Effects of an arbitrarily oriented rotating field in magnetic resonance

Effects of an arbitrarily oriented rotating field in magnetic resonance are analyzed by using a dressed atom theory.     

Complex magnetic susceptibility of uniaxial superparamagnetic particles in a strong static magnetic field

The magnetic relaxation of a system of single-domain ferromagnetic particles in the presence of a strong static magnetic field directed at an arbitrary angle relative to the particle anisotropy axis is investigated. A system of linear difference-differential equations for the moments (averaged spherical harmonics) is derived without recourse to the Fokker-Planck equation by averaging Gilbert’s equations with a fluctuating field. An exact solution (in terms of matrix continuous fractions) is found for this system. The relaxation times and spectra of the complex magnetic susceptibility are calculated. Fiz. Tverd. Tela (St. Petersburg) 40, 1642–1649 (September 1998)     

Electromagnetic waves in a round rod in the presence of an arbitrarily directed external magnetic field or anisotropy axis

A rigorous theory of the propagation of electromagnetic waves in round anisotropic and semiconductor rods in the presence of an arbitrarily directed anisotropy axis or external magnetic field is developed. New types of independent waves are discovered. Exact dispersion equations are obtained for them, which define the dependence of their spectral characteristics on the parameters of the semiconductor or anisotropic crystal and on the magnitude and direction of the constant external magnetic field. The results of numerical investigations for rods made from a semiconductor or a uniaxial crystal are presented. Zh. Tekh. Fiz. 67, 86–91 (July 1997)     

An optimal analytical expression for the superparamagnetic relaxation rate under conditions of a stochastic resonance for the model of bistable systems

An analytical expression for the superparamagnetic relaxation rate of uniaxial particles modulated by a radio-frequency field is chosen that describes adequately the magnetic moment dynamics under conditions of a stochastic resonance in the approximation of discrete orientations. The complex magnetic susceptibility components are calculated for a weak external variable field. A comparison is made with the results of numerical modeling based on a solution of the Fokker-Planck equation with a periodic drift term. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 54–58, May, 2007.     

The effect of an oblique magnetic field on the superparamagnetic relaxation time

The effect of a constant magnetic field, applied at an angle ψ to the easy axis of magnetization, on the Néel relaxation time τ of a single domain ferromagnetic particle with uniaxial anisotropy is investigated by calculating the smallest non-vanishing eigenvalue of the appropriate Fokker-Planck equation as a function of ψ. The curve of λ₁ versus ψ is symmetric about ψ = 90°. Thus the maximum decrease in τ occurs at ψ = 45° with maximum increase to a value exceeding that at ψ = 0, at ψ = 90°, the ψ = 0 value being again attained at ψ = 180°. The results are shown to be consistent with the behaviour predicted by the Kramers theory of the rate of escape of particles over potential barriers. This theory when applied to the potential barriers for the ψ = 90° orientation for rotation in space yields a simple approximate formula for the escape rate which is in reasonable agreement with the exact λ₁ calculated from the Fokker-Planck equation.     

Electron transport across a microconstriction in an arbitrarily oriented homogeneous magnetic field

An analysis is made of ballistic electron transport in three-dimensional microconstrictions of elliptic cross section located in an arbitrarily oriented magnetic field. The model of a parabolic confinement potential is used to obtain and analyze the dependence of hybrid frequencies on the magnitude and direction of the magnetic field. An expansion of the conductance as a Fourier series is obtained and used to study Aharonov-Bohm and Shubnikov-de Haas oscillations as a function of the field and the stepped quantization structure of the conductance. A possible explanation is given for the experimentally observed effect of conductance quantization at fairly high temperatures.     

Impurity effects on the magnetization and magnetic susceptibility of an electron confined in a quantum ring under the presence of an external magnetic field

The magnetization and the magnetic susceptibility of a single electron confined in a two-dimensional (2D) parabolic quantum ring under the effect of external uniform magnetic field and in the presence of an acceptor impurity have been studied. The shifted 1/N expansion method was used to solve the Hamiltonian quantum ring within the effective mass approximation. The computed energy spectra, the magnetization and magnetic susceptibility have been displayed as a function of the quantum ring parameters: confinement strength ω₀, magnetic field strength (ω_c), and temperature (T). The obtained energy results show level-crossings, in the absence and presence of acceptor impurity, which are manifested as oscillations in the magnetization and magnetic susceptibility curves.     

Saturation of two-level systems under pulsed stochastic resonance conditions

Pulsed stochastic excitation of a two-level system described by Bloch equations is studied. An equation for the average powers of the components of the state vector of the system is obtained on the basis of the theory of stochastic differential equations, and solved. The dynamical and nonlinear properties of the response of a system under pulsed stochastic resonance conditions are analyzed and the results are compared with the corresponding stationary state. The results obtained can be used in spectroscopy and for analysis of nonlinear filters based on the saturation effect and intended for processing rf and light range signals. Zh. Tekh. Fiz. 69, 65–69 (December 1999)     

Effect of stochastic resonance on bone loss in osteopenic conditions

We investigate the effect of noise on the remodelling process of the inner spongy part of the trabecular bone. Recently, a new noise-induced phenomenon in bone formation has been reported experimentally. We propose the first conceptual model for this finding, explained by the stochastic resonance effect, and provide a theoretical basis for the development of new countermeasures for bone degeneration in long space flights, which currently has dramatic consequences on return to standard gravity. These results may also be applicable on Earth for patients under osteopenic conditions.     

Entropic stochastic resonance of a self-propelled Janus particle

Entropic stochastic resonance is investigated when a self-propelled Janus particle moves in a double-cavity container. Numerical simulation results indicate the entropic stochastic resonance can survive even if there is no symmetry breaking in any direction. This is the essential distinction between the property of a self-propelled Janus particle and that of a passive Brownian particle, for the symmetry breaking is necessary for the entropic stochastic resonance of a passive Brownian particle. With the rotational noise intensity growing at small fixed noise intensity of translational motion, the signal power amplification increases monotonically towards saturation which also can be regarded as a kind of stochastic resonance effect. Besides, the increase in the natural frequency of the periodic driving depresses the degree of the stochastic resonance, whereas the rise in its amplitude enhances and then suppresses the behavior.     

Diffusional coagulation of superparamagnetic particles in the presence of an external magnetic field

In this paper, we investigate the diffusional coagulation of colloidal superparamagnetic (SP) latex particles that are under the influence of an external magnetic field. The cluster size distributions (CSDs) that evolve with time were determined using an optical set-up that permits the direct visualization of particle clusters. Following the dynamic scaling analysis of van Dongen and Ernst (Phys. Rev. Lett. 54 (1985) 1396), we find that the CSDs all collapse onto a master curve when properly scaled. The bell-shape of this master curve indicates that large clusters preferentially scavenge small clusters in our system. From the time evolution of the average cluster size we infer that the reactivity between large clusters diminishes with increasing cluster size. These results are consistent with a simple mathematical formulation of the coagulation rate constant, or kernel, for the Brownian coagulation of magnetic particles. Moreover, our results support a growing body of evidence that the dynamic scaling theory developed by van Dongen and Ernst is a useful framework with which to study the microscale processes governing particle coagulation.     

Orientation of aligned alkali atoms under magnetic resonance in a weak magnetic field

It is shown that, upon excitation of the magnetic resonance in a system of aligned atoms with unequally spaced sublevels, the atoms should become oriented. This orientation was detected experimentally for Cs atoms pumped by unpolarized light in the magnetic field of the Earth.     

Effect of boundary conditions on quantum-anti-dots in the presence of a magnetic field

Many people have studied the conductance properties through an array of anti-dots, especially since the observation of Weiss oscillations. In most cases, however, in which the recursive Greenźs functions are used on a spatial lattice, periodic boundary conditions are employed. In this paper, we analyse the effects of boundary conditions and magnetic field on the conductance behavior in a number of anti-dot-shaped, GaAs/AlGaAs 2DEG quantum systems. The effect of periodic boundary conditions causes a reduction in the overall conductance. The effect of changing the boundary conditions is more profound for lower numbers of anti-dots.     

垂直场下磁性薄膜中的铁磁共振现象

将铁磁共振频率看成外磁场的函数, 讨论了垂直场下磁性膜中的铁磁共振现象. 结果显示: 当外磁场平行于膜面, 并考虑磁膜具有垂直磁晶各向异性情形时, 其磁共振频率随外磁场的变化分为高频支和低频支两种情况, 具体的依赖关系取决于磁膜内磁晶的各向异性; 当外磁场垂直于膜面, 其磁共振频率随外磁场的关系仅存在一支, 一般地, 磁共振频率随外磁场的增加单调地非线性减小, 但当立方磁晶各向异性场H_k1 与单轴磁晶各向异性场H_a之比值介于2/3 < H_k1/H_a <1时, 其磁共振频率随外磁场的增加单调增加, 这与相关的实验结果一致. 研究结果表明: 磁薄膜中有无垂直于膜面的磁各向异性可以通过其磁共振谱的测量进行辨析.     

Intensity of electromagnetic energy radiation by a quiescent ferromagnetic spherical particle placed in a permanent magnetic field

Technical Physics - The classical problem of calculating the energy of electromagnetic energy radiated from a ferromagnetic particle is solved for the case when the vector of spontaneous...     

均匀磁化永磁棒的磁场

本文综合运用分子电流观点及磁荷观点,系统地计算了沿轴向、横向及其它任意方向均匀磁化的有限长永磁棒在轴线上的磁场以及无限长永磁棒在全空间的磁场.     

Deformation of Si under conditions of the combined influence of an electric current and a magnetic field

Changes in the physical and mechanical properties of single crystals of n- and p-type silicon are investigated under the combined influence of a constant electric current and a magnetic field and an electric current separately. There is a slight increase in the resistivity of Si as pressure is applied. Increased resistance to compressive deformation is observed under the combined influence of a magnetic field and an electric cur- rent during compression, while increased plasticity is seen under the sole influence of an electric current for p-Si samples. There is an opposite effect for samples of n-Si. Increased plasticity is observed under the combined influence of a magnetic field and an electric current during compression, while increased strength is seen under the sole influence of an electric current. Surface microstructures of deformed samples are studied. A possible physical explanation for the observed phenomena is proposed.     

Influence of an electric field on the ferromagnetic resonance in a plane-layered magnetic system

The influence of an electric field on the ferromagnetic resonance (FMR) in a multilayer magnetic system consisting of two magnetic layers separated by a thin nonmagnetic interlayer has been investigated. It has been shown that, upon the excitation of magnetization oscillations by a microwave magnetic field, the eigenfrequencies of the ferromagnetic resonance depend on the stationary electric field applied in the plane of the layers. It has also been demonstrated that, in this system, high-frequency magnetization oscillations can be excited by an electric microwave field. The results of the investigation of the polarization properties of the excitation mechanism indicate that this effect can be observed experimentally.     

The dynamic behavior of magnetic fluid adsorbed to small permanent magnet in alternating magnetic field

The dynamic behavior of a magnetic fluid adsorbed to a small NdFeB permanent magnet subjected to an alternating magnetic field was studied with a high speed video camera system. The directions of alternating magnetic field are parallel and opposite to that of the permanent magnet. It was found that the surface of magnetic fluid responds to the external alternating magnetic field in elongation and contraction with a lot of spikes. Generation of a capillary magnetic fluid jet was observed in the neighbourhood of a specific frequency of alternating field. The effect of gravitational force on surface phenomena of magnetic fluid adsorbed to the permanent magnet was revealed.     

Inverse beta decay of arbitrarily polarized neutrons in a magnetic field

We calculate the cross-section of the inverse beta decay process,v _e+n → p+e, in a background magnetic field which is much smaller than m _p ^2/e . Using exact solutions of the Dirac equation in a constant magnetic field, we find the cross-section for arbitrary polarization of the initial neutrons. The cross-section depends on the direction of the incident neutrino even when the initial neutron is assumed to be at rest and has no net polarization. Possible implications of the result are discussed.