电光与磁光效应的互补特性及其传感应用

研究了兼有电光效应和磁光效应的晶体内电光与磁光效应的互补特性及其传感应用. 在光强度调制条件下, 晶体中偏振光波的电光调制与磁光调制具有互相补偿的效果, 从而能够使输出光强度保持为一个固定值. 基于这种互补特性, 提出了一种利用单块闪烁锗酸铋(Bi₄Ge₃O₁₂, BGO)晶体的电光补偿型光学电流(磁场)传感器, 其光学传感单元由两个偏振器和一块平行四边形BGO晶体组成. 该晶体自身能够产生π/2的光学相位偏置, 同时兼用作电流传感和电光补偿元件, 通过控制BGO晶体的外加电压, 能够实时补偿被测电流(磁场)变化引起的磁光旋转角和输出光强度的变化, 从而实现电流(磁场)的闭环光学测量. 实验测量了5.0 A范围内的工频交流电流, 所需要的电光补偿电压约为21.2 V/A, 补偿电压与被测电流之间具有良好的线性关系, 其非线性误差低于1.7%.     

Cole-Cole Analysis of the Superspin Glass System Co 80 Fe 20 /Al 2 O 3

Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co 80 Fe 20 ( t )/Al 2 O 3 (3 nm)] 10 , t = 0.9 and 1.0 nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al 2 O 3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 h T h 100 K at different dc fields and as a function of frequency 0.01 h f h 1000 Hz. The spectral data were successfully analyzed by use of the phenomenological Cole-Cole model, giving a power-law temperature dependence of the characteristic relaxation time c and a high value for the polydispersivity exponent, f , 0.8, typical of spin glass systems.     

Large polarization rotation via atomic coherence

We report significant enhancement of the nonlinear Faraday rotation in optically thick Rb vapor. Polarization rotation angles as large as 10 rad were observed for what is believed to be the first time for sub-Gauss magnetic fields. The use of this effect for high-precision magnetometry is also discussed.     

Confinement of chiral magnetic modulations in the precursor region of FeGe

We report on magnetic susceptibility and specific heat measurements of the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at TC = 278.2(3) K. Pronounced anomalies in the field-dependent χac(H) data as well as in the corresponding imaginary part χ'ac(H) reveal a precursor region around TC in the magnetic phase diagram. The occurrence of a maximum at T0 = 279.6 K in the zero-field specific heat data indicates a second-order transition into a magnetically ordered state. A shoulder evolves above this maximum as a magnetic field is applied. The field dependence of both features coincides with crossover lines from the field-polarized to the paramagnetic state deduced from χac(T) at constant magnetic fields. The experimental findings are analyzed within the standard Dzyaloshinskii theory for cubic helimagnets. The remarkable multiplicity of modulated precursor states and the complexity of the magnetic phase diagram near the magnetic ordering are explained by the change of the character of solitonic inter-core interactions and the onset of specific confined chiral modulations in this area.     

CPT Magnetometer with Atomic Energy Level Modulation

We propose and experimentally investigate a coherent population trapping state based magnetometer prototype with ^87Rb atoms. Through modulating Zeeman sublevels with an ac magnetic field, not only a phase sensitive detection scheme suitable for miniature magnetometer is realized, but also the detection resolution of magnetic field intensity could be improved by a factor of two. Our study result indicates that it is a promising low power consumption miniature sensitive low magnetic field sensor offering spatially resolved measurement at the sub-millimetre level.     

Magneto and Electro-Optic Intensity Modulator Based on Liquid Crystal and Magnetic Fluid Filled Photonic Crystal Fiber

We propose a novel light intensity modulator based on magnetic fluid and liquid crystal(LC) filled photonic crystal fibers(PCFs). The influences of electric and magnetic fields on the transmission intensity are theoretically and experimentally analyzed and investigated. Both the electric and magnetic fields can manipulate the molecular arrangement of LC to array a certain angle without changing the refractive index of the LC. Therefore, light loss in the PCF varies with the electric and magnetic fields whereas the peak wavelengths remain constant. The experimental results show that the transmission intensity decreases with the increase of the electric and magnetic fields. The cut-off electric field is 0.899 V/um at 20 Hz and the cut-off magnetic field is 195 m T. This simple and compacted optical modulator will have a great prospect in sensing applications.     

Hall effect measurements using low ac magnetic fields and lock-in technique on field effect transistors with molybdenum disulfide channels

Hall effect measurements conventionally rely on the use of dc magnetic fields. For electronic devices made of ultrathin semiconducting materials, such as molybdenum disulfide (MoS₂), the dc Hall effect measurements have practical difficulties. Here, we report the results of the Hall effect measurements using ac magnetic fields and a lock-in detection of the Hall voltage for field effect transistors with ultrathin MoS₂ channels. The ac Hall effect measurements have some advantages over the dc measurements. The carrier concentration and the Hall mobility were estimated as a function of gate voltage from the results of the ac Hall effect measurements. They used a magnetic field strength that was lower by two orders of magnitude than those used in prior studies on MoS₂ devices, which relied on dc magnetic fields.     

Spin-polarized current in double quantum dots

We analyze the transport through asymmetric double quantum dots with an inhomogeneous Zeeman splitting in the presence of crossed dc and ac magnetic fields.A strong spin-polarized current can be obtained by changing the dc magnetic field.It is mainly due to the resonant tunnelling.But for the ferromagnetic right electrode,the electron spin resonance also plays an important role in transport.We show that the double quantum dots with three-level mixing under crossed dc and ac magnetic fields can act not only as a bipolar spin filter but also as a spin inverter under suitable conditions.     

Current oscillation and chaotic dynamics in superlattices driven by crossed electric and magnetic fields

We have theoretically studied current oscillation and chaotic dynamics in doped GaAsAlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.     

Spin-wave modes and their intense excitation effects in Skyrmion crystals

We theoretically study spin-wave modes and their intense excitations activated by microwave magnetic fields in the Skyrmion-crystal phase of insulating magnets by numerically analyzing a two-dimensional spin model using the Landau-Lifshitz-Gilbert equation. Two peaks of spin-wave resonances with frequencies of ～1 GHz are found for in-plane ac magnetic field where distribution of the out-of-plane spin components circulates around each Skyrmion core. Directions of the circulations are opposite between these two modes, and hence the spectra exhibit a salient dependence on the circular polarization of irradiating microwave. A breathing-type mode is also found for an out-of-plane ac magnetic field. By intensively exciting these collective modes, melting of the Skyrmion crystal accompanied by a redshift of the resonant frequency is achieved within nanoseconds.     

Effect of a dc magnetic field on the magnetization relaxation of uniaxial single-domain ferromagnetic particles driven by a strong ac magnetic field

The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker-Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is demonstrated that for a nonzero bias field as the magnitude of the ac field increases the reversal time first increases and having attained its maximum at some critical value of the ac field, decreases exponentially.     

Angular dependence of the photothermally modulated magnetic resonance in Gd thin films

In this paper we use electron spin resonance and photothermally modulated magnetic resonance techniques to investigate gadolinium thin films as a function of the orientation of the film surface with respect to the external magnetic field and of the temperature, around the magnetic phase transition temperature. We observe that, in the ferromagnetic phase, the resonance line is shifted up to higher external magnetic fields when the angle between the film surface and the field increases, revealing the magnetic anisotropy of the sample. At the same time, when the temperature is augmented to values higher than the phase transition temperature, the external field of the resonance collapses back to the expected value in the paramagnetic phase for all orientations. We also demonstrated that, even for the perpendicular orientation (magnetic field perpendicular to the sample surface), the photothermally modulated magnetic resonance signal is maximized near the magnetic phase transition temperature. Furthermore, in the ferromagnetic phase the photothermally modulated magnetic resonance intensity is very sensitive to the orientation, showing a significant enhancement in the perpendicular direction.     

Microwave solitons in molecular magnets via electromagnetically induced transparency

Using the Schrödinger-Maxwell equations, we investigate the formation of microwave solitons in a crystal of molecular magnets. This system is subjected to one dc magnetic field and two (probe and coupling) ac resonant magnetic fields. The results show that the probe magnetic field can freely propagate in a crystal of molecular magnets due to quantum interference. Furthermore, within certain parameter range, both bright and dark microwave solitons can occur in such a highly resonant medium. We also obtain the analytical expressions for the phase shift and absorption coefficient of the probe magnetic field.     

Field-induced magnetic transition in a mixed rare-earth aluminum garnet Er<Subscript>2</Subscript>HoAl<Subscript>5</Subscript>O<Subscript>12</Subscript>

The temperature dependence of the ac magnetic susceptibility of a single-crystal mixed rare-earth garnet Er₂HoAl₅O₁₂ has been investigated within the range from 1.8 to 300 K in a zero constant field and in applied bias fields of up to 9 T. In the absence of a constant magnetic field the magnetic susceptibility followed the Curie–Weiss law. The application of a constant magnetic field caused a magnetic phase transition, the temperature of which increased with increasing magnetic field. The temperature of the maximum of the ac magnetic susceptibility, which is a characteristic of the phase transition, did not show a noticeable dependence on the frequency of the alternating magnetic field.     

Spin dynamics of magnetic resonance with parametric modulation in a potassium vapor cell

A typical magnetic-resonance scheme employs a static bias magnetic field and an orthogonal driving magnetic field oscillating at the Larmor frequency, at which the atomic polarization precesses around the static magnetic field. Here we demonstrate both theoretically and experimentally the variations of the resonance condition and the spin precession dynamics resulting from the parametric modulation of the bias field. We show that the driving magnetic field with the frequency detuned by different harmonics of the parametric modulation frequency can lead to resonance as well. Also, a series of frequency sidebands centered at the driving frequency and spaced by the parametric modulation frequency can be observed in the precession of the atomic polarization. We further show that the resonant amplitudes of the sidebands can be controlled by varying the ratio between the amplitude and the frequency of the parametric modulation. These effects could be used in different atomic magnetometry applications.     

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

We analyze uniform orientation phases in soft ferronematics (suspensions of magnetic nanoparticles in nematic liquid crystals) induced by electric and magnetic fields. It is shown that the competition between the electric and magnetic fields can lead to various sequences of orientation transitions in a ferronematic depending on the energy of coupling between the director and magnetization. We obtain and analyze phase diagrams of these transitions. A sequence of re-entrant transitions in the orientation structure (angular phase-homeotropic phase-angular phase-planar phase) is predicted for a certain range of the coupling energies and electric field strengths.     

Phase composition and magnetism of combustion products in Ba–Fe–O compounds synthesized under applied DC electric field

The effect of non-contact electric fields applied directly during combustion synthesis of barium ferrites is presented. The microstructure, phase composition and magnetic characteristics of the combustion products were obtained by scanning electronic microscopy (SEM), X-ray diffraction and vibrating sample magnetometry (VSM). A correlation was found between the observed magnetic properties of the product and the magnitude of the applied electric field. The role of oxygen exchange activated by the electric field during the combustion synthesis is discussed.     

Features of motion of interacting magnetic liquid drops

The features of simultaneous motion of two identical drops of magnetic liquid, which are determined by their hydrodynamic and magnetic interaction, are considered. The changes in the trajectories of drops initially moving in the same direction, caused by a variation of direction and magnitude of the applied dc and ac magnetic field, are studied experimentally and substantiated theoretically. The possibility of effective control of the motion of magnetic liquid drops by applying magnetic fields is demonstrated.     

Vortex pump for dilute bose-einstein condensates

The formation of vortices by topological phase engineering has been realized experimentally to create the first two- and four-quantum vortices in dilute atomic Bose-Einstein condensates. We consider a similar system, but in addition to the Ioffe-Pritchard magnetic trap we employ an additional hexapole field. By controlling cyclically the strengths of these magnetic fields, we show that a fixed amount of vorticity can be added to the condensate in each cycle. In an adiabatic operation of this vortex pump, the appearance of vortices into the condensate is interpreted as the accumulation of a local Berry phase. Our design can be used as an experimentally realizable vortex source for possible vortex-based applications of dilute Bose-Einstein condensates.     

On the possibility for flux penetration into a Campbell regime near the onset of irreversibility in high temperature superconductors

The onset of dissipation and of harmonic generation in response to an ac magnetic field of high temperature superconductors are used for the identification of the irreversibility line below which dissipation occurs due to flux motion in the sample. We demonstrate, however, that in the presence of a static magnetic field and for low amplitude ac fields, the penetration close to the onset may occur into the Campbell regime in which the vortex motion induced by the ac field occurs inside the pinning potential well.