Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Field dependent shape variation of magnetic fluid droplets on magnetic dots

The morphology of magnetic fluid droplets on magnetic thin film dots is studied experimentally, including the aspect ratio and the contact angle variation of the droplets. Under a uniform external magnetic field, the droplet's aspect ratio increases with the external field and with the diameter of the magnetic dot due to the concentrated magnetic flux inside the magnetic fluid droplet. Similar to the electrical wetting phenomenon, the induced magnetic dipoles in the magnetic film and in the magnetic fluid near the solid–liquid interface change the solid–liquid interfacial tension, and in consequence reduce the apparent contact angle of the magnetic fluid droplet.     

软磁材料磁导率测量实验探索

软磁材料磁滞回线细长,计算精度不高时忽略其磁滞,可定义磁感应强度与磁场强度的比为磁导率。利用霍尔效应实验仪对螺线管加载软磁材料前后一端的磁感应强度进行测量,计算出该端点处的磁场强度即可计算出该种软磁材料的磁导率,实验表明在材料磁饱和后,磁导率迅速降低。     

Fabrication,properties, and applications of flexible magnetic films

Flexible magnetic devices, i.e., magnetic devices fabricated on flexible substrates, are very attractive in applications such as detection of magnetic field in an arbitrary surface, non-contact actuators, and microwave devices, due to their stretchable, biocompatible, light-weight, portable, and low cost properties. Flexible magnetic films are essential for the realization of various functionalities of flexible magnetic devices. To give a comprehensive understanding for flexible magnetic films and related devices, recent advances in the study of flexible magnetic films are reviewed, including fabrication methods, magnetic and transport properties of flexible magnetic films, and their applications in magnetic sensors, actuators, and microwave devices. Our aim is to foster a comprehensive understanding of these films and devices. Three typical methods have been introduced to prepare the flexible magnetic films, by deposition of magnetic films on flexible substrates, by a transfer and bonding approach or by including and then removing sacrificial layers. Stretching or bending the magnetic films is a good way to apply mechanical strain to them, so that magnetic anisotropy, exchange bias, coercivity, and magnetoresistance can be effectively manipulated. Finally, a series of examples is shown to demonstrate the great potential of flexible magnetic films for future applications.     

磁记忆检测的力磁耦合型磁偶极子理论及解析解

磁偶极子理论在缺陷漏磁场解释中被成功广泛使用.由于磁荷密度等参数不易定量,磁偶极子理论在应用中常常进行归一化处理,被认为不适用于对应力相关的磁记忆信号做量化分析.本文通过建立力磁耦合型磁偶极子理论模型,以适用于分析磁记忆检测中应力对磁信号的影响.基于铁磁学理论确定应力和磁场联合作用下的等效场强度,基于弱磁化状态的一阶近似,获得了各向同性铁磁材料微弱环境磁场下的应力磁化解析解.结合磁信号二维问题中矩形和V形磁荷分布假定,建立了光滑与破坏试件表面磁信号、矩形和V形表面缺陷所诱导磁信号的力磁耦合型磁偶极子理论分析模型,并获得其解析解.基于力磁耦合型磁偶极子理论的解析解,对拉伸实验中试件破坏前后的信号差异、矩形和V形表面缺陷诱导磁信号,以及磁信号的影响因素和规律等进行了详细分析.理论研究表明,基于本文理论模型的解析解可实现对磁记忆检测中的一些基本实验现象和规律的解释.     

霍尔推力器磁场位形及其优化的数值研究

基于麦克斯韦方程,在轴对称假设下建立了霍尔推力器磁场的数学模型.用有限差分方法对模型进行了离散.给出了数值求解模型的迭代法.通过对模型的数值求解,得到了相应的数值结果.通过对所得数值结果的分析,研究了磁场线圈电流变化对霍尔推力器磁场位形的影响.通过调整磁场线圈电流的大小找到了理想磁场位形.研究表明,对于理想磁场位形,内通道的磁镜比在3—3.5之间,外通道的磁镜比在0.4—0.9之间;增加磁场线圈的电流,出口的磁场强度随着增加,但不能增加磁镜比.通道内部的磁场强度几乎不随着磁场线圈电流的变化而变化. 关键词： 霍尔推力器 磁场位形 磁场线圈电流 磁镜比     

脉冲压缩电路磁开关动态特性

提出在典型一阶磁脉冲压缩电路的基础上,测取磁芯在实际工作条件下的动态磁滞回线和饱和磁导率等磁参数,再根据所获得的动态参数指导磁开关设计,进行一阶磁压缩实验。实验选取国内外被广泛应用的非晶磁芯和纳米晶磁芯进行测试,根据实测动态磁参数设计磁开关。实测结果表明:用国产非晶磁芯做磁开关可得到上升沿73 ns、电压幅值28.3 kV、半高宽为503 ns的脉冲,用日本产的纳米晶磁芯做磁开关可得到上升沿30 ns、电压幅值28.4 kV、半高宽为193 ns的脉冲。     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

Magnetorheological and deformation properties of magnetically controlled elastomers with hard magnetic filler

Viscoelastic and deformational behavior of soft magnetic elastomers with hard magnetic fillers under the influence of a magnetic field is studied by different experimental techniques. The magnetic elastomers used in this work were synthesized on the basis of silicone rubber filled with FeNdB particles and were magnetized in a field of 3 and 15 kOe. We have shown that due to high residual magnetization the materials demonstrate well pronounced non-elastic behavior already in the absence of any external magnetic field. In particular, in contrast to magnetic elastomers based on soft magnetic fillers their elastic modulus is strain-dependent. Under the influence of external magnetic field the storage and loss moduli of magnetic elastomers with hard magnetic filler can both increase and decrease tremendously.     

一种新型的基于磁性液体的光纤Sagnac磁场传感器

提出一种使用磁性液体的新型光纤Sagnac磁场传感器.磁性液体具有磁致可变双折射效应和二向色性,在外加磁场作用下,液体中的磁性纳米粒子沿磁场方向结链规则排列,形成各向异性.将其制成液体薄膜,放入具有一段保偏光纤的Sagnac环中,使光纤Saganc干涉仪的正弦形状干涉光谱可随外磁场变化.光纤中传输光垂直经过磁性液体薄膜...     

Two-dimensional Monte Carlo simulations of structures of a suspension comprised of magnetic and nonmagnetic particles in uniform magnetic fields

The structures of suspensions comprised of magnetic and nonmagnetic particles in magnetic fields are studied using two-dimensional Monte Carlo simulations. The magnetic interaction among magnetic particles, magnetic field strength, and concentrations of both magnetic and nonmagnetic particles are considered as key influencing factors in the present work. The results show that chain-like clusters of magnetic particles are formed along the field direction. The size of the clusters increases with increasing magnetic interaction between magnetic particles, while it keeps nearly unchanged as the field strength increases. As the concentration of magnetic particles increases, both the number and size of the clusters increase. Moreover, nonmagnetic particles are found to hinder the migration of magnetic ones. As the concentration of nonmagnetic particles increases, the hindrance on migration of magnetic particles is enhanced.     

磁流体粘度的实验研究

采用毛细法粘度计测量了水基Fe磁流体的粘度,分析了磁性粒子份额、表面活性剂含量以及外加磁场强度和方向对粘度的影响。实验结果表明,磁流体粘度随着磁性粒子和表面活性剂浓度的增加而增加;随着外加磁场强度的增大而增大,对于相同的磁流体,在外加磁场方向垂直于流动方向时的粘度大于外加磁场方向平行于流动方向时的粘度;表面活性剂含量的增大将减弱外加磁场对磁流体粘度的影响。     

磁性材料的磁结构、磁畴结构和拓扑磁结构

首先简要地介绍了磁性材料中磁结构、磁畴结构和拓扑磁结构以及相互之间的关系. 一方面, 磁畴结构由材料的磁结构、内禀磁性和微结构因素决定; 另一方面, 磁畴结构决定了材料磁化和退磁化过程以及技术磁性. 拓扑学与材料物理、材料性能的联系越来越紧密. 最近的研究兴趣集中在一些拓扑磁性组态, 如涡旋、磁泡、麦纫、斯格米子等. 研究发现这些拓扑磁结构的拓扑性质与磁性能密切相关. 然后从尺寸效应、缺陷、晶界三个方面介绍国际学术界在磁结构、磁畴结构和拓扑磁结构方面的进展. 最后介绍了在稀土永磁薄膜材料的微观结构、磁畴结构和磁性能关系、交换耦合纳米盘中的拓扑磁结构及其动力学行为方面的工作. 通过对文献的评述, 得到以下结论: 开展各向异性纳米复合稀土永磁材料的研究对更好地利用稀土资源具有重要的意义. 可以有目的地改变材料的微结构, 可控地进行磁性材料的磁畴工程, 最终获得优秀的磁性能. 拓扑学的概念正在应用于越来越多的学科领域, 在越来越多的材料中发现拓扑学的贡献. 研究磁畴结构、拓扑磁性基态或者激发态的形成规律以及动力学行为对理解量子拓扑相变以及其他与拓扑相关的物理效应是十分重要的. 也会帮助理解不同拓扑学态之间相互作用的物理机制及其与磁性能之间的关系, 同时拓展拓扑学在新型磁性材料中的应用.     

磁流体中Helmholtz和Kelvin力的界定

磁流体磁彻体力的两种简化形式Helmholtz力和Kelvin力具有一定的适用范围.在推导磁流体中的磁彻体力表达式基础上,分析Helmholtz力和Kelvin力在磁流体中的起源,得出两种形式的成立条件.计算结果表明:当磁流体磁导率与外磁场强度无关时,磁流体磁彻体力可由Helmholtz力表示;当磁流体中磁性颗粒的平均磁矩与磁流体比体积无关时,Kelvin力为磁彻体力的简化形式;在磁流体磁化系数与其密度成正比情况下,Helmholtz力可转换为Kelvin力. 关键词： 磁流体 磁彻体力 Helmholtz力 Kelvin力     

不闭合磁芯的Tesla变压器初次级线圈电感估算

对具有不闭合磁芯的Tesla变压器磁路进行了研究,计算了Tesla变压器磁路中磁力线分布以及各处磁感应强度分布,结果表明:在磁芯所在位置,磁力线主要集中在磁芯内部; 在内外筒磁芯之间,磁力线主要分布在初次级线圈以外磁芯两端之间的空隙中。实际的Tesla变压器漏磁较小,分析了在没有漏磁的理想情况下Tesla变压器不闭合磁芯对初、次级线圈电感的影响,并给出了初、次级线圈电感的估算公式,利用估计公式得出的结果与实际测量对比,误差范围在15%以内,该公式在Tesla变压器设计和参数估算时不失为一种简便易行的处理方法。     

Magnetic correction to the anomalous magnetic moment of electrons

We investigate the leading order correction of anomalous magnetic moment (AMM) to electrons in a weak magnetic field and find that the magnetic correction is negative and magnetic field dependent, indicating a magnetic catalysis effect for the electron gas. In the laboratory, to measure the g − 2, the magnitude of the magnetic field B is several T, and correspondingly the magnetic correction to the AMM of electron/muon is around 10⁻³⁴/10⁻⁴², therefore the magnetic correction can be safely neglected in the current measurement. However, when the magnitude of the magnetic field strength is comparable with the electron mass, the magnetic correction of the electron's AMM will become considerable. This general magnetic correction to the charged fermion's AMM can be extended to study quantum chromodynamic matter under a strong magnetic field.     

Magnetic Textures and Dynamics in Magnetic Weyl Semimetals

Recent theoretical and experimental attempts have been successful in finding magnetic Weyl semimetal phases, which show nodal-point structure in the electronic bands as well as magnetic orders. Beyond uniform ferromagnetic or antiferromagnetic orders, nonuniform magnetic textures, such as domain walls and skyrmions, enrich the properties of the Weyl electrons even more in such materials. Here, a topical review on interplay between Weyl electrons and magnetic textures in those magnetic Weyl semimetals is given. The basics of magnetic textures in nontopological magnetic metals are reviewed first, and then the effect of magnetic textures in Weyl semimetals is discussed, regarding the recent theoretical and experimental progress therein. The idea of the fictitious “axial gauge fields” is pointed out, which effectively describes the effect of magnetic textures on the Weyl electrons and can well account for the properties of the electrons localized around magnetic domain walls.     

基于三维磁成像的近场磁性体探测

提出一种近场条件下未知磁源的三维磁成像方法.考虑到大多数磁性体不仅受背景磁场磁化，本身也带有较强剩磁，将观测面上的磁场转换为磁场矢量异常模量，并建立目标函数进行最优化求解，以得到符合观测磁场特征的磁性体磁化模型.仿真和实验表明：此方法可有效消除剩磁对反演结果的影响，能够实现对近场多个磁源磁化率分布的成像，验证了所提方法用于探测隐含磁体位置和形状的可行性.     

Magnetic properties of Co-P-based trilayers

The magnetic properties of a film trilayer consisting of hard magnetic and soft magnetic Co-P layers separated by a nonmagnetic Ni-P spacer have been studied. The features of the hysteresis shift relative to the zero exchange magnetic field and of the coercivity of the soft magnetic layer in the dependence on thicknesses of the hard magnetic layer and spacer have been considered. The dynamic changes in the shift of the hysteresis of the soft magnetic layer versus the magnetization reversal time after saturation of the hard magnetic layer have been found and investigated.     

两个非磁性颗粒在磁流体中的沉降现象研究

在磁场作用下,在磁流体里添加非磁性颗粒(non-magnetic particles,NPs),可以使得NPs形成不同的结构,操控NPs的运动从而影响磁流体的特性,这种应用逐渐受到了研究者的关注.为了更好地操控磁流体里NPs的运动,本文采用一种多物理模型研究在外加磁场作用下,磁流体中两个NPs沉降的运动过程.其中,用格子玻尔兹曼方法模拟磁流体的运动,外加磁场对磁流体的影响用一种自修正方法求解泊松方程,这个自修正方法可以使欧姆定律满足守恒定律.NPs之间的偶极干扰力采用偶极力模型,同时采用一种相对过渡平滑的共轭边界条件处理NPs与磁流体交界面的流固干扰以避免磁场密度过渡的突变.本文主要探究两个NPs在磁流体中的沉降,揭示磁场作用下NPs的相互干扰原理;同时,对控制NPs运动时的参数进行调节,得到NPs不同的运动轨迹,达到操控颗粒运动的目的.本研究可对NPs在磁流体中的应用提供定量的分析结果,对NPs在工业上的应用提供有力的理论支撑.