Magnetooptical images of inhomogeneous magnetic fields in metallic films with in-plane anisotropy

The magnetooptical (MO) images of the inhomogeneous field created by permanent magnets in magnetic metallic films with in-plane anisotropy are experimentally studied. The MO images recorded using the longitudinal Kerr effect are the superposition of two pictures, namely, a polar-sensitivity MO image and a longitudinal-sensitivity MO image. An analysis of these images after separation shows that the polar-sensitivity MO image reflects the distribution of the inhomogeneous field component that is normal to the surface of an indicator film in an analog manner. The longitudinal-sensitivity MO image reflects the angular distribution of the in-plane component of a stray field in an analog manner. The coincidence of the experimental and corresponding simulated MO images makes it possible to interpret the experimental images. In particular, it is shown that the specific features detected in the topological characteristics of the inhomogeneous field correspond to experimental singular points. Hidden magnetic images (magnetic bar codes) are shown to be visualized with metallic CoFe films. As an example, the stray field of a magnetic system made of cylindrical magnets is mapped.     

Radiofrequency magnetic field gradient echoes have reduced sensitivity to susceptibility gradients

The amplitudes of gradient-echoes produced using static field gradients are sensitive to diffusion of tissue water during the echo evolution time. Gradient-echoes have been used to produce MR images in which image intensity is proportional to the self-diffusion coefficient of water. However, such measurements are subject to error due to the presence of background magnetic field gradients caused by variations in local magnetic susceptibility. These local gradients add to the applied gradients. The use of radiofrequency (RF) gradients to produce gradient-echoes may avoid this problem. The RF magnetic field is orthogonal to the offset field produced by local magnetic susceptibility gradients. Thus, the effect of the local gradients on RF gradient-echo amplitude is small if the RF field is strong enough to minimize resonance offset effects. The effects of susceptibility gradients can be further reduced by storing magnetization longitudinally during the echo evolution period. A water phantom was used to evaluate the effects of background gradients on the amplitudes of RF gradient-echoes. A surface coil was used to produce an RF gradient of between 1.3 and 1.6 gauss/cm. Gradient-echoes were detected with and without a 0.16 gauss/cm static magnetic field gradient applied along the same direction as the RF gradient. The background static field gradient had no significant effect on the decay of RF gradient-echo amplitude as a function of echo evolution time. In contrast, the effect of the background gradient on echoes produced using a 1.6 gauss/cm static field gradient is calculated to be significant. This analysis suggests that RF gradient-echoes can produce MR images in which signal intensity is a function of the self-diffusion coefficient of water, but is not significantly affected by background gradients.     

Auxiliary phase encoding in multi spin-echo sequences: application to rapid current density imaging

Multi spin-echo sequences such as single-shot RARE are very sensitive to the initial phase of the transverse magnetization, and they can preserve only the transverse magnetization component which is aligned with the axis of the refocusing pulse rotation. Therefore, two separate single-shot RARE experiments with phases of refocusing pulses 90 degrees apart have to be run and their complex images summed to obtain an error-free phase map of the initial transverse magnetization. This is particularly useful when auxiliary phase encoding is integrated in the preparation period of the RARE sequence, such as when encoding flow, displacement, susceptibility, pH or temperature. In this paper, the two-shot RARE approach is verified first theoretically and then experimentally by demonstrating its application to rapid current density imaging (CDI). The sequence consists of the preparation period which triggers electric pulses in the sample followed by the RARE acquisition period. Electric currents through the sample induce a magnetic field change in the direction of the static magnetic field and a phase change of the initial magnetization proportional to it. To calculate one component of current density two orthogonal components of magnetic field change must be measured. In general, for 2D non-symmetrical samples, this can be done by rotating the sample to a perpendicular orientation. The proposed CDI method allows much for faster magnetic field change mapping than the standard spin-echo based CDI.     

利用法拉第效应进行永磁体磁场二维光学成像

借助于法拉第磁致旋光效应,以ZF6玻璃为磁旋光介质,设计了适当的光学系统,对永磁体磁极附近磁场的分布进行了光学二维成像.利用自编的图像采集软件采集了一系列原始图片,并利用自编的图像处理软件对这些原始图片进行了处理,完成了检偏过程.分别得到了透射成像方式和反射成像方式下永磁体磁场的分布图像,这些图像正确反映了永磁体磁场的实际分布.本研究工作为宏观尺度磁场的观测与测量提供了一种有效的手段.     

Spin-polarized scanning tunneling microscopy with antiferromagnetic probe tips

We have performed low temperature spin-polarized scanning tunneling microscopy (SP-STM) of two monolayers Fe on W(110) using tungsten tips coated with different magnetic materials. We observe stripe domains with a magnetic period of 50 +/- 5 nm. Employing Cr as a coating material we recorded SP-STM images with an antiferromagnetic probe tip. The advantage of its vanishing dipole field is most apparent in external magnetic fields. This new approach resolves the problem of the disturbing influence of a ferromagnetic tip in the investigation of soft magnetic materials and superparamagnetic particles.     

Investigation of micromagnetism and magnetic reversal of Ni nanoparticles using a magnetic force microscope

Isolated Ni nanoparticles were studied in situ by atomic and magnetic force microscopy in the presence of an additional external field up to 300 Oe. By comparing topographic and magnetic images, and also by computer modeling of magnetic images, it was established that particles smaller than 100 nm are single-domain and easily undergo magnetic reversal in the direction of the applied external magnetic field. For large magnetic particles, the external magnetic field enhances the magnetization uniformity and the direction of total magnetization of these particles is determined by their shape anisotropy. Characteristics of the magnetic images and magnetic reversal of particles larger than 150 nm are attributed to the formation of a vortex magnetization structure in these particles. Fiz. Tverd. Tela (St. Petersburg) 40, 1277–1283 (July 1998)     

Correction of concomitant gradient artifacts in experimental microtesla MRI

Magnetic resonance imaging (MRI) suffers from artifacts caused by concomitant gradients when the product of the magnetic field gradient and the dimension of the sample becomes comparable to the static magnetic field. To investigate and correct for these artifacts at very low magnetic fields, we have acquired MR images of a 165-mm phantom in a 66-microT field using gradients up to 350 microT/m. We prepolarize the protons in a field of about 100 mT, apply a spin-echo pulse sequence, and detect the precessing spins using a superconducting gradiometer coupled to a superconducting quantum interference device (SQUID). Distortion and blurring are readily apparent at the edges of the images; by comparing the experimental images to computer simulations, we show that concomitant gradients cause these artifacts. We develop a non-perturbative, post-acquisition phase correction algorithm that eliminates the effects of concomitant gradients in both the simulated and the experimental images. This algorithm assumes that the switching time of the phase-encoding gradient is long compared to the spin precession period. In a second technique, we demonstrate that raising the precession field during phase encoding can also eliminate blurring caused by concomitant phase-encoding gradients; this technique enables one to correct concomitant gradient artifacts even when the detector has a restricted bandwidth that sets an upper limit on the precession frequency. In particular, the combination of phase correction and precession field cycling should allow one to add MRI capabilities to existing 300-channel SQUID systems used to detect neuronal currents in the brain because frequency encoding could be performed within the 1-2 kHz bandwidth of the readout system.     

Magnetic duplication and contact printing method

We present a simple and effective method of magnetic duplication. It is based on the printing of a patterned microstructure from a medium with low coercivity into a medium with high coercivity by subjecting both media to the external magnetic field. The prints obtained are studied by magnetic force microscopy and magnetooptic visualisation techniques. The images of duplicated prints show high efficiency of the method. Theoretical estimates of the external magnetic field range suitable for duplication are in good agreement with the magnetic field range obtained in experiment. Numerical calculations of the field distribution during duplication are also presented.     

Time-resolved measurement technique for pulsed electron beam envelope basing on framing and streaking principle

The time-resolved electron beam envelope parameters, including cross sectional distribution and beam centroid position, are very important for the study of beam transmission characteristics in a magnetic field and for verifying the rationality of the magnetic field parameters employed. One kind of high time-resolved beam envelope measurement system has recently been developed, constituted of a high-speed framing camera and a streak camera.It can obtain three panoramic images of the beam and time continuous information along the given beam profile simultaneously. Recently obtained data has proved that several fast vibrations of the beam envelope along the diameter direction occur during the front and the tail parts of the electron beam. The vibration period is several nanoseconds. The effect of magnetic field on the electron beam is also observed and verified. Beam debugging experiments have proved that the existing beam transmission design is reasonable and viable. This beam envelope measurement system will establish a good foundation for beam physics research.     

Multi-objective optimization of gradient coil for benchtop magnetic resonance imaging system with high-resolution

Significant high magnetic gradient field strength is essential to obtaining high-resolution images in a benchtop mag- netic resonance imaging （BT-MRI） system with permanent magnet. Extending minimum wire spacing and maximum wire width of gradient coils is one of the key solutions to minimize the maximum current density so as to reduce the local heating and generate higher magnetic field gradient strength. However, maximum current density is hard to optimize together with field linearity, stored magnetic energy, and power dissipation by the traditional target field method. In this paper, a new multi-objective method is proposed to optimize the maximum current density, field linearity, stored magnetic energy, and power dissipation in MRI gradient coils. The simulation and experimental results show that the minimum wire spacings are improved by 159% and 62% for the transverse and longitudinal gradient coil respectively. The maximum wire width increases from 0.5 mm to 1.5 mm. Maximum gradient field strengths of 157 mT/m and 405 mT/m for transverse and lon- gitudinal coil are achieved, respectively. The experimental results in BT-MRI instrument demonstrate that the MRI images with in-plane resolution of 50 ~tm can be obtained by using the designed coils.     

Effect of Local Magnetic Field in G Measurement with Time-of-Swing Method

The effect of the local time- varying magnetic field in our G measurement with the time-of-swing method is studied by magnifying the magnetic field to cause a perceptible change in the pendulum＇s period. The experimental result shows that the coefficients of the change in the period to the magnetic field are 37（1） and 12（1） ms/gauss in the two horizontal directions respectively, which means that the systematic uncertainty due to the local magnetic field is less than 0.4ppm in our G measurement.     

Electron mirror as a corrector of electron microscope objective aberrations

We describe a new optoelectronic system for correcting aberrations of the objective lens with the help of an electron mirror. The system is based on the implementation of a special focusing regime (so-called regime of superimposed images) in which two images of the object are formed in the plane passing through the center of the curvature of the mirror. One of these images is formed by the beams (with aberrations) emerging from the objective lens, and the other is formed by the beams (with cancelled aberrations) reflected from the mirror. The separation of the two superimposed images and visualization of the image with cancelled aberrations are performed by deflection of the electron beam in the axisymmetric magnetic field whose symmetry axis passes through the center of curvature of the electron mirror. The magnetic field distribution ensuring aberration-free deflection of the electron beam is calculated.     

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.     

时间分辨电子显微镜的数值模拟

对时间分辨电子显微镜进行了数值模拟。通过求解从样品透射出来的电子在静态磁场和动态电场的混合场中的运动,评价时间分辨电子显微镜的动态时空特性。根据该数值模拟,时间分辨电子显微镜能够在荧光屏上获得样品在不同时刻的6幅显微分幅图像。     

Fourier transform imaging of spin vortex eigenmodes

Thin-circular lithographically defined magnetic elements with a spin vortex configuration are excited with a short perpendicular magnetic field pulse. We report the first images of excited magnetic eigenmodes up to third order, obtained by means of a phase sensitive Fourier transform imaging technique. Both axially symmetric and symmetry breaking azimuthal eigenmodes are observed. We observe strong oscillations of the magnetization in the central part of the magnetic elements. The experimental data are in good agreement with micromagnetic simulations.     

Use of a Phase Reference for Field Mapping with Amplitude Images at Low Field

We present a method to obtain MRI amplitude images that can picture the magnetic field due to arbitrary shaped magnetized objects. The method employees the gradient recalled echo sequence and two sets of data obtained in separate experiments, one of which provides a phase reference image making it possible to eliminate the effect of theB₀field inhomogeneities. The final magnitude images have a good signal-to-noise even at low fields, and provide qualitative as well as quantitative information about the magnetic field produced by the ferromagnetic object. As an example the method is applied to study the field produced by a small metal piece in a 500-G scanner, and the experimental results are compared with numerical simulations.     

两种外磁场形式对介质面次级电子倍增的抑制

利用自编1D3V PIC程序,数值研究了不同外加磁场方式对次级电子倍增抑制的物理过程,给出了次级电子数目、平均能量、密度、运动轨迹、渡越时间、介质表面静电场及沉积功率等物理量时空分布关系。模拟结果表明:不同方向外加磁场抑制次级电子倍增的机理有所不同。轴向外加磁场利用电子回旋运动干扰微波电场对电子加速过程,使其碰壁能量降低以达到抑制二次电子倍增的效果;横向外加磁场利用电子回旋漂移过程中,电子半个周期被推离介质表面（不发生次级电子倍增）,半个周期被推回介质表面（降低电子碰撞能量）的作用机理,达到抑制二次电子倍增的效果。讨论了横向磁场在回旋共振下,电子回旋同步加速导致回旋半径增大,电子能量持续增加的特殊过程。两种外加磁场方式都可以通过增加磁场达到进一步抑制次级电子倍增的目的。轴向外加磁场加载容易,但对磁场要求较高;横向外加磁场需要磁场较低,但加载较为困难。     

线电流与条形铁磁质板所形成的磁场

利用已有的儒可夫斯基变换结论,通过圆柱镜像法,计算线电流与薄条形铁磁质板所形成的磁场,给出其磁矢势的分布和磁感线方程,并利用数学软件MATLAB绘制出磁感线图.     

Periodic structure of acicular magnetic clusters in a magnetic liquid

The behavior of magnetic clusters in a magnetic liquid placed in a circular capillary is considered. When a uniform magnetic field is applied to the system, acicular clusters grow from the sediment, being aligned with the field. The interaction of the clusters as magnetic dipoles with one another and with an external gradient magnetic field is considered theoretically. In a nonuniform symmetric magnetic field with a peak, the cluster distribution is uniform near the peak. Such a distribution is fairly stable when the magnetic field gradient is varied over certain limits. The ordered (periodic) cluster configuration is realized experimentally, and it is shown that its period can be controlled. As the magnetic field gradient exceeds a certain threshold, the clusters are redistributed, forming a multirow hexagonal array.     

极化中子照相磁场量化技术方案比较与分析

极化中子照相技术通过分析极化中子束的自旋相移对样品磁场进行成像,目前已发展出多种成像技术方案,其中能量选择法和自旋回波法极化中子成像技术从不同的原理出发,解决了极化中子照相中磁场量化的周期解问题,同时避免装置极化效率等参数的影响,可以实现较高的量化精度.本文对两种极化中子照相技术方案进行研究,通过对单色器能量分辨率和装置极化效率等关键参数的分析和模拟,确定在研究堆上开展相关实验的可行性,并初步明确其量化能力和适用范围.相关结果可为极化中子照相的实验数据处理技术研究及装置设计提供参考.