Antimagnons in ferrimagnets

Purely antiferromagnetic intrinsic oscillations of magnetic ions in a tetragonal ferrimagnet are considered. The magnetic ions occupy two double positions (forming four magnetic sublattices) so that the center of symmetry for each position is not a closed element. Not involving the total magnetization vector, the oscillations are not excited by the magnetic field; however, the oscillations can be excited by an alternating electric field or by a combination of a constant electric field and the alternating magnetic field of a certain frequency. This phenomenon is a dynamic manifestation of the magnetoelectric interaction. These oscillations, representing a new special type of spin waves (magnons), were called antimagnons. The intrinsic frequencies of antimagnons, as well as the corresponding susceptibilities, were determined. Quantitative estimates were obtained for a Mn₂Sb-based ferrimagnetic phase in both easy-axis and easy-plane orientation states.     

Excitation of spin remagnetization waves in systems with spin-orbit coupling

Elementary excitations of a new type in paramagnetic materials with spin-orbit interaction are predicted theoretically within the Rashba model and named the spin remagnetization waves. These normal modes arise from rotation of the spin magnetic moment in a medium where the electric field contains a constant component and a traveling wave component. A method is proposed for exciting these vibrations by illuminating the sample with an oscillating interference pattern. The spin remagnetization waves can be experimentally detected by measuring the dependence of the current through the sample on the oscillation frequency and wave vector of the interference pattern.     

Hybrid helicity and magneto-vorticity flux conservation in superdense npe-plasma

In this paper, it is shown that the magnetic helicity dissipation per unit volume, coupled with the longitudinal conductivity, causes enhancement of the kinematic rotation of the electric (and magnetic) lines if the npe-plasma vorticity vector aligns with the electric (or the magnetic) field. In the case of a rigidly rotating npe-plasma under the influence of a strong magnetic field, the electric lines are rotating faster than the magnetic lines. It is deduced that the orthogonality of the electric and magnetic fields is an essential condition for the conduction current to remain finite in the limit of infinite electric conductivity of the npe-plasma. In this case, the magnetic field is not frozen into the npe-plasma, but the magnetic flux in the magnetic tube is conserved. The hybrid helicity is conserved if the “magneto-vorticity” vector is tangent to the level surfaces of constant entropy per baryon. The “magneto-vorticity” lines are rotating on the level surfaces of constant entropy per baryon due to the electromagnetic energy flow in the direction of the npe-plasma vorticity and the chemical potential variation locked with the kinematic rotation of the npe-plasma flow lines. In the case of an isentropic npe-plasma flow, there exists a family of timelike 2-surfaces spanned by the “magneto-vorticity” lines and the npe-plasma flow lines. In this case, the electric field is normal to such a family of timelike 2-surfaces. Maxwell like equations satisfied by “magneto-vorticity” bivector field are solved in axially symmetric stationary case. It is shown that the npe-plasma is in differential rotation in such a way that its each plasma shell (i.e., plasma surface spanned by “magneto-vorticity” lines) is rotating differentially without continually winding up “magneto-vorticity” lines frozen into the npe-plasma. It is also found that gravitational isorotation and Ferraro’s law of isorotation are intimately connected to each other because of coexistence of both the plasma vorticity and the magnetic field due to interaction between the electromagnetic field and npe-plasma flows.     

Faraday rotation in a single-mode fiber with controlled birefringence

Faraday rotation within a polarization non-conserving single-mode optical fiber is used for ac magnetic field measurement. For that purpose the intrinsic fiber birefringence on the path to and from the Faraday rotation section is controlled. As control devices externally applied stress birefringence and phase retarder plates were used. With respect to magnetic field measurements on high electric potentials all control devices are placed at fiber input and output only.     

Threshold Frequency of Helical Electron–Hole Plasma Instability

Experimental evidence on the dependence of the threshold frequency of silicon oscillistors on the threshold electric field strength, magnetic induction, temperature, and injecting-contact separation is presented. In the temperature interval, where the weak magnetic field criterion is roughly satisfied, the experimental results are shown to be adequately explained by the classical theory of the bulk helical instability of an extrinsic plasma. The threshold frequency in this temperature interval is determined by the sum of two components. One component is due to the ambipolar drift of helical plasma perturbations, and the other results from the presence of the charge-carrier concentration gradient in a direction normal to the vectors of the electric field strength and magnetic induction. In short oscillistors (0.85·10^–3, 2.38·10^–3 m) at 77 K, a semiconductor plasma, wherein the helical instability is excited, approximates an intrinsic plasma, and the threshold frequency is determined by the rotation rate of helical perturbations.     

Rotational properties of annulus dusty plasma in a strong magnetic field

The collective dynamics of an annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radiofrequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In the absence of the magnetic field, the dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in the anticlockwise direction with increasing magnetic field (B > 0.02 T ), and the constant value of the angular frequency at various strengths of the magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B > 0.6 T ) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B > 1 T ) lowers the angular frequency. Low value of the angular frequency is expected by reducing the width of the annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of a magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of a magnetic field.     

Higher Harmonics and Transport Coefficients of Plasmas in Circulary Polarized Magnetic Fields and Additional Electromagnetic Fields

With the methods of kinetic theory on the basis of the Boltzmann and Fokker-Planck kinetic equations the behaviour of a Lorentz plasma in a circularly polarized (rotating) magnetic field (rotation frequency ω), an alternating electric field (frequency ω′) and additional constant electromagnetic fields is investigated. By means of a generalized Fourier expansion it is shown that the above fields create in the plasma currents of the frequencies 0, ω′, ω–ω′,ω, ω+ω′, 2ω–ω′, 2ω, and 2ω+ω′. Transport coefficients are calculated explicitly and the validity of Onsager reciprocity relations and that of Kronig-Kramers relations is discussed. The special case of the electric field induced by the rotating magnetic field is treated separately. Finally, problems of plasma containment are discussed.     

Probing Dual Asymmetric Transverse Spin Angular Momentum in Tightly Focused Vector Beams in Optical Tweezers

The spin-orbit interaction (SOI) of light generated by tight focusing in optical tweezers is regularly employed in generating angular momentum - both spin and orbital - the effects being extensively observed in trapped mesoscopic particles. Specifically, the transverse spin angular momentum (TSAM), which arises due to the longitudinal component of the electromagnetic field generated by tight focusing is of special interest, both in terms of fundamental studies and associated applications. This study provides an effective and optimal strategy for generating TSAM in optical tweezers by tightly focusing first-order radially and azimuthally polarized vector beams with no intrinsic angular momentum (AM) into a refractive index stratified medium. The choice of such input fields ensures that the longitudinal spin angular momentum (LSAM) arising from the electric (magnetic) field for the radial (azimuthal) polarization is zero. As a result, the effects of the electric and magnetic TSAM are exclusively observed separately in the case of input first-order radially and azimuthally polarized vector beams on single optically trapped birefringent particles. This research opens up new and simple avenues for exotic and complex particle manipulation in optical tweezers.     

垂直入射条件下金属环的谐振特性

本文研究垂直入射条件下水平极化和垂直极化时金属开口谐振环(split ring resonator, SRR)的电磁响应行为. 通过分析这两种情况下的透射系数、介电常数和磁导率, 发现垂直极化时, SRR可以产生电谐振实现负介电常数, 其频段远高于磁谐振频段; 水平极化时, SRR只能产生磁谐振实现负磁导率, 其频段与水平入射时的SRR的磁谐振频段相对应. 通过仿真对此进行了证明, 并对产生电谐振和磁谐振的原因进行了分析.     

Equivalent representation of orthogonal-field electrodynamics by system of covariant lines of force

It is shown that the electrodynamics of orthogonal magnetic and electric fields can be represented as the dynamics of covariant lines of force. Such a representation is provided for the Lienard-Wichert field of an arbitrary moving charge and the field of a charge that moves uniformly about a circle. The four vector of the electric lines of force is written as the sum of the four vector of the charge and the radius four vector directed along the light cone to the observation point. This vector is a solution of an equation that formally coincides with the equation of motion of the magnetic moment in external fields for a zero intrinsic magnetic moment. The electromagnetic field is reconstructed according to a system of lines that correspond to the total equation of motion of the magnetic moment. Such a field for a uniformly circulating charge is examined.Erevan Physics Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, Nos. 3, 4, pp. 313–323, March–April, 1992.     

抽运-检测型非线性磁光旋转铷原子磁力仪的研究

报道了一种抽运-检测型的非线性磁光旋转铷原子磁力仪.其原理是线偏振光通过处于外磁场环境中被极化的原子介质后,由于原子对线偏振光中左、右圆偏成分不同的吸收和色散,导致光的偏振方向会产生与磁场相关的转动.分析了该磁力仪的工作原理,并测试了它对不同磁场大小的响应.测试结果表明,磁力仪测量范围为100—100000 nT,极限灵敏度为0.2 p T/Hz~(1/2),磁场分辨率为0.1 p T.进一步研究了不同磁场下原子系综极化态的横向弛豫时间,讨论了原子磁力仪高磁场采样率的获得方法.本文的原子磁力仪在5000—100000 n T的磁场测量范围内磁场采样率可实现1—1000 Hz范围内可调,能够测量低频的微弱交变磁场.本文的研究内容为大磁场测量范围、高灵敏度、高磁场采样率的原子磁力仪研制提供了重要参考.     

Determination of electric-field, magnetic-field, and electric-current distributions of infrared optical antennas: a near-field optical vector network analyzer

In addition to the electric field E(r), the associated magnetic field H(r) and current density J(r) characterize any electromagnetic device, providing insight into antenna coupling and mutual impedance. We demonstrate the optical analogue of the radio frequency vector network analyzer implemented in interferometric homodyne scattering-type scanning near-field optical microscopy for obtaining E(r), H(r), and J(r). The approach is generally applicable and demonstrated for the case of a linear coupled-dipole antenna in the midinfrared spectral region. The determination of the underlying 3D vector electric near-field distribution E(r) with nanometer spatial resolution and full phase and amplitude information is enabled by the design of probe tips with selectivity with respect to E(∥) and E(⊥) fabricated by focused ion-beam milling and nano-chemical-vapor-deposition methods.     

Forced magnetic reconnection and field penetration of an externally applied rotating helical magnetic field in the TEXTOR tokamak

The magnetic field penetration process into a magnetized plasma is of basic interest both for plasma physics and astrophysics. In this context special measurements on the field penetration and field amplification are performed by a Hall probe on the dynamic ergodic divertor (DED) on the TEXTOR tokamak and the data are interpreted by a two-fluid plasma model. It is observed that the growth of the forced magnetic reconnection by the rotating DED field is accompanied by a change of the plasma fluid rotation. The differential rotation frequency between the DED field and the plasma plays an important role in the process of the excitation of tearing modes. The momentum input from the rotating DED field to the plasma is interpreted by both a ponderomotive force at the rational surface and a radial electric field modified by an edge ergodization.     

Magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet MnI2

Magnetic and dielectric properties with varying magnitude and direction of magnetic-field H have been investigated for a triangular-lattice helimagnet MnI_{2}. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-plane H. With every 60° rotation of H around the [001] axis, discontinuous 120° flop of the P vector is observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane H above 3 T, however, the stable q direction changes from q‖(110[ ˉover 0]) to q‖(110), leading to a change of P-flop patterns under rotating H. At the critical field region (～3 T), due to the phase competition and resultant enhanced q flexibility, the P vector smoothly rotates clockwise twice while the H vector rotates counterclockwise once.     

Electric-field-induced in-plane effective 90° magnetization rotation in Co_2FeAl/PMN-PT structure

The in-plane effective 90° magnetization rotation of Co_2 Fe Al thin film grown on PMN-PT substrate induced by the electric field is investigated at room temperature. The magnetic hysteresis loops under different positive and negative electric fields are obtained, which reveals remanent magnetization can be mediated by the electric field. Moreover, under positive electric fields, the obvious 90° magnetization rotation can be observed, while remanent magnetization is nearly unchanged under negative electric fields. The result is consistent with the electric field dependence of effective magnetic field,which can be attributed to the piezostrain effect in Co_2 Fe Al/PMN-PT structure. In addition, the piezostrain-mediated 90° magnetization rotation can be demonstrated by the result of resonance field changing with electric field in the measurement of ferromagnetic resonance, which is promising for the design of future multiferroic devices.     

Optical properties of a polarizable linear magneto-cosserat fluid

General optical properties of polarizable linear magneto-Cosserat fluids are investigated. The constitutive dielectric tensor is generalized to include the effects of magnetic field and rotation vector, and the general expression for the refractive index is obtained. If the optical effects of the magnetic field and rotation vector are negligible, the fluid shows the same birefringent phenomena as a stokesian fluid and if the effect of magnetic field or rotation vector is dominant, the fluid shows double circular refraction. The coupling effect of the magnetic field and rotation vector is considered and special cases of slow motion and low conductivity are also discussed.     

Der wendelnde Wasserstoffbogen

A hydrogen arc with a fixed lower and a movable upper end abruptly changes from its straight static form into a rotating conical helix when a critical current is exceeded. The chances of its winding becoming left or right-hand are equal if the earth magnetic field is compensated. A helix with a left winding, however, is always turning clockwise, a helix with a right winding counter-clockwise if looking axially in the direction of the expanding helical arc. The frequency of the rotation depends only on the current strength and is explained by the balance of the driving Lorentz forces due to the intrinsic magnetic field of the entire electrical circuit on one hand and the friction of the moving arc with its environment on the other. A corresponding model theory applied to measurements on a wall-stabilized H₂-Arc with 10–15 A in a quartz tube of 10 mm radius yields a reasonable arc diameter of 2.23 mm. The observed independence of the rotation frequency on the length of the arc can be derived from the balance equations as well. By superimposing an external axial magnetic field to the rotating arc its rotation frequency and simultaneously its expansion increase if the external field is parallel to the intrinsic one and viceversa. The experimental results are in full agreement with calculations based upon the model including the effect of the external field as an additional independent variable.     

Fe_3O_4单晶薄膜磁性电场调控的微磁学仿真研究

磁性薄膜磁学特性电场调控的相关研究对开发新型低功耗磁信息器件具有突出意义.本文基于电场调控磁性的基本理论,以OOMMF(Object Oriented Micro-Magnetic Frame)微磁学仿真软件为工具,研究了电场对生长于PZN-PT单晶衬底上Fe_3O_4单晶薄膜磁学特性的调控.研究结果显示:无外加电场时,薄膜表现出典型的软磁特性;沿衬底[001]方向施加的外加电场可以使得薄膜矫顽力、矩形比等磁学特性发生显著改变:当外加磁场沿[100]([010])时,施加正值(负值)电场强度可以显著增大薄膜的矫顽力与矩形比,当电场强度不小于0.6 MV/m时薄膜矩形比达到1.这是因为外加电场导致薄膜产生单轴应力各向异性,使得薄膜的等效磁各向异性发生了从无外电场下的面内四重磁晶各向异性向高电场下的近似单轴磁各向异性的过渡.外加1 MV/m与-1 MV/m的电场时等效易磁化轴分别沿[100]与[010]方向.另外,外加1 MV/m(-1 MV/m)的电场强度可以使得铁磁共振的频率增大(减小)接近1 GHz.     

Instability of electromagnetic waves in transversely magnetised semiconductor-plasmas

Using the hydrodynamic model of plasmas the general dispersion relation is derived in the collisiondominated regime when a d.c. magnetic field is applied (Y-axis) transversly to the propagation vector k (Z-axis), and the d.c. electric field is inclined to the Z-axis in the X-Z plane. The dispersion relation is solved for intrinsic and extrinsic semiconductors to explore the possibility of wave instability. The threshold conditions of wave oscillations are obtained. In n-InSb the frequency of the oscillation attains a maximum value when the electron cyclotron frequency is equal to the electron collision frequency. In intrinsic InSb instability is possible only in the long wavelength region for E₀ ? 10 kVm^?1 when B₀> 0.2 T, while for lower values of B₀, E₀ should be greater 20kVm^?1. The energy dependent collision frequency has a significant effect on the threshold frequency of oscillation.     

Magnetic-field-induced insulator-conductor transition in SU(2) quenched lattice gauge theory

We study the correlator of two vector currents in quenched SU(2) lattice gauge theory with a chirally invariant lattice Dirac operator with a constant external magnetic field. It is found that in the confinement phase the correlator of the components of the current parallel to the magnetic field decays much slower than in the absence of a magnetic field, while for other components the correlation length slightly decreases. We apply the maximal entropy method to extract the corresponding spectral function. In the limit of zero frequency this spectral function yields the electric conductivity of quenched theory. We find that in the confinement phase the external magnetic field induces nonzero electric conductivity along the direction of the field, transforming the system from an insulator into an anisotropic conductor. In the deconfinement phase the conductivity does not exhibit any sizable dependence on the magnetic field.