铁基非晶磁环磁开关设计

分析了磁开关的工作原理及其工作过程进行了分析,推导了磁开关磁芯体积公式,给出了多级磁脉冲压缩电路所需的条件,使用Pspice对磁开关的工作过程进行了模拟。根据理论分析和数值模拟的结果,设计了一台10 kV级,压缩比为8.3的磁开关,并在实验中将10 kV,9.2μs的脉冲压缩为10 kV,1.3μs的脉冲。针对该实验平台设计了一种便捷的磁芯动态磁滞回线测试方法,运用该方法不需搭建测试平台,直接在单级磁脉冲压缩电路平台上即可完成磁芯参数的测量和磁滞回线的绘制。     

Effect of magnetic field on self-assembling of colloidal Co magnetic nanoparticles

In this paper the formation of 3-D structures composed of Co nanoparticles (NPs) is reported. Structures were obtained by drying a droplet of a colloidal solution of NPs in a magnetic field perpendicular to the substrate. The Co nanoparticles were prepared by thermolysis of Co₂(CO)₈. The 3-D NP structures were characterized by scanning electron microscopy (SEM) and atomic and magnetic force microscopy (AFM/MFM). It has been found that at the border of the droplet, NPs assemble into hexagonally ordered 3-D columns or they form a labyrinthine structure. The formation of the 3-D structures can be explained by the outflow of NPs to the border of the droplet during the drying process. Within this model the pattern formation is dependent on the concentration of the NPs and the degree of alignment of the magnetic moments of NPs in the 3-D columns.     

Heat-treatment effect on structure and magnetic properties of hard magnetic Co-W films for perpendicular magnetic recording

The variations of coercivity for different preferred orientation (PO) of electrodeposited Co-W films with perpendicular magnetic anisotropy are demonstrated by manipulating the inter-particle separation through the heat-treatment effect. The character of the dependences corresponds to magnetization reversal of the films with (002) PO by the rotational mechanism.     

扫场对核磁共振测量的影响

在连续波核磁共振实验中，通常采用低频扫场产生重复再现的共振吸收信号。当处于磁场中的射频线圈引线回路等效面积不为零时，简谐扫场必然引起该闭合回路的磁通量变化，由此而产生同频简谐感生电动势并与核磁共振信号叠加共同构成振荡器输出信号。由于扫场上升和下降过程的感生电动势方向相反，从而扫场前半周和后半周所对应的共振吸收测量信号存在差异。实验结果表明：扫场强度及方向对核磁共振测量影响来源于实验测量技术缺陷，而非核磁共振的物理本质。     

Influence of magnetic fields on taylor vortex formation in magnetic fluids

The flow of a magnetic fluid placed inside a small gap between concentric rotating cylinders is investigated for axial, radial and azimuthal magnetic fields. An equation of motion is derived phenomenologically to describe the hydrodynamics of magnetic fluids. Studied are the changes in the critical Taylor numberT _c and wave numberT _c which characterize the instability of primary circular Couette flow towards Taylor vortices. It is found that all above magnetic fields have a stabilizing effect on circular Couette flow and thatT _c increases or decreases, depending on the direction of the magnetic field. Besides this, the influence of the magnetic fields on the correlation length ₀, the wave number of maximal growthk _m and the linear growth rate amplitude ₀ is determined.     

Annealing effects on magnetic properties of silicone-coated iron-based soft magnetic composites

This paper focuses on novel iron-based soft magnetic composites synthesis utilizing high thermal stability silicone resin to coat iron powder. The effect of an annealing treatment on the magnetic properties of synthesized magnets was investigated. The coated silicone insulating layer was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. Silicone uniformly coated the powder surface, resulting in a reduction of the imaginary part of the permeability, thereby increasing the electrical resistivity and the operating frequency of the synthesized magnets. The annealing treatment increased the initial permeability, the maximum permeability, and the magnetic induction, and decreased the coercivity. Annealing at 580 °C increased the maximum permeability by 72.5%. The result of annealing at 580 °C shows that the ferromagnetic resonance frequency increased from 2 kHz for conventional epoxy resin coated samples to 80 kHz for the silicone resin insulated composites.     

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.     

Localised spin modes of decorated magnetic clusters on a magnetic surface

A method for the calculation of the energies of spin modes of surface magnetic clusters on a magnetic surface, using broken translation symmetry in three dimensions (3D), is employed to determine the spin mode energies for a variety of planar clusters. The cluster is considered to be supported on a magnetically ordered substrate such that the localised spins of the cluster and the substrate interact via magnetic exchange. No electronic effects are considered. The analytical approach solves for the 3D evanescent crystal spin field in the bulk and the surface domains around the cluster. This spin field arises owing to the breakdown of magnetic translation symmetry caused by the surface cluster. The analytical approach can be applied to any cluster configuration, underlying the general character of the calculation. In particular, we consider here a 4-, 5-, and 9-atoms planar clusters supported on the surface of a ferromagnetic simple cubic lattice, the spin order being in the direction normal to surface boundary. The method is applied to calculate the spin mode energies of these planar clusters consisting of Gd atoms interacting via Anti-ferromagnetic exchange with an Fe(1 0 0) surface. These results are compared with the calculated energies of the spin modes of the free clusters, and also with those for the same planar clusters when the cluster-substrate exchange is considered hypothetically ferromagnetic.     

A study on the magnetic properties of two-phase particulate magnetic composites

The magnetic properties of two-phase particulate magnetic composites with a hard ferromagnetic component are studied theoretically and experimentally. The magnetic properties considered here are phase-distribution sensitive properties, including remanence, coercivity and the shape of hysteresis loop. These properties depend mainly on the properties of its constituents, volume fractions, phase distribution, packing fraction and orientation distribution for anisotropic particles. With fixed packing fraction and orientation distribution, the magnetic properties of the two phase mixture can be calculated in terms of its component properties, volume fractions and phase distribution. Here, the component properties include not only remanence B_r and coercivity H_c but also a variable m which is the rate of change of magnetic induction B with respect to field intensity H. For two-phase systems satisfying B - H relation of the type B = B_r + mH where m is a constant, the equations for calculating the magnetic properties B_r, H_c, etc., in terms of m are derived. The method for calculating m is also developed for the cases of parallel and series distributions. Bounds for m-values were also established. A modified Landauer's type equation is developed to calculate m-values in terms of the component properties of the mixture. Experiments were conducted to verify the theoretical calculations. Good agreements between the theoretical calculations and experimental results were obtained.     

微观组织结构对铂钴永磁合金磁性能的影响

系统研究了Pt-Co合金磁性能与其微观组织结构间的关系.合金铸锭的X射线衍射结果表明：熔炼后的Pt-Co合金铸锭沿冷却方向存在明显的织构;扫描电镜照片显示合金的组织结构为柱状晶结构,柱状晶的生长方向平行于冷却方向;合金铸锭经塑性变形和再结晶处理后柱状晶组织消失,电镜照片显示处理后的合金晶粒尺寸变小且均匀;对不同组织结构的Pt-Co合金磁性能的测试结果表明,经塑性变形及再结晶处理后合金的磁性能有了明显提高,说明该合金中晶粒尺寸和取向是影响其矫顽力的重要因素. 关键词： 铂钴永磁合金 磁性能 织构     

Influence of background carriers on magnetic properties of Mn-doped dilute magnetic Si

Using hybrid exchange density functional calculations we show that the type of background carriers has profound effects on magnetic interactions in Mn doped dilute magnetic Si. The p- and n-type Si were simulated by introducing an extra hole and an extra electron, respectively in the 64 atoms Si supercell. In case of p-type Si compensated by a homogeneous background potential and 1.6% Mn, the ground state is ferromagnetic, whereas other conditions remaining the same, the ground state becomes antiferromagnetic for the n-type Si. The exchange energies in Mn-doped extrinsic Si are higher by about 1 eV/Mn atom compared to the Mn doped intrinsic Si. Calculated electronic structures reveal that in p-type Si:Mn the hole localises over Mn and the short range magnetic coupling increases. Our calculations indicate that localisation of magnetic polarons at the Mn site is likely, which in turn enhances long range magnetic interaction between Mn ions and responsible for FM stabilisation. On the other hand, in the n-type host electron–electron repulsion increases within Mn–Si impurity band and the short range coupling decreases, which destroys the long range spin polarisation. These calculations explain the observed ferromagnetism in the p-type Si:Mn at higher temperatures than in the n-type Si:Mn and the magnetic moments of the systems compare well with experiments.     

磁屏蔽罩在梯度静磁场中的受力研究

在有梯度的静磁场中,对由软磁材料制作的磁屏蔽罩所受的静磁力的量级进行了评估。使用有限元分析和实验测试的方法研究了纯铁方形屏蔽罩在梯度静磁场中的受力情况,结果表明：把内腔尺寸100mm×100mm×150mm、罩体厚度为20mm的方形屏蔽罩放置在强度约为0.2T、梯度为0.03T?m?1的磁场中,其所受的静磁力约为184N,与其自身重力相当;静磁力随磁场强度的升高而增大;有限元分析与实验测试结果的偏差在0.2T背景磁场下约为21%。因此,在考虑一定余量的情况下,使用有限元计算评估屏蔽罩在静磁场中的受力情况是一种有效和比较可靠的方法。     

Particle size effect on phase and magnetic properties of polymer-coated magnetic nanoparticles

Polymer-coated magnetic nanoparticles are hi-tech materials with ample applications in the field of biomedicine for the treatment of cancer and targeted drug delivery. In this study, magnetic nanoparticles were synthesized by chemical reduction of FeCl₂ solution with sodium borohydride and coated with amine-terminated polyethylene glycol (aPEG). By varying the concentration of the reactants, the particle size and the crystallinity of the particles were varied. The particle size was found to increase from 6 to 20 nm and the structure becomes amorphous-like with increase in the molar concentration of the reactant. The magnetization at 1 T field (M_1T) for all samples is > 45 emu/g while the coercivity is in the range of 100-350 Oe. When the ethanol-suspended particles are subjected to an alternating magnetic field of 4 Oe at 500 kHz, the temperature is increased to a maximum normalized temperature (3.8 °C/mg) with decreasing particle size.     

Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles

The effect of surface anisotropy on the magnetic ground state of hollow maghemite nanoparticles is investigated using atomistic Monte Carlo simulation. The computer modeling is carried on hollow nanostructures as a function of size and shell thickness. It is found that the large contribution of the surface anisotropy imposes a “throttled” spin structure where the moments located at the outer surface tend to orient normal to the surface while those located at the inner surface appear to be more aligned. For increasing values of surface anisotropy in the frame of a radial model, the magnetic moments become radially oriented either inward or outward giving rise to a “hedgehog” configuration with nearly zero net magnetization. We also show the effect of the size of hollow nanoparticle on the spin behavior where the spin non-collinearity increases (for fixed value of surface anisotropy) as the diameter of the hollow nanoparticle increases due to the significant increase in surface-to-volume ratio, the thickness being constant. Moreover, the thickness of the hollow nanoparticle shell influences the spin configuration and thus the relation between surface anisotropy and the size or the thickness of the hollow nanoparticle is established.     

Influence of eddy currents on magnetic hysteresis loops in soft magnetic materials

In this paper an attempt has been made to extend the Jiles and Atherton (J–A) quasi-static hysteresis model to describe magnetisation of a material with an alternating magnetic field. In low – industrial – and medium frequency of magnetic field it is possible to ignore the magnetic relaxation and resonance. The field penetration is assumed to be uniform through the material. The influence of eddy currents on the hysteresis loop could be considered and calculated using the method of successive reactions of eddy currents, where a reaction is an additional magnetic field, called reaction H_d, induced in the material by the eddy currents according to rot J=γ∂B/∂t where γ is the electrical conductivity. The reaction field H_d was added to the basic field H₀∝Iz_1, where I is the current intensity in the magnetising coil of z₁ number of windings. By solving the J–A equation for the magnetic field H_w=H₀+H_d it has achieved an extension of the hysteresis loop at an increased frequency of the current, caused by increased losses of the eddy currents. At the frequency f→0 Hz , the hysteresis loop approaches the shape of the quasi-static one.     

Effect of noncrystallographic magnetic anisotropy on magnetization reversal in multilayer magnetic films

A study has been carried out of the magnetization of a layered system consisting of two exchange-coupled easy-axis ferromagnetic layers separated by a nonmagnetic spacer and having mutually perpendicular anisotropy axes. It is shown that the magnetization of such a system can undergo stepwise decrease with increasing magnetic field. The field dependence of the magnetization has a bifurcation point. The magnetization orientation can exhibit jumps in excess of 90° depending on the angle of magnetic-field orientation. Fiz. Tverd. Tela (St. Petersburg) 41, 660–664 (April 1999)     

磁极和磁场实验的创新设计

利用非磁性透明塑料袋和有机玻璃板等材料收集铁粉,解决了磁极和磁场实验中铁粉不易收集且不卫生的问题、实验装置制作简单,易操作,现象明显.     

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在有梯度的静磁场中,对由软磁材料制作的磁屏蔽罩所受的静磁力的量级进行了评估.使用有限元分析和实验测试的方法研究了纯铁方形屏蔽罩在梯度静磁场中的受力情况,结果表明:把内腔尺寸100mm×100mm×150mm、罩体厚度为20mm的方形屏蔽罩放置在强度约为0.2T、梯度为0.03T?m-1的磁场中,其所受的静磁力约为184N,与其自身重力相当;静磁力随磁场强度的升高而增大;有限元分析与实验测试结果的偏差在0.2T背景磁场下约为21%.因此,在考虑一定余量的情况下,使用有限元计算评估屏蔽罩在静磁场中的受力情况是一种有效和比较可靠的方法.