微波铁氧体材料自旋波共振线宽ΔH_k的测量

一、引 言 自旋波共振线宽ΔHk是微波铁氧体材料的重要参数,不仅材料的高功率性能与它有关,而且在实用中当材料工作在远离共振区时,它也表征损耗的大小.为了全面标定材料的性能,测量它是必要的. 在横向激励的微波铁氧体材料中的自旋波现象早在50年代初就已发现并作了全面的实验研究,而且有了理论解释.到了50年代末,Schlomann等人[1]观察到纵向激励的微波铁氧体中的自旋波幅度不稳定增长的现象.当直流偏置磁场H较低时,自旋波具有一定的波数k,随着H的增加,k减小;在特征磁场Hc下,k为零;这时,在与直流偏置磁场H方向垂直的方向上传播的、频串…     

杂质散射引起的亚铁磁体的共振线宽

本文得到了具有二个等价次点阵的椭球亚铁磁体的二支自旋波频谱,它们的频率对波矢的依赖关系与椭球铁磁体的相类似。基于此频谱和交换作用涨落引起的自旋波散射机制,计算了一致共振和交换共振的线宽。对一致共振,用等效铁磁模型得到的结论仍然成立:非磁性杂质离子和自旋等于同一次点阵上基质离子自旋的杂质离子,对一致共振线宽没有贡献。但这二类杂质离子对交换共振线宽有贡献。用我们的简化亚铁磁模型对各种铁氧体算得的一致共振线宽差别很大。对许多尖晶石型铁氧体,交换作用涨落可能是引起一致共振线宽的主要机制。对于铁氧体,由此机制引起的交换共振线宽其数量级为几十奥。     

基于外腔反馈二极管线阵列的smile效应测量方法

采用外建激光谐振腔,在低于原芯片阈值的电流激励下对LDA的每个发光点进行单独测量,从而分析整个半导体激光阵列(LDA)的smile效应。实验中利用镀膜反射率大于半导体前腔面的外腔镜形成外腔半导体激光器。在外腔中插入曲面平行于p-n结的柱面镜,使只在光轴上的发光点与外腔镜形成外腔激光器,降低该发光点的激光阈值,从而使其在正常的阈值以下的电流激励下输出激光,在平行于p-n结的方向移动柱面镜,可以逐个对半导体激光器中的发光点进行选择测量,从而获得LDA smile效应的测量值。测量中的低电流激励产生的热量对芯片寿命没有影响,对LDA的发光点的单个测量也避免了其他发光点对CCD的影响。     

锯齿波信号激励增强超声造影剂的次谐波信号

提出了一种利用锯齿波激励以增强超声造影剂微气泡激发的次谐波信噪比的方法。基于修正的Church方程,采用数值计算分析了锯齿波激励下微气泡产生的次谐波的声压阈值及变化特性,并与正弦波激励进行了比较。实验中采用声压相等的基频为2 MHz的锯齿波及正弦波激发一种自制的包膜微气泡造影剂,测量了产生次谐波的激励声压阈值及与激励声压的关系。结果表明,锯齿波信号激励可以增强次谐波信号的强度,相对于正弦波激励增强约13 dB,并且激发声压阈值较低。实验结果与数值计算结果相符。     

基于MI自零差法的窄线宽激光器线宽测量

提出了基于迈克尔逊干涉仪的延时自零差法,讨论了该方法测量激光线宽的原理.搭建了线宽测量系统,对窄线宽激光器的线宽进行了测量.测量系统运用数据采集卡获取拍频信号,通过软件算法分析拍频信号自功率谱半峰值宽度实现激光线宽测量.实验测得RIO公司某型窄线宽激光器的线宽为2.7kHz,与该激光器的标称线宽相符合.同时对另一台激光器不同输出光功率下的线宽进行测量,结果表明其线宽和功率成反比关系,进一步验证了基于迈克尔逊干涉仪的延时自零差法测试的准确性.     

铁磁/反铁磁双层膜中自旋波交换偏置的理论与实验分析

考虑铁磁/反铁磁界面粗糙度的随机性及局域外单向的界面能的随机性,考虑原子尺度在交换偏置强度上的至关作用,提出随机场效应概念.对(Fe/FeF2)研究表明,交换偏置和自旋波线宽频率在铁磁/反铁磁双层界面是依赖于外部磁场的.应用玻恩近似,计算了交换偏置对随机场的依赖,由于界面的粗糙度,证明了在反铁磁层出现时在界面自旋波线宽的增加,但自旋波随机场线宽随着外磁场增加而减小,在较大的外场下,从实验能观察到扩展的峰.     

金属多层膜与超晶格中的自旋波及布里渊散射研究

评述了近年来迅速发展的金属多层膜与超晶格中的磁性激发理论和实验。着重于借助 布里渊散射在双层、多层膜中新发现的自旋波模式。首先介绍了磁性介质中自旋波模式的 一般理论,比较了观测自旋波的实验方法,然后,对单层、双层、多层膜中的理论结果作 一总结,并与实验进行了比较,最后讨论了一种新的磁性超晶格——反平行磁化超晶格中 的自旋波。     

金属多层膜与超晶格中的自旋波及布里渊散射研究

评述了近年来迅速发展的金属多层膜与超晶格中的磁性激发理论和实验。着重于借助 布里渊散射在双层、多层膜中新发现的自旋波模式。首先介绍了磁性介质中自旋波模式的 一般理论,比较了观测自旋波的实验方法,然后,对单层、双层、多层膜中的理论结果作 一总结,并与实验进行了比较,最后讨论了一种新的磁性超晶格——反平行磁化超晶格中 的自旋波。     

交换作用涨落所引起的铁磁共振线宽

本文同时考虑自旋矩和交换积分的室间涨落,计算了在一个次点阵上有二类阳离子无规分布的铁氧体的铁磁共振线宽。这种不均匀性使由微波场所激发的一致进动散射为与之简併的自旋波。Clogston等用来解释此线宽的赝偶极作用涨落是估计过高了,它与我们现在所考虑的交换作用涨落相比实际上可以略去。我们计算的结果不仅给出了足够大的线宽,它在数量级上与许多反型尖晶石型铁氧体的实验结果相同,并且自然地解释了镁锰铁氧体和无序锂铁氧体的线宽非常小的原因。     

平面约束下金属铁磁条中平行抽运微波磁场的非稳定阈值研究

运用Landau-Lifshitz方程,边界处动态磁化强度由有效偶极边界条件约束,研究了无限长金属磁条中自旋波传播的特征,得到了该体系抽运微波磁场的阈值曲线以及色散曲线的解析式,揭示出自旋波激发谱与磁条宽度的具体关系.结果表明,平面约束下磁条的阈值曲线中出现了明显的扭结现象.并且随着磁条宽度的增加,其扭结数近似地呈指数形式增加,而最低扭结处两阈值的跳跃差值几乎呈反比形式变化. 关键词： Landau-Lifshitz方程 有效偶极边界条件 阈值曲线 色散曲线     

Instability of magnetoelastic waves in ferrites

The magnetoelastic instability that arises in parallel pumping is studied by methods of quantum theory. A general expression for the instability threshold and a dispersion equation for magnetoelastic waves for any phonon polarization and any direction of magnetization in crystals with cubic symmetry'are obtained. Some special cases are examined.The author thanks A. I. Pil'shchikov for useful discussion.     

垂直泵的结构对YIG单晶薄膜的第二级自旋波不稳定性临界场的影响

本文采用Schl?mann和Patton方法,进一步讨论铁氧体椭球样品的第二级自旋波不稳定性过程,给出薄膜样品的第二级自旋波不稳定性临界场的表示式,它与Anderson-Suhl公式有所不同。对YIG单晶薄膜样品在3种不同垂直泵结构下临界场的计算表明:对于3种结构计算的临界场与实验结果一致,而且理论上和实验上给出的临界场值与线性极化微波磁场的取向有密切关系。 关键词：     

Threshold fields for stimulated Brillouin scattering in spatially limited plasma

Brillouin scattering in an infinite medium is anisotropic, in this case the threshold of absolute instability is caused by attenuation of scattered waves. If the collision attenuation mechanism prevails, the minimum threshold value is observed during backward scattering. For a scattering region limited in the longitudinal direction (parallel to the direction of pumping wave propagation), the backward scattering threshold will be greater than for an infinite medium due to convective loss associated with energy removal by scattered waves. In this paper, the scattering of a wide wave beam in plasma is considered, whose dimension in the transverse direction to the pumping wave propagation substantially exceeds the dimension in the longitudinal direction. It was revealed that in this case, during angle scattering the instability threshold can be less than the threshold for backward scattering due to the increased time of radiation removal from the interaction region. This effect was not taken into account previously. In turn, the decrease of the threshold leads to increasing the radiation loss, which is important in plasma heating problems. The results can also be used for plasma diagnostics.     

Modified temperature-anisotropy instability thresholds in the solar wind

The proton and electron temperature anisotropies in the solar wind are constrained by the instability thresholds for temperature-anisotropy-driven kinetic plasma instabilities. The modifications to the marginal instability conditions from accounting for the influence of damping connected with the collisional effects in the solar wind plasma are calculated for right- and left-handed polarized parallel propagating Alfvén waves and mirror and firehose fluctuations. These modifications provide tighter threshold constraints compared to the marginal thresholds but do not fully explain the observations at small values of the parallel plasma beta.     

Parallel pump spin wave instability in thin ferromagnetic films

The spin wave instability generated by parallel pumping in a tangentially magnetized ferromagnetic film is considered, with simultaneous regard for both the dipole and exchange fields. A dispersion equation and some expressions for the critical microwave threshold of the spin wave parametric excitation have been obtained. The dependence of the critical field on the magnetizing field has an unusually oscillating character. This is connected both with the discreteness of spin wave spectrum and the peculiarities of spin wave polarization in a tangentially magnetized film.     

Parametric excitation of spin waves in ferromagnets by longitudinal pumping localized in space

The equations of motion for the slowly varying complex amplitudes of spin waves parametrically excited by a localized pumping magnetic field have been derived. A solution of these equations satisfying given boundary and initial conditions has been obtained. The energy dissipated by spin waves decreases with the pumping intensity beyond a certain pumping power, which can be termed the regeneration threshold. The losses vanish and change sign at the instability threshold. Both thresholds depend heavily on the linear dimension L of the pumping zone, increasing with decreasing L. Owing to the regeneration process, the dissipation length of spin waves increases without bound as the pumping power approaches the instability threshold. Consequently, perturbations of a uniform state due to the boundary penetrate throughout the pumping zone, regardless of the dimension L. As a result, the full pattern of parametric instability is strongly affected by the zone boundary: 1) the spatial distribution of wave amplitudes becomes nonuniform everywhere inside the zone; 2) the amplitude growth rate in the unstable regime decreases at all points when perturbations due to the boundary reach these points; 3) the instability threshold is independent of the spin-wave frequency offset from the parametric resonance frequency. The calculated minimum instability threshold as a function of the bias magnetic field (the “butterfly” curve) changes shape with L, in agreement with the available experimental data. Zh. éksp. Teor. Fiz. 111, 199–219 (January 1997)     

Instability of the steady-state regime of generation in laser diodes with a broad gain spectrum

The laser-diode parameters at which the steady-state regime of generation becomes unstable are analyzed within the framework of the mode-locking model. The crucial role of the transverse inhomogeneity of the field, pumping intensity, and spectrum width in developing the instabilities of the steady-state regime of generation is demonstrated. The calculated values of the instability threshold are shown to be consistent with the experimental results. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

Supercritical dynamics of magnetoelastic wave triad in a solid

Parametric coupling of three traveling waves is studied numerically on an example of magnetoelastic waves in a highly anharmonic antiferromagnetic crystal. The physical mechanism of coupling is explained as a result of modulation of the nonlinear elastic moduli of the crystal by RF electromagnetic pumping. Parametric interaction of a coupled wave triad with homogeneous pumping field results in an instability of explosive type. Above the threshold of instability, the amplitudes of waves increase to occurrence of singularity in finite time. Explosion is accompanied by spatial localization of wave envelopes. The supercritical dynamics of a wave triad is simulated numerically taking into account the third- and fourth-order magnetoelastic anharmonicity of the medium. Violation of the explosive scenario by nonlinear phase mismatch between the coupled waves and pumping field is demonstrated. Modulation of the pumping phase in time is considered as a tool to compensate for the nonlinear mismatch and recondition the explosive amplification and spatial localization of wave triads. A proper phase modulation law is found in a numerical experiment.     

并联管两相流不稳定性的研究

在高压汽水回路上对垂直并联管中汽液两相流不稳定性进行了试验研究．确定了压力、质量流速、入口过冷度、热负荷及热负荷不对称分布、进口及出口节流、可压缩容积等对不稳定性的影响．得出了压力降型和密度波型不稳定性的界限，给出了并联管中计算不稳定性起始条件的无因次方程，为大型直流锅炉和蒸汽发生器的设计提供依据．     

二维不可压流体瑞利-泰勒不稳定性的非线性阈值公式

分析了二维情况下平面几何、柱几何和球几何中瑞利-泰勒不稳定性发生非线性偏离的阈值问题，给出了三种几何中密度扰动振幅的定义，以及与界面扰动振幅的关系.由此得到了三种几何中密度扰动的非线性阈值公式，用高精度流体程序对三种几何中的不稳定性进行了数值模拟，验证了得到的非线性阈值公式. 关键词： 瑞利-泰勒不稳定性 非线性阈值 密度振幅