip are considered. Contributions of the apex and the conical part of the tip into the field are taken into account. It is supposed that this process passes through the same stages as the corresponding process in homogeneous external fields. However, the character of these stages may differ significantly. We consider all the stages of this process: formation of a nucleus, its growth, and the equilibrium domain parameters. Quantitative analysis is carried out for barium titanate. It is shown that the activation energy of nucleation strongly decreases with the applied voltage, and it is too small to limit the rate of the process even under low voltages. Dynamic equations for the time dependence of the domain length and radius are constructed and solved. Comparison of the calculated domain sizes with those observed in experiment is carried out. Calculated results obtained using different models for the field of the tip are compared.     

The interaction energy of parallel Bloch walls

The paper gives an exact calculation of the interaction energy of parallel 180° Bloch walls of an unbounded uniaxial ferromagnetic as a function of their distance s and of the external field. The general relation, valid for a periodic domain structure, is specialised for the case of two Bloch walls.The author wishes to thank F. Kroupa and V. Janovec, of the Institute, for their valuable suggestions.     

Interaction of a shear wave with a moving domain wall in a ferromagnetic crystal with allowance for the external magnetic field

The boundary-value problem of the magnetoelastic wave interaction with a moving domain wall in a ferromagnetic crystal is solved in the nonexchange magnetostatic approximation with allowance for the external magnetic field. It is shown that the difference introduced by magnetic field between the ferromagnetic resonance frequencies of the domains does not cause any noticeably departure of the refraction characteristics of reflected and transmitted waves from those observed at zero frequency mismatch. By contrast, the magnitudes of the transmission and reflection coefficients strongly depend on the external magnetic field and on the mobility of the domain wall. The dependence of the magnitude of the reflection coefficient on the external magnetic field at a fixed angle of shear wave incidence is found to possess two ferromagnetic resonance peaks. The positions and heights of the peaks may vary depending on the mobility of the domain wall.     

Nonlinear electromechanical analysis: a constitutive theory for perovskite type ferroelectrics

A constitutive theory is established in this paper to describe the nonlinear electromechanical behavior of perovskite type ferroelectrics subjected to external stress and external electric field. In the proposed theory, each domain is considered as an inclusion. The Helmholtz free energy and Gibbs free energy of a constituent element, that are derived by using micromechanics approaches, are functionals of the orientation distribution function (ODF) that depicts the domain distribution patterns. By applying the internal variable theory and expanding ODF in Fourier series, the yielding condition, the evolution of ODF, and the constitutive relation are obtained. In terms of the simplification of the constitutive relation, theoretical predictions are compared with experimental results. There is an agreement between the theoretical and the experimental results, indicating that the theory is reasonable and applicable. It should be pointed out that the constitutive model proposed in this paper is restricted to ferroelectric materials exhibiting transformation from cubic to tetragonal only.     

Ferromagnetic domain structure dynamics and microeddy currents in high-frequency fields

An expression is found for the energy loss of an external alternating field in a ferromagnetic conducting plate that contains 180° domain walls by means of a self-consistent solution of the equation of motion of a domain wall and Maxwell's equations; the shielding effect of microeddy currents is taken into consideration.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 95–99, August, 1977.     

Influence of an external electric field on the motion of a ferroelectric domain wall

The study of the influence of an external electric field on the motion from rest of a domain wall in ferroelectric crystals is presented. The solution of the problem representing the translational and rotational motions of the chain is sought in terms of solitary waves. Both electric and mechanical state of the structure in domains can be determined. The evolution of the velocity of the wall altered by the applied field is determined by means of energy arguments accounting for electromechanical couplings. A numerical simulation is given which illustrates the transient motion from rest of a wall separating two ferroelectric domains.     

Bifurcation and stability of dynamical structures at a current instability

We investigate bifurcation and stability of nonuniform current states at a voltage-controlled current instability. We consider a model which exhibits bulk negative differential conductivity due to Bragg scattering of hot electrons. The system is described by balance equations for momentum and energy densities of the carriers. These transport fields are coupled to Maxwell's equations. The uniform stationary current state is unstable against long-wavelength dielectric relaxation modes at a critical field. We find that the softening of these modes gives rise to a family of periodic travelling waves and to a solitary solution (dipole domain). We show that the periodic travelling waves are unstable, wheras the dipole domain can be stabilized by coupling the sample to a suitable external circuit, if the static impedance of the sample in the domain state is negative. The model describes therefore a discontinuous nonequilibrium transition to a large amplitude domain state.Work Supported by the Swiss National Science Foundation     

ZnSe在外电场下的基态性质和激发特性研究

以LANL2DZ为基组, 采用Hartree-Fock(HF)方法研究了不同外电场(-0.025–0.040 a.u.)对ZnSe分子的基态几何结构、电荷分布、能量、电偶极矩、最高占据轨道(HOMO)能级、最低空轨道(LUMO)能级、能隙、红外光谱特性的影响; 继而采用含时的TD-HF方法研究了ZnSe分子在外电场下前9 个激发态的吸收谱、激发能、振子强度等激发特性. 研究结果表明: 当电场从-0.025 a.u.变化到0.04 a.u. 时, 键长先减小后增加; 分子偶极矩先由正减小到0, 然后又反向增加; 体系总能量一直减小; 谐振频率先增加后减小, 红外光谱强度先减小后又增加. ZnSe分子的LUMO能级一直增加, HOMO能级先增加后又减小, 变化趋势较小, 而能隙一直增大. 外电场对ZnSe分子的激发特性影响较大, 当电场从-0.025 a.u.变化到0.04 a.u.时, 激发能增加, 相应的激发波长减小; 对应的振子强度也受到很大影响, 原来振子强度最强的激发态变得很弱, 而原来振子强度很弱的激发态变得最强. 因此, 可以通过改变电场来控制ZnSe的激发特性.     

Phase-transition kinetics with the formation of topological defects in superconductors with a multicomponent order parameter

This paper discusses the kinetics of phase transitions to superconductivity with a multicomponent order parameter in zero external field. It is shown that as it approaches equilibrium the superconductor passes through an intermediate vortex-like state containing domain walls, single-quantum, and multiquantum axially nonsymmetric vortices and antivortices. The energy and other parameters of the domain walls are derived. Rigid superconducting bubbles are discussed and criteria are established for their local stability. Zh. éksp. Teor. Fiz. 112, 1351–1373 (October 1997)