Investigation of applicability of extrapolation method for sample field determination in single-yoke measuring setup

In this work the single-yoke measuring technique is proposed to be optimized by extrapolation of a magnetic field profile to the sample surface for determination of the “real” field inside the sample. It has been shown that this approach gives reasonable values of magnetic parameters and allows to solve the well-known problem of considerable fluctuations of the measurement results due to imperfections of the yoke–sample contact. The magnetization process with the single-yoke setup is considered on basis of the surface field measurements around the sample and their extrapolation to the sample surfaces. Advantages as well as drawbacks of the measuring procedure and of the suggested optimization method are discussed.     

脉冲磁场对水热法制备Mn掺杂ZnO稀磁半导体的影响

本文以Zn(CH₃COO)₂·2H₂O, Mn(CH₃COO)₂·4H₂O和氨水缓冲溶液为原料, 在4 T脉冲磁场下利用水热法制备了Mn掺杂ZnO稀磁半导体晶体, 通过X射线衍射、 扫描电子显微镜、透射电子显微镜、拉曼光谱、荧光分光光度计及振动样品磁强计等对样品的微观结构及磁性能等进行了表征, 结果表明: Mn掺杂ZnO稀磁半导体晶体仍保持ZnO六方纤锌矿结构, 4 T脉冲磁场下合成的Mn掺杂ZnO稀磁半导体晶体具有明显的室温铁磁性, 其饱和磁化强度(M_s)为0.028 emu/g, 比无脉冲磁场下制备的样品提高一倍以上, 且4 T 脉冲磁场将样品的居里温度提高了15 K.     

Dependence of developing magnetic hysteresis characteristics on stages of evolving microstructure in polycrystalline yttrium iron garnet

The microstructure evolution in several polycrystalline yttrium iron garnet samples as a result of a sintering scheme was studied in detail, in parallel with the changes in their magnetic properties. Samples with nanometer sized starting powder were synthesized by employing the High-Energy Ball Milling technique and then sintering toroidal compacts of the milled powder. Nine sintered samples were obtained, each corresponding to a particular sintering from 600 °C to 1400 °C. The samples were characterized for their evolution in crystalline phases, microstructure and magnetic hysteresis-loops parameters. The results showed an increasing tendency of the saturation magnetization and saturation induction with grain size, which is attributed to crystallinity increase and to reduction of demagnetizing fields in the grains. The variation in coercivity could be related to anisotropy field changes within the samples due to grain size changes. In particular, the starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped B-H loops seems to be dependent on a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. Viewed simultaneously, the hysteresis loops appear to belong to three groups with different magnetism-type dominance, respectively dependent on phase purity and three different groups of grain size distributions.     

Fabrication and magnetic properties of hexagonal BaFe12O19 ferrite obtained by magnetic-field-assisted hydrothermal process

High magnetic field effects on the microstructure and magnetic properties of BaFe₁₂O₁₉ hexaferrites synthesized hydrothermal method have been investigated. The obtained results indicate that the lattice constant decreases gradually as the magnetic field strength increases, which may be attributed to the lattice distortion resulted from the high magnetic field. Polycrystalline BaFe₁₂O₁₉ samples prepared under magnetic field strength at zero and 5 T are single phase. It is found that application of external magnetic field during synthesis can induce orientated growth of the hexaferrite crystals along the easy magnetic axis. The magnetic properties can be effectively regulated by an application of high magnetic fields. It is observed that the BaFe₁₂O₁₉ prepared under a 5 T magnetic field exhibits a higher room-temperature saturation magnetization (66.3 emu/g) than that of the sample (43.6 emu/g) obtained without magnetic field. The results can be explained as the enhanced crystalline, improvement of Fe³⁺ ions occupancy and the oriented growth induced by the external magnetic field. The growing orientation of particles gives rise to increased coercivity due to the enhancement in shape anisotropy. It is expected that an application of magnetic field during the formation of magnetic nanoparticles could be a promising technique to modify magnetic properties with excellent performance.     

Formation rate of vapor bubbles in magnetic fluid boiling at a single vaporization center: Measuring technique and experimental setup

A technique for measuring the bubbling rate during magnetic fluid boiling at a single vaporization center in an external uniform magnetic field is suggested and proved, and an experimental setup is designed based on this technique. It is shown that the magnetic field substantially influences the rate of detachment of the bubbles in a wide range of temperatures below the saturation temperature. The diameter of the vapor bubble at which it separates in the magnetic field (separation diameter) is estimated.     

Development of a magnetic measurement device for thin ribbon samples

This paper presents a magnetic measurement device for thin ribbon samples, which are produced by rapid cooling technique. This device enables us to measure magnetic properties easily by only inserting a ribbon sample into a sample holder. The sample holder was made by bakelite to fix any width sample. A long solenoid coil was used to generate a uniform magnetic field and the sample holder was placed at the mid part of the solenoid. The magnetic field strength was measured using a shunt resistor and the magnetic flux density and magnetization in sample ribbons were evaluated by using search coils. The accuracy of measurement was verified with an amorphous metal ribbon sample. Next, we have measured magnetic properties of some magnetic shape memory alloys, which have different compositions. The measured results are compared and we clarified the effect of Sm contents on the magnetic properties.     

Effect of pulsed magnetic field annealing on the microstructure and texture of grain-oriented silicon steel

The effect of pulsed magnetic primary annealing on the microstructure and texture of two-stage cold-rolled silicon steel is investigated. Specimens are annealed at 700 °C for 1 h under a 1 T pulsed magnetic field along different directions with respect to the sample coordinate system. Crystallographic orientation and grain size are identified by analyzing electron backscattered diffraction pattern. The effects of magnetic field treatment are related to the magnetic field direction. Based on the anisotropy energy of ferromagnetic material during magnetic annealing, a hypothesis is proposed. All of the experimental results in this work support the proposed model.     

The influence of external magnetic field and femtosecond laser pulses on the anomalous Kerr loops in GdFeCo amorphous film

The anomalous loops obtained by applying the traditional Kerr loop technique, in which the sample was pumped by continuous pulses using the method of successive increment scanning external magnetic field, contained memory and accumulation effects originating, respectively, from external field history and multiple pulse excitations. To eliminate the memory effect and study the influence of external magnetic field on the anomalous loops, an initializing field scanning approach was used to replace the method of successive increment scanning external field. And to confirm the existence of accumulation effect and study the influence of pump pulses on the anomalous loops, a controllable pump-pulse-number magneto-optical Kerr technique was used to replace continuous pulses pump Kerr technique. In both ways, measured with different pump fluence and different pump pulses, the results showed clearly the dependence of the hot coercivity and the degree of magnetization reversal on the external field, pump pulse numbers and pump fluence.     

Analysis of magnetic nanoparticles using quadrupole magnetic field-flow fractionation

The new technique of quadrupole magnetic field-flow fractionation is described. It is a separation and characterization technique for particulate magnetic materials. Components of a sample are eluted from the separation channel at times dependent on the strength of their interaction with the magnetic field. A quadrupole electromagnet allows a programmed reduction of field strength during analysis of polydisperse samples.     

磁场成型对锶铁氧体磁性能影响的模型研究

磁场成型是锶铁氧体获得高性能永磁材料的关键工艺.利用已建立的数学模型,研究锶铁氧体磁畴中心和材料几何中心相对位置变化对锶铁氧体磁性能的影响.结果表明,磁畴中心向下偏移比其向上偏移时,外磁场对磁畴中心的作用效果明显;同时当试样呈平面时,外磁场全部垂直作用于试样上,有效作用为最佳,并可以得到高矫顽力各向异性锶铁氧体.     

Development of neutron spin phase contrast imaging

We present an imaging technique utilizing a neutron spin interferometer. Neutron spin phase contrast is achieved in spatial resolved measurements of the phase difference between two superposed neutron spin states introduced by passing through a magnetic sample. Since the phase difference of spin states parallel and anti-parallel to the magnetic field is proportional to the magnetic field integral, it is possible to record images of the internal magnetic field distribution of the sample. Taking advantage of high transmission probabilities, neutron spin phase contrast provides non-destructive images of internal magnetic structures.     

强磁场对不同厚度Fe80Ni20薄膜的微观结构及磁性能的影响

有无6 T强磁场条件下利用分子束气相沉积方法制备了不同厚度的Fe₈₀Ni₂₀薄膜. 研究发现, 薄膜的面内矫顽力随厚度增加而降低且符合Neel理论; 矩形比随厚度的增加先快速增大后缓慢降低; 6 T磁场抑制了颗粒团聚及异常长大, 并降低了薄膜表面的粗糙度, 这使薄膜的矫顽力要小于无磁场作用的薄膜, 矩形比大于无磁场作用的薄膜; 而且薄膜在垂直于基片表面的6 T磁场作用下由0 T下的面内磁各向异性转变为磁各向同性. 关键词： 强磁场 气相沉积 微观结构 磁性能     

Training, constraints, and high-cycle magneto-mechanical properties of Ni-Mn-Ga magnetic shape-memory alloys

Magneto-mechanical experiments with a rotating magnetic field of 0.97 T were performed with a Ni-Mn-Ga single crystal. Periodic strains exceeding 1% were recorded over a hundred million magneto-mechanical cycles. The twin microstructure of the cycled crystal was characterized using atomic force microscopy (AFM) and magnetic force microscopy (MFM). In the center of the sample, no twin boundaries were found. At the sample edges, the microstructure shows a dense twin pattern. The results are compared with previous experiments of differently trained crystals. It is useful to distinguish between “ineffective training”, which results in a nearly self-accommodated martensite, and “effective training”, which results in a nearly single-variant crystal. The evolution of twin structure is discussed in terms of training, magneto-mechanical cycling, and extrinsic constraints imposed by the experimental setting. It is concluded that the response of a magnetic shape memory alloy to an alternating excitation depends strongly on the initial twin microstructure established through training. In particular, ineffective training results in a twin microstructure which can adapt to extrinsic constraints resulting in continued large periodic magnetic-field-induced deformation. In contrast, the twin microstructure of an effectively trained crystal can not adapt to extrinsic constraints resulting in early failure by fracture.     

Ni0.8Co0.2-SiO2颗粒膜中的Hall效应

对用离子束溅射制备的Ni_0.8Co_0.2-SiO₂颗粒膜的Hall效应进行了研究.当Ni_0.8Co_0.2合金的含量为43vol%时,在9.5×10⁵A/m的外磁场下,样品的室温Hall电阻率达6.3μΩ·cm ,高于普通磁性金属两个数量级以上,还研究了不同NiCo组分样品的电阻率与温度的关系（95—300K）,发现x＜x_c(逾渗组分)时,电阻率与-logT成正比,电阻温度系数为负;当x＜x_c时,逐渐呈现金属性;而当x≈x_c时,为过渡态.还用透射电子显微镜和振动样品磁强计等手段对样品的微结构和磁性进行了对比研究,表明金属-绝缘体颗粒膜中在逾渗阈值附近有较大的Hall效应,这与样品的微结构有密切关系. 关键词：     

Magnetodeformation of ferrogels and ferroelastomers. Effect of microstructure of the particles’ spatial disposition

The work deals with the theoretical study of ferrogel/ferroelastomer deformations under the action of a homogeneous magnetic field. The compositions with linearly magnetizable non-Brownian particles are considered. Under the field action the ferrogel/ferroelastomer samples can either elongate or contract in the field direction. The type of deformation is determined by the sample shape and microstructure of the particles’ spatial disposition. Two kinds of systems are studied. The first one is the composition with the particles homogeneously distributed, like in an ideal gas, in the polymer matrix. The second kind is the system of linear chain-like aggregates of densely situated particles. The results show that the particles’ spatial disposition in the matrix is crucially important for the type of macroscopical deformation of the magnetopolymer compositions. The systems with the gas-like distribution of particles, depending on the sample shape, can either elongate or contract, whereas the chains stimulate the sample elongation.     

1-D EPR imaging of conducting and lossy-dielectric samples

Radiation power absorbed by a half-infinite sample irradiated by microwave field incident normally to the sample surface is calculated. Local properties of the sample, such as electric conductivity, magnetic susceptibility, concentration of localized spins, etc., are supposed to be nonhomogeneous in the direction perpendicular to the surface. In linear on magnetic susceptibility approximation, algorithms of electron paramagnetic resonance imaging are suggested for the case of conducting and lossy-dielectric materials. These algorithms are tested by numerical simulations. Obtained results can be applied to nuclear magnetic resonance imaging as well.     

High-field ESR on aligned membranes: a simple method to record spectra from different membrane orientations in the magnetic field

A combination of isopotential spin-dry ultracentrifugation (ISDU) and microtome techniques was used to facilitate the collection of high field/high frequency (170 GHz) ESR spectra corresponding to different orientations of the membrane normal relative to the magnetic field. This technique is particularly valuable for aligned biological samples in vitro. At 170 GHz, conventional sample preparation techniques based solely on ISDU constrained the sample to be oriented so that the membrane normal was parallel to the applied magnetic field due to the geometry and the millimeter wave field distribution of the Fabry-Pérot resonator used in these experiments. This orientational constraint limited the information that could be obtained from aligned membranes at high field. The combined ISDU/microtome technique overcame this limitation. Spectra from spin-labeled Gramicidin A and the spin label cholestane in aligned DPPC membranes provide a demonstration of the technique. We also discuss some virtues of high field/high frequency ESR on aligned membranes compared to X-band.     

Sm_2Co_(17)型永磁合金的辐照效应研究

Sm_2Co_(17)型永磁合金大量使用在上海同步辐射光源储存环的永磁型波荡器上,在受到长期辐照后会发生磁性能损失的现象,进而影响同步辐射光的品质.为了探索其潜在的微观机理,本文首先对Sm_2Co_(17)型永磁合金所处混合辐射场的粒子及相关物理量进行了计算分析,确定引发磁性能损失的主要粒子是中子.然后采用Ar离子模拟中子辐照损伤的方法对其进行辐照,采用透射电镜对其辐照前后的微观形貌及微观结构进行了研究探讨,采用振动样品磁强计对永磁合金辐照前后的饱和磁化强度进行了分析对比,并讨论了微观结构演化与宏观磁性能变化的联系.结果表明,Ar离子辐照后Sm_2Co_(17)型永磁合金饱和磁化强度的不可逆损失与微观结构变化有直接的关系,其2:17相从单晶结构转变为非晶结构是造成其磁性能损失可能的微观机制.     

Synthesis of magnetite nanoparticles for AC magnetic heating

Magnetite particles with different average diameter (D_m) suitable for magnetic fluid hyperthermia (MFH) were synthesized by controlled coprecipitation technique. In this method, the reaction pH was stabilized using the pH buffer and the average particle diameter decreased with increasing reaction pH. The size-dependent magnetic behavior of the magnetite nanoparticles was studied and the optimum size range required for magnetic fluid hyperthermia (MFH) has been arrived at. Among the samples studied, the maximum specific absorption rate of 15.7 W/g was recorded for the magnetite sample with D_m of 13 nm, when exposed to an AC magnetic field strength of 3.2 kA/m and a frequency of 600 kHz. The AC magnetic properties suggested that the size distribution of the sample was bimodal with average particle size less than ∼13 nm.     

强磁与应力场耦合作用下AZ31镁合金塑性变形行为

研究强磁场对AZ31镁合金塑变能力和微观组织的作用,在3 T脉冲强磁场条件下对合金进行磁场耦合应力时的拉伸实验.采用电子背散射衍射、Ⅹ射线衍射和透射电镜分析等方法研究材料的微观组织.结果表明:与0 T拉伸试样相比,3 T拉伸试样抗拉强度和延伸率分别提高了2.2%和28.7%,说明将强磁场耦合作用于材料塑性变形过程时,能在不降低材料强度的同时提高镁合金的塑性变形能力,有助于同步改善材料强韧性.磁场作用机理主要表现为磁致塑性效应,计算表明主要合金相β(Mg_(17)Al_(12))为顺磁性,有助于发挥磁场作用效果.磁场提高了位错运动灵活性并促使位错增殖,晶界处位错堆积和应力集中促进了再结晶形成,晶粒发生细化,发挥细晶强韧化效果;同时磁场诱发塑性变形时的晶粒转动,新生成非基面取向的晶粒弱化了镁合金(0001)基面织构,该组织特征有助于提高材料的塑变能力.