Estimation of magnetic field growth and constraction of adaptive mesh in corner domain for magnetostatic problem

A magnetostatic problem arises in searching for the distribution of the magnetic field generated by magnet systems of many physics research facilities, e.g., accelerators. The domain in which the boundaryvalue problem is solved often has a piecewise smooth boundary. In this case, numerical calculations of the problem require the consideration of the solution behavior in the corner domain. In this paper an upper estimate is obtained for the maximum possible growth of the magnetic field in the corner domain of vacuum. Based on this estimate, we propose a method of condensing the differential grid near the corner domain of vacuum. An example of the modeling problem calculation in the corner domain is given.     

Computation of demagnetizing fields and particle distribution in magnetic fluid with inhomogeneous density

A new algorithm for calculating magnetic fields in a concentrated magnetic fluid with inhomogeneous density is proposed. Inhomogeneity of the fluid is caused by magnetophoresis. In this case, the diffusion and magnetostatic parts of the problem are tightly linked together and are solved jointly. The dynamic diffusion equation is solved by the finite volume method and, to calculate the magnetic field inside the fluid, an iterative process is performed in parallel. The solution to the problem is sought in Cartesian coordinates, and the computational domain is decomposed into rectangular elements. This technique eliminates the need to solve the related boundary-value problem for magnetic fields, accelerates computations and eliminates the error caused by the finite sizes of the outer region. Formulas describing the contribution of the rectangular element to the field intensity in the case of a plane problem are given. Magnetic and concentration fields inside the magnetic fluid filling a rectangular cavity generated under the action of the uniform external filed are calculated.     

Shape Anisotropy and Resonance Mode Guided Reliable Interconnect Design for In-plane Magnetic Logic

Dipole coupled nanomagnets controlled by the static Zeeman field can form various magnetic logic interconnects.However, the corner wire interconnect is often unreliable and error-prone at room temperature. In this study, we address this problem by making it into a reliable type with trapezoid-shaped nanomagnets, the shape anisotropy of which helps to offer the robustness. The building method of the proposed corner wire interconnect is discussed,and both its static and dynamic magnetization properties are investigated. Static micromagnetic simulation demonstrates that it can work correctly and reliably. Dynamic response results are reached by imposing an ac microwave field on the proposed corner wire. It is found that strong ferromagnetic resonance absorption appears at a low frequency. With the help of a very small ac field with the peak resonance frequency, the required static Zeeman field to switch the corner wire is significantly decreased by ~21 m T. This novel interconnect would pave the way for the realization of reliable and low power nanomagnetic logic circuits.     

Length of magnetic field lines in turbulent plasmas

An estimation of the length of any magnetic field line in a two-dimensional periodic magnetohydrodynamic problem is provided. This is done by using some classical function theory results on the analytic extension of the vector potential. The essential parameter, the maximum of this extension, may be analyzed in the case of turbulent plasmas by admitting the Iroshnikov-Kraichnan statistics, establishing in this way a relation between the length of any magnetic field line and the energy dissipation scale. (c) 2002 American Institute of Physics.     

A contribution to the theory of the Faraday effect of excitons in rare-gas crystals

An expression for the Faraday rotation of excitons in rare-gas crystals was found for the case of a weak magnetic field. The wave functions of an exciton in a magnetic field were found in first order of perturbation theory in an approximation including spin-orbit interaction and overlapping of atomic wave functions. The general theory was used for the estimation of Faraday rotation in crystalline argon.It is a pleasure for the author to acknowledge her debt to Dr. M. Trlifaj for suggesting the problem and for helpful discussions.     

Understanding magnetic force microscopy

Magnetic force microscopy is a new method for imaging ferromagnetic domains with a high lateral resolution (10 nm). In this paper we give the basic tip parameters that have to be taken into account to achieve a quantitative image interpretation. For the electrochemically otched polycrystalline iron, nickel and cobalt wires, the tip-apex domain is found to be oriented along the tip axis, because of shape anisotropy. The stray field emerging from the tip apex is comparable to the size of the tip saturation field. The effective domain lengthL determines the image formation: the force due to magnetization patterns of scales which are large compared toL follow the point-dipole approximation. In the opposite case, a single-pole model is more appropriate. While a cobalt tip can be treated as an isolated domain, for nickel and iron a net polarization in the tip wire induced by the front apex-domain has to be considered. A new analytical theory provides an overall understanding of the image formation and allows the determination of the magnetic field vector and the estimation of its magnitude from measurements.     

Influence of a magnetically permeable surface layer on transient electromagnetic field of a vertical magnetic dipole on a two-layer conducting earth

The transient electromagnetic field, in the frequency domain, on a two-layered conducting earth is reexamined when the usually neglected magnetic permeability contrast is considered. It is shown that for two-layered earth model, where the upper is permeable, the electromagnetic induction response is modified over the nonpermeable case. It is obtained as a summation of waveguide modes plus contributions from branch-cuts in the complex plane of the longitudinal wave number. The results from this paper can be used to evaluate numerical solution of other more complicated forward modeling algorithms. It can also be used in developing fast and simple interpretation tools, which can deliver quick estimation of the earth structure at the field.     

黏性各向异性磁流体Kelvin-Helmholtz不稳定性:二维数值研究

利用corner transport upwind和constrained transport算法求解非理想磁流体动力学方程组,对匀强平行磁场作用下,黏性各向异性等离子体自由剪切层中的Kelvin-Helmholtz不稳定性进行了数值模拟.从流动结构、涡结构演化、磁场分布、横向磁压力、抗弯磁张力等角度对各向同性和各向异性黏性算例结果进行了讨论,分析了黏性各向异性对Kelvin-Helmholtz不稳定性的影响.结果表明,黏性各向异性比黏性各向同性更利于流动的稳定.其稳定性作用是由于磁感线方向上剪切速率降低导致界面卷起程度和圈数的降低,并使卷起结构中小涡产生增殖、合并,破坏了涡的常规增长,从而导致流动的稳定.黏性各向异性对横向磁压力的影响比对抗弯磁张力更大.     

Effect of fluctuations on the forced motion and interactions of domain walls in one-dimensional magnetic systems

Forced motion of a domain wall in the presence of fluctuations of external magnetic field and those of the parameters of the magnetic medium is studied. Calculations for the models of magnetic systems described by the sine-Gordon and Landau-Lifshitz equations are presented. It is shown that the driven motion of domain walls is characterized by the time-independent velocity distribution function which is used to calculate various statistical characteristics of the domain wall. Analysis of the mean velocity of the steady motion of the domain wall leads to the conclusion that the presence of a fluctuating magnetic field results in an increase of the effective relaxation constant of the magnetic system. In case of the sine-Gordon model the mean radiation power accompanying the forced motion of the domain wall is calculated. Inelastic interactions of two domain walls of opposite polarities are described.     

Theory of cylindrical magnetic domains of submicron size

By successively taking account of the domain boundary thickness and the contribution of the magnetic field energy to the boundary energy, an expression is obtained for the total energy of a cylindrical magnetic domain in the case when the domain size is comparable to its boundary thickness. The boundary of the stability region is determined for the cylindrical magnetic domain with respect to the field as a function of the domain boundary thickness. Analysis of the results obtained permits making a deduction about the degree of influence of the boundary-layer thickness on the static properties of the cylindrical magnetic domain.Translated from lzvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 61–67, February, 1984.In conclusion the author is grateful to Yu. I. Gorobets for interest in the research and for valuable discussion.     

Frequency-selective quantification of biomedical magnetic resonance spectroscopy data

In this paper the possibility of obtaining accurate estimates of parameters of selected peaks in the presence of unknown or uninteresting spectral features in biomedical magnetic resonance spectroscopy (MRS) signals is investigated. This problem is denoted by frequency-selective parameter estimation. A new time-domain technique based on maximum-phase finite impulse response (FIR) filters is presented. The proposed method is compared to a number of existing approaches: the application of a weighting function in the time domain, frequency domain fitting using a polynomial baseline, and the time-domain HSVD filter method. The ease of use and low computational complexity of the FIR filter method make it an attractive approach for frequency-selective parameter estimation. The methods are validated using simulations of relevant (13)C and (31)P MRS examples.     

Electronic states and interband light absorption in semi-spherical quantum dot under the influence of strong magnetic field

In the framework of effective mass approximation the electronic states in semi-spherical quantum lens under the influence of strong magnetic field are investigated. We have used the adiabatic approximation for the case of strong magnetic field. The eigenfunctions and eigenvalues of this problem are determined. In strong confinement regime interband optical absorption of light is investigated in quantum lens from InAs. The threshold frequencies of absorption are determined. The comparison with the case of film under the influence of strong magnetic field with infinitely high confinement potential is performed.     

State-independent hypothesis to model the behavior of magnetic materials

The problem of modeling the magnetic behavior due to a magnetization process is usually dealt with using the classical scalar Preisach model (CSPM). Recently, it has been shown that in the case of soft magnetic materials better results can be obtained by means of the modified scalar Preisach model (MSPM). The problem of model identification is common to both these two approaches. In the past we used a Lorentzian approximation for CSPM identification. In this paper, after reviewing the method, which allows, in particular, the efficient determination of the parameters controlling the reversible magnetization, we propose an extension of the Lorentzian approximation to the case of MSPM identification. It will be shown that an accurate estimation of the model parameters is possible also in this case, as it is confirmed by comparing the model with experimental data.     

Dynamics of solenoidal magnetic structures in soft magnetic thin-film elements

We report on the dynamics of magnetic domain structure conversions exhibited by soft magnetic thin-film elements of elementary geometrical shape (square, disc, triangle) when exposed to a strong external magnetic field. Starting from flux closure vortex patterns, the magnetic structures evolve towards an in-plane saturated state under the influence of an external field. This irreversible and nucleation-free magnetization process occurs on the time scale of picoseconds. The details of this conversion are investigated by means of a time-resolved micromagnetic finite element modeling. We find a sensitive dependence of the temporal evolution of the magnetic structure on the value of the damping parameter in Gilbert's equation of motion. In the case of high damping, domain wall motion dominates the process, while lower damping leads to the formation of a 360° wall which collapses by emitting magnetization waves. It is shown that the mobility of vortices is generally much lower than that of domain walls. The calculations indicate that at a low damping, a magnetic vortex can act almost as a source for concentric waves in ferromagnetic thin-film elements.     

Ground-state entropy of the q-state Potts antiferromagnet in a magnetic field

We present a simple method to obtain reliable ground-state entropies of the q-state Potts antiferromagnet in an external magnetic field. As an example, the ground-state entropy for the triangular lattice is established for all q. In the particular case q = 2, our method gives results which coincide with the first-order approximation obtained by the corner transfer matrix method.     

Fulde-Ferrell-Larkin-Ovchinnikov state in a perpendicular field of quasi-two-dimensional CeCoIn5

A Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state was previously reported in the quasi-2D heavy fermion CeCoIn5 when a magnetic field was applied parallel to the ab plane. Here, we conduct 115In NMR studies of this material in a perpendicular field, and provide strong evidence for FFLO in this case as well. Although the topology of the phase transition lines in the H-T phase diagram is identical for both configurations, there are several remarkable differences between them. Compared to H parallelab, the FFLO phase for H perpendicularab is confined in a much narrower region at the low-T-high-H corner in the H-T plane, and the critical field separating the FFLO and non-FFLO superconducting states almost ceases to have a temperature dependence. Moreover, directing H perpendicularab results in a notable change in the quasiparticle excitation spectrum within the planar node associated with the FFLO transition.     

Zur Theorie der Bewegung geladener Teilchen in richtungskonstanten Magnetfeldern exponentieller Zeitabhängigkeit. III. Allgemeine Schaltprozesse

The motion of a charged particle in a magnetic field is calculated for the following case: the spatially homogeneous magnetic field having a constant direction is a superposition of a field constant in time and one decreasing exponentially in time; taken into account is the influence of the electric field induced by the time dependent magnetic field and a friction force proportional to the particle velocity. The higher transcendental functions appearing in the exact solutions are approximated in various ways in dependence on the values of argument and parameters. In this manner approximated formulae of a very simple form are obtained for position, velocity, kinetic energy and magnetic moment of the particle, and the domain of validity of these formulae is determined. The particle orbits are classified, and their dependence on the initial values, parameters of the magnetic field and on the magnitude of the friction force is studied. A comparison between our results and a rectangular variation of the field is given. It is shown that the electric field induced by the time dependent magnetic field has an important influence on the particle motion.     

理想磁流体中激波与矩形密度界面相互作用的数值研究

发展一套采用三阶WENO格式和混合GLM方法的理想磁流体数值方法,并对激波与矩形密度界面相互作用进行数值研究.通过圆极化阿尔芬波和旋转激波管问题对数值方法的稳定性和可靠性进行验证.在入射激波马赫数为10,界面内外气体密度比为10的情况,对比不同磁场中矩形密度界面的演变过程.结果表明,磁场能够减少界面上涡量的生成从而抑制界面不稳定性,并且磁场对界面的加速过程以及界面内外气体混合率有影响;而界面的存在将会使波后部分区域磁场增强;由于尖角的存在,矩形界面的发展与圆形界面不同.     

Landau electron in a rotating environment: A general factorization of time evolution

For the Landau problem with a rotating magnetic field and a confining potential in the (changing) direction of the field, we derive a general factorization of the time evolution operator that includes the adiabatic factorization as a special case. The confining potential is assumed to be of a general form and it can correspond to nonlinear Heisenberg equations of motion. The rotation operator associated with the solid angle Berry phase is used to transform the problem to a rotating reference frame. In the rotating reference frame, we derive a natural factorization of the time evolution operator by recognizing the crucial role played by a gauge transformation. The major complexity of the problem arises from the coupling between motion in the direction of the magnetic field and motion perpendicular to the field. In the factorization, this complexity is consolidated into a single operator which approaches the identity operator when the potential confines the particle sufficiently close to a rotating plane perpendicular to the magnetic field. The structure of this operator is clarified by deriving an expression for its generating Hamiltonian. The adiabatic limit and non-adiabatic effects follow as consequences of the general factorization which are clarified using the magnetic translation concept.     

S-N-D corner junctions in magnetic field

The dependence of the critical current of a highly transparent S-N-D corner junction on the applied magnetic field is determined for different orientations of a d-wave superconductor relative to the interface plane. It is shown that this dependence exhibits characteristic plateaus in a certain range of magnetic fields at low temperatures. These plateaus do not appear in the S-N-S corner junctions, indicating the presence of a superconductor with a sign-variable order parameter.