Temperature dependence of the magnetic susceptibility of magnetic dispersed nanosystems

The temperature dependence of the complex magnetic susceptibility of different magnetic nano-colloids and finely dispersed magnetite powder is studied. The results are explained with allowance for the magnetic moment relaxation in single-domain particles and phase transitions in a system of interacting dipoles.     

Polarization and transmission of microwaves in the magnetic suspension in the applied magnetic field

The production method of magnetic suspension consisting of ferromagnetic particles dispersed in cedarwood oil is presented at the beginning of this article. Next, the set-up for microwaves generation using a klystron is described. The main part of this paper concerning microwave transmission and polarization during its passage in samples of the produced magnetic suspension placed in a magnetic field is based on the following parameters: induction of this field, filling factor of magnetic suspension by ferromagnetic particles, dimensions of particles, viscosity of liquid carrier, and ratio of the magnetic field changes. Conducted investigations show that microwaves are damped and polarized in these magnetic suspensions. Obtained results are discussed and observed effects are explained by ordering of ferromagnetic particles in magnetic suspension by applied magnetic field.     

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.     

A simple method to estimate the magnetic moment of magnetic micro-particles

Micro-particles in suspension in a fluid are an example of a very low Reynolds number problem. In this case, no inertial effects are observed. Magnetic micro-particles with magnetic moment m, suspended in a fluid orient to applied external magnetic fields B due to the interaction between the field and the magnetic moment. In this work, we present a simple method to estimate the total magnetic moment of magnetic micro-organisms. The method is based on the application of an external oscillating magnetic field in the sites where the micro-organisms are. In this case, it is possible to obtain theoretically the solution of the equation of motion (rotation of the organism and its trajectory). The solution is a transcendental equation relating the orientation angle and m and can be solved by numerical methods. Changing the frequency and/or the field intensity, it is possible to obtain a situation in which the crystal rotates uninterruptedly (a resonance regime). This condition is related to the applied field intensity, to the frequency, to the medium viscosity, to the crystal dimension, and to the micro-crystal magnetic moment m. The method can be used to estimate the total cellular magnetic moment of magnetic micro-particles.     

Estimation of the chiral magnetic effect considering the magnetic field response of the QGP medium

The magnetic field plays a major role in searching for the chiral magnetic effect in relativistic heavy-ion collisions. If the lifetime of the magnetic field is too short, as predicted by simulations of the field in vacuum, the chiral magnetic effect will be largely suppressed. However, the lifetime of the magnetic field will become longer when the QGP medium response is considered. We give an estimate of the effect, especially considering the magnetic field response of the QGP medium, and compare it with the experimental results for the background-subtracted correlator H at RHIC and LHC energies. The results show that our method explains the experimental results better at the top RHIC energy than at the LHC energy.     

Screening of the magnetic field by magnetic monopoles in spin ice

The screening of the external magnetic field by magnetic monopoles in spin ice has been considered. The polarization of the magnetic system with moving monopoles has been shown to result in the incomplete screening of the external magnetic field. The static permeability of spin ice and the magnetic-field screening length have been calculated and numerically estimated and the physical meaning of introducing monopoles is discussed.     

In situ magnetic hysteresis measurement of magnetic tips in magnetic force microscope

In situ magnetic hysteresis measurements of magnetic tips in a magnetic force microscope (MFM) are demonstrated using alternating gradient force magnetometry. The measured magnetic moments of MFM tips are estimated in the range from 10⁻⁶ to 10⁻⁵ emu by this technique and the whole MFM tips in cantilevers are considered to be measured from the value of measured magnetic moments. The relationship between the magnetic hysteresis loops of MFM tips and those of coated magnetic films is discussed.     

Measurement of the magnetic field-dependent refractive index of magnetic fluids in bulk

An optical alignment-free and highly accurate method is employed to measure the magnetic field-dependent refractive index of magnetic fluid(MF) in bulk.The measured refractive index decreases significantly with the increasing magnetic strength and then tends to saturate in the high intensity range.By applying a tunable magnetic field ranging between 0 and 1661 Oe,the maximum shift of the refractive index of MF in bulk iS found to be 0.0231.     

Ni-Zn铁氧体的动态磁特性

设计并制作了基于没有附加磁芯复位电路的单级、双级磁脉冲压缩系统电路,用于测试Ni-Zn铁氧体磁芯在μs及亚μs级脉冲激励下的动态磁特性。磁芯的磁滞回线由测量到的磁开关两端电压和电流数据经计算得到,由磁滞回线可知磁芯在μs及亚μs级脉冲激励下的各种特性参数如饱和磁感应强度、剩余磁感应强度、矩形比、磁通密度跳变、矫顽力、饱和磁场强度及单位体积材料磁滞损耗;通过比较两块磁芯在μs及亚μs级脉冲激励下的各种动态磁特性参数可知:两块磁芯随激励脉冲宽度变窄磁芯磁性能有不同程度的下降,亚μs级脉冲激励下的矫顽力和单位体积材料磁滞损耗都比μs级脉冲激励下增大约3倍;饱和磁感应强度小、剩余磁感应强度大的Ni-Zn铁氧体磁芯动态磁特性性能优异,适合用于更窄脉冲的压缩电路中。     

利用智能手机磁传感器测量亥姆霍兹线圈磁场

通过预实验确定智能手机磁传感器的位置后,用其测量亥姆霍兹线圈轴线的磁场分布,验证毕奥-萨伐尔定律及磁场的叠加原理,提供一种较为精确而简便的测量磁场的新方法.     

Influence of switchable magnetic field on the modulation property of nanostructured magnetic fluids

Magnetic fluid is a kind of colloidal material with tunable microstructure and unique optical properties. The tunable magneto-optical modulation property of magnetic fluid under externally switchable magnetic field with various modulation periods is investigated theoretically and experimentally. The transitional modulation period (lower limit of the working frequency) between the square-like and oscillation-like modulation is achieved and found to be magnetic-field- and sample-concentration-dependent. The modulation mechanism is analyzed and ascribed to the dynamic microstructure of magnetic fluid under different modulation periods of external magnetic fields. The result of this work may be helpful for the pragmatic applications of magnetic fluid based on the square-like modulation.     

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基于磁流体理论利用CIP方法数值对电阻磁流体中的磁岛演化动力学行为进行了长时间模拟。研究发现,磁岛饱和后流体演化并未结束,磁场重联将导致流体的宏观剪切流动。这一物理图象可用于解释托卡马克等离子体L-H模转换过程中观察到的磁流体行为。     

Soft X-ray resonant magnetic scattering of magnetic nanostructures

Soft X-ray resonant magnetic scattering offers a unique element-, site- and valence-specific probe to study magnetic structures on the nanoscopic length scale. This new technique, which combines X-ray scattering with X-ray magnetic circular and linear dichroism, is ideally suited to investigate magnetic superlattices and magnetic domain structures. The theoretical analysis of the polarization dependence to determine the vector magnetization profile is presented. This is illustrated with examples studying the closure domains in self-organising magnetic domain structures, the magnetic order in patterned samples, and the local configuration of magnetic nano-objects using coherent X-rays. To cite this article: G. van der Laan, C. R. Physique 9 (2008).     

Distribution of micrometer-size particles in magnetic fluids in the presence of steady uniform magnetic field

In this paper, distribution of suspended micrometer-size particles in magnetic fluids is investigated. Microstructure formation of particles in magnetic fluids is simulated by using the discrete particle method based on the simplified Stokes dynamics. Not only magnetic particles but also nonmagnetic particles are rearranged in the field direction and form chain-like clusters due to the apparent magnetization in magnetic fluids in the presence of magnetic field. When the diameter of nonmagnetic particles is smaller than that of magnetic particles, nonmagnetic particles move into the empty space of microstructure of magnetic particles, and they are rearranged in the field direction. Uniformity of distribution of particles on the plane perpendicular to the field direction is maintained even after microstructure formation.     

Micromagnetic simulation of the magnetic dispersion angle and effective damping factor for the single-phase soft magnetic films

The micromagnetic structure of the single-phase soft magnetic films was simulated using the model of two-dimensional hexagonal lattices by micromagnetic method. The typical micromagnetic ripple structure of magnetic films was obtained. Thus, the magnetic dispersion angle was calculated from the static magnetic structure of the film. Furthermore, the relationship between the magnetic dispersion angles and the corresponding magnetic parameters of the film was discussed. The technique also demonstrated the microwave permeability of the films and the magnetic spectra well fitted by the permeability equation, which was deduced from the Landau–Lifshitz–Gilbert (LLG) function when the film was considered as a single domain. The fitting data of effective damping factor as a function of the magnetic dispersion angle were investigated.     

Structure and magnetic field of periodic permanent magnetic focusing system with open magnetic rings

The magnetic field along the central axis for an axially magnetized permanent magnetic ring was investigated by analytical and finite element methods. For open magnetic rings, both calculated and measured results show that the existence of the radial magnetic field creates a remarkable cosine distribution field along the central axis. A new structure of periodic permanent magnet focusing system with open magnetic rings is proposed. The structure provides a satisfactory magnetic field with a stable peak value of 120 mT for a traveling wave tube system.     

Site-directed research of magnetic nanoparticles in magnetic drug targeting

The targeting of ferrofluids composed of 20 nm magnetic particles was studied through simulation and animal experiment. The results showed that some magnetic particles were concentrated in the target area depending on the applied magnetic field. Through theoretical analysis, the retention of the magnetic nanoparticles in a target area is due to large magnetic liquid beads formed by the magnetic field.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

磁场及磁矩的测量实验

对地磁水平分量测量实验加以改进,将其扩充为能够测量亥姆霍兹线圈磁场分布和磁针的转动惯量以及磁针磁矩的实验.使其成为一个综合性较强,适用于开展研究性实验的项目.     

The role of the exchange and magnetic dipole interactions in the superconductors with helical magnetic ordering

The helical ordering of localized moments in the magnetic superconductors is considered by taking into account the exchange interaction of localized moments and electrons as well as their interaction via the magnetic field. We show that the structure of superconducting phase with helical magnetic order is determined mainly by the exchange interaction. The magnetic dipole interaction fixes only the direction of the helix wave vector so that it is transverse.