Interaction between large and small signals in a resonant transmission line based on magnetostatic waves

The influence of a large signal on the characteristics of a small signal near the frequency of the former is experimentally studied for the case of their simultaneous propagation through a resonant transmission line based on backward volume magnetostatic waves. The effects observed are caused by the excitation of spin waves in a ferromagnetic film at different large-signal frequencies and by the back influence of excited spin-wave packets on the amplitude and phase characteristics of the small signal.     

Spatial–frequency selection of magnetostatic waves in a two-dimensional magnonic crystal lattice

Features of the propagation of magnetostatic waves in a tangentially magnetized magnonic crystal structure based on iron–yttrium garnet with a two-dimensional array of grooves on the surface are studied. Numerical simulation is performed by the finite element method and the dispersion characteristics and the spatial distribution of fields of eigenmodes of surface magnetostatic waves propagating in this structure are calculated. The characteristics of waves in the magnonic crystal structure are experimentally studied by means of Brillouin scattering. It is shown that the formation of waveguide channels is possible when the frequency of the input signal is close to the frequency of the band gap of the structure.     

Emission of noise-like spin wave packets in the presence of three-magnon decay processes and the kinetic instability of waves in a ferromagnetic film

The features of spin wave emission from a ferromagnetic film in the direction of the propagation of a surface magnetostatic wave have been experimentally investigated at various input signal powers. Radiation in the form of two noise-like spin wave packets has been detected near the frequency corresponding to half the pump frequency. This radiation is caused by three-magnon processes of the decay of a surface magnetostatic wave and by the kinetic instability of spin waves.     

Magnetization dynamics in epitaxial films induced by femtosecond optical pulses near the absorption edge

Peculiarities of the magnetization dynamics induced in iron garnet films by laser pulses with a frequency detuning near the absorption edge have been studied experimentally. It has been found that the dependence of the observed signal amplitude on the pumping energy becomes nonmonotonic with an increase in the pumping frequency. At the same time, the pumping energy corresponding to the maximum amplitude of the signal and this maximum signal amplitude decrease. Moreover, the signal amplitude starts to decrease with an increase in the pumping energy at frequencies within the absorption band. The observed phenomena are possibly caused by generation of magnetostatic spin waves and the effect of ultrafast optically induced demagnetization.     

Wave front reversal of surface magnetostatic waves

The wave front reversal (WFR) of non-reciprocal waves has been investigated. The experiment was performed using surface magnetostatic waves (SMSW) excited by a pulsed microwave signal of the carrier frequency ∼4.7 GHz in an epitaxial yttrium–iron garnet (YIG) film. The WFR was realized by pulsed parametric pumping of a double frequency. It was shown that WFR with high efficiency can be achieved for SMSW having relatively small wavenumbers k∼10² rad/cm.     

圆柱状铁磁体中的偶极-交换自旋波

对于轴向磁化的圆柱状铁磁体,同时考虑偶极作用和交换作用,得到了偶极-交换自旋波的本征方程,给出了频谱的数值结果。当自旋波波矢β较小,即偶极能起主要作用时,相当于静磁模的结果,β较大,即交换能起主要作用时,过渡到宏观自旋波交换模理论的结果。     

Reflection of electromagnetic waves from the surface of a cubic ferrite

The reflectivity of electromagnetic waves incident on the surface of a semi-infinite nonconducting cubic ferrite was found both analytically and numerically with inclusion of spin-wave damping. The frequency and field dependences of the reflectivity were found for various values of the damping parameter and the anisotropy and magnetostriction constants in a region far from the point of the orientational phase transition and at this point. It is shown that the reflectivity has peaks near the frequencies of ferromagnetic, magnetoacoustic, and magnetostatic resonances. The peak amplitude decreases with increasing spin-wave damping. At frequencies below the magnetoelastic gap, the reflectivity can take on anomalously small (down to zero) and anomalously large (up to unity) values. These frequencies can lie in the microwave range.     

Wideband chaotic oscillation in a self-oscillatory system with a nonlinear transmission line on magnetostatic waves

Wideband chaotic microwave oscillation in a ring self-oscillatory system is studied. The system includes a solid-state power amplifier and a wideband nonlinear transmission line with a ferromagnetic film in which magnetostatic waves of different types are excited. It is found that the eigenmodes of the self-oscillatory system excited in the passband of the transmission line on magnetostatic waves become noisy because of spin wave parametric excitation due to the magnetostatic wave and nonlinearity of the power amplifier. A continuous spectrum of modes observed in the wideband chaotic signal is associated with the presence of a descending portion in the dynamic characteristic of the nonlinear transmission line, which arises when a magnetostatic surface wave is excited.     

Spectrum of magnetostatic waves in a ferromagnet with a moving superlattice of domain walls

The spectral properties of magnetostatic waves in a ferromagnet with a moving periodic domain structure are considered within the exchange-free magnetostatic approximation. It is demonstrated that the Doppler frequency shift caused by the domain-wall motion leads to the splitting of the spectrum of each magnetostatic wave mode into two dispersion branches, namely, into high-frequency and low-frequency branches. It is established that the larger the mode number, the larger the separation between these branches with respect to the mode spectrum in the presence of the static domain structure.     

Nonlinear absorption and refractive index of Brillouin scattered mode in piezoelectric semiconductor plasmas by an applied magnetic field

With the aid of hydrodynamic model, a detailed analytical investigation is made of the stimulated Brillouin scattering (SBS) of the Stokes component of the scattered wave in piezoelectric-doped semiconductor plasma subjected to a magnetostatic field. The origin of the SBS process lies in the third-order nonlinear optical susceptibility arising due to the induced nonlinear current density and acoustic perturbations internally generated due to crystal properties such as piezoelectricity and electrostriction. Using the coupled mode theory of plasmas the effective refractive index and absorption coefficient are determined via the effective susceptibility. The influence of piezoelectricity, magnetostatic field and doping concentration has been explored. The analysis has been applied to both noncentrosymmetric and centrosymmetric crystals. Numerical estimates are made for n-type InSb crystal duly irradiated by a frequency doubled 10.6 μm CO₂ lasers. Results are found to be well in agreement with available literature. The analysis establishes that a large nonlinear refractive index and small absorption coefficient can easily be obtained under moderate excitation intensity in piezoelectric doped magnetized semiconducting crystal, which proves its potential as candidate material for the fabrication of cubic nonlinear devices.     

Comparative Study on Large and Small Signal Theory for Intensity Modulation Response of Semiconductor Lasers Including Higher-Order Dispersion Terms

In this article, the comparison of large signal theory and small signal theory has been done with dispersive propagation of optical signal with IMDD (Intensity Modulation Direct Detection) systems for semiconductor lasers with higher-order dispersion terms. The expressions for an exact large signal theory and small signal theory including higher-order dispersion terms for propagation of an optical wave with sinusoidal amplitude and frequency modulation in a dispersive fiber have been derived. It is observed that small signal theory is more sensitive compared to large signal theory in terms of intensity modulation/direct detection systems. Also, it is reported that for large signal analysis the higher-order effects of dispersion can be ignored, whereas for small signal theory, the higher-order effects can be ignored for lower modulation frequencies only. The variation in the transfer function for various values of modulation indices are greater for small signal analysis than for large signal analysis. Also, as the intensity modulation index is increased, there is a decrease in the value of transfer function. The large signal model approximates the small signal model for lower values of the intensity modulation index.     

Comparative Study on Large and Small Signal Theory for Intensity Modulation Response of Semiconductor Lasers Including Higher-Order Dispersion Terms

In this article, the comparison of large signal theory and small signal theory has been done with dispersive propagation of optical signal with IMDD (Intensity Modulation Direct Detection) systems for semiconductor lasers with higher-order dispersion terms. The expressions for an exact large signal theory and small signal theory including higher-order dispersion terms for propagation of an optical wave with sinusoidal amplitude and frequency modulation in a dispersive fiber have been derived. It is observed that small signal theory is more sensitive compared to large signal theory in terms of intensity modulation/direct detection systems. Also, it is reported that for large signal analysis the higher-order effects of dispersion can be ignored, whereas for small signal theory, the higher-order effects can be ignored for lower modulation frequencies only. The variation in the transfer function for various values of modulation indices are greater for small signal analysis than for large signal analysis. Also, as the intensity modulation index is increased, there is a decrease in the value of transfer function. The large signal model approximates the small signal model for lower values of the intensity modulation index.     

Effects of metal and “magnetic wall” on the dispersion characteristic of magnetostatic waves

The dispersion relation of magnetostatic waves tangentially magnetized to saturation ferrite film, with a “magnetic wall” condition (tangential component of microwave magnetic field is equal to zero) on one of the film surface and with a metal condition on the opposite surface is analyzed. The dispersion characteristics show that unidirectional magnetostatic waves appear in this structure: they can transfer energy in one direction only and fundamentally cannot transfer energy in the opposite direction. The dispersion-free propagation of magnetostatic waves also is possible in the structure in a wide frequency interval.     

Magnetostatic interactions in dense nanowire arrays

We demonstrate the importance of magnetostatic interactions in dense arrays of ferromagnetic nanowires. Beginning from a simple micromagnetic model, we have calculated the interaction field for saturated magnetization in the plane of the array (perpendicular to the axes of the wires) and normal to the plane, using a hybrid (numerical and analytical) strategy. The slope of interaction field versus wire length changes dramatically at the transition between a dipolar regime (at very small lengths) and a monopolar regime (for longer nanowires). We present the interaction fields and the applied fields at saturation for large nanowire arrays. These results are compared with experiment for electrodeposited arrays, and very good agreement is obtained. This shows that the high field behavior of such arrays is dominated by magnetostatic effects and that a nanowire array behaves like a double-sided distribution of magnetic monopoles.     

静磁波法研究材料的磁性

设计了可在片状样品上宽频带内(2—8GHz)激发,接收静磁波的器件。在此基础上建立了测量静磁波频率-磁场关系的系统。用该系统不仅可以研究材料中的静磁波的激发,传播特性,也可以用来测量材料的基本磁性。用于基本研究测量时该系统显示出与铁磁共振方法不同的特点:样品不置于谐振腔内,可以在对样品进行某些处理的同时作测量。 关键词：     

北极跨冰层水中声源方位估计实验与分析

为了获取北极冰下声源方位信息,在北极冰层上方布放检波器和冰下投放声弹声源开展了冰下声源的分频段方位估计实验.研究结果表明,冰下声源方位估计的误差、准确率与检波器接收信号频段(或冰下声源信号频段)有关.采用低频段的接收信号进行方位估计时准确率较高,误差均值较小,所用接收信号频段≤250 Hz时,方位估计准确率可达72.9...     

掺铒光纤放大器的频率响应和瞬态增益

根据掺铒光纤放大器（ＥＤＦＡ）的速率方程和传播方程,忽略自发辐射噪声,推导出了小信号调制时频率响应函数的解析表达工以及大信号调制时描述瞬态增益的方程。数值计算表明,当输入信号的调制频率大于５ｋＨｚ时,放大器的瞬态增益变化很小,频率响应很弱。通过计算机仿真,描述了关闭一个信道时的放大器的增益变化过程。     

Array anisotropy in structured thin film arrays: Influence on the magnetodynamics

The dynamic magnetic properties of soft-ferromagnetic thin film element arrays are strongly influenced by long range interelement magnetostatic interaction. In order to estimate the effective array dipolar field a quantitative model is presented, which is based on the superposition of stray fields that arise from the neighborhood of a reference element. Kittel's equation, that describes the magnetodynamics, is generalized for magnetically saturated arrays by additional array dipolar field terms. Measurements of the magnetodynamic response of quasi-saturated arrays with a rectangular base agree with theoretical predictions. Thus, our model allows the estimation of the frequency of the uniform precessional mode in mesoscopic thin film arrays with non-negligible magnetostatic interaction.     

分数阶双稳系统中的非周期振动共振

在受二进制非周期信号和周期方波信号激励的分数阶双稳系统中,研究了非周期振动共振问题,用于微弱非周期信号的检测和增强.当非周期信号脉宽较大时,系统为小参数,通过调节周期方波信号的幅值,能够实现非周期振动共振.当非周期信号脉宽较小时,分别通过变尺度法和二次采样法实现了非周期振动共振.使用变尺度法,得到的大参数等价系统能够匹配任意小的非周期信号脉宽,其中变尺度系数是该方法在使用过程中需要选择的关键参数.使用二次采样法,二次采样后得到的非周期信号具有较大的脉宽,能够匹配原先的小参数系统,其中二次采样频率比是该方法使用过程中的关键参数.这两种方法虽然实现非周期振动共振的物理过程不同,但能够达到相同的效果.系统阶数对振动共振产生影响,随着阶数的增大,发生最佳振动共振时所需要的辅助信号幅值变大,同时系统输出的最佳时间序列与输入非周期信号之间的相似性增强.     

On the accuracy of FFT based magnetostatic field evaluation schemes in micromagnetic hysteresis modeling

Micromagnetic hysteresis models for large, bulk like samples are useful for the identification of relations between microscopic material properties and macroscopic magnetic behavior. To bridge the gap between the nanometer space scale of the micromagnetic theory and the large sample dimensions, time and memory efficient numerical schemes are needed. In micromagnetic computations, fast Fourier transforms (FFTs) have been widely adopted to speed up magnetostatic field computations. In this paper, two FFT schemes are compared. The first scheme evaluates the magnetostatic field directly starting from the magnetization and has a large accuracy, while in the second scheme the magnetostatic field is derived from the scalar magnetic potential resulting in a reduced accuracy but also in a CPU time reduction for a magnetostatic field evaluation to 65% and a reduction of memory requirements to 55%. The influence of the low accuracy evaluations on the simulated macroscopic hysteresis behavior is studied. Therefore, comparison is made with the influence of thermal effects in hysteresis simulations. It is found that the resulting changes in macroscopic hysteresis behavior are of the same order of magnitude as the ones obtained when thermal fluctuations are taken into account in the high accuracy computations.