SEM investigation of the temperature dependence of magnetic domain structure of cobalt monocrystals

The magnetic domain structure of a cobalt monocrystal is observed by means of a scanning electron microscope (SEM). It is revealed by the so-called type-I magnetic contrast [1]. The dependence of the magnetic domain structure on temperature up to about 700 K is investigated and discussed. Digital image processing (image restoration, enhancement and analysis) is used on the images obtained directly from the SEM. The main reasons for the application of digital image processing are: poor resolution of type-I magnetic contrast due to the diffuseness of the leakage magnetic fields above the specimen surface, and the complex character of the magnetic domain structure. Statistical distributions of magnetic domain width are also calculated and presented.     

Observation of the domain structure of Nd–Fe–B magnets using the SEM type-I magnetic contrast

In this paper, we present the first observation of the domain structure of Nd–Fe–B magnets with the type-I magnetic contrast in a scanning electron microscope (SEM). The applied method was supported with digital image recording, enhancement and analysis. Observations were made at the surfaces perpendicular to the alignment axis. The domain pattern is revealed in the form of undulated stripes magnetized alternately in the two directions along the alignment axis. However, because of insufficient spatial resolution of the SEM type-I magnetic contrast we could not observe reverse spike domains of about 0.5 μm in diameter, the presence of which was proved by Bitter pattern technique and magnetic force microscopy (MFM). The smallest resolvable domain was 0.8 μm in width, being the best result so far obtained with the type-I magnetic contrast method. Some aspects related to the domain observation with the method applied are discussed in more detail. It is anticipated that the spatial resolution of the method can be improved to 0.2–0.3 μm by employing SEMs with high-brightness electron guns.     

Magnetic contrast in the scanning electron microscope: An appraisal of techniques and their applications

The SEM represents a comparatively new technique for the imaging of magnetic domains and a “state of the art” review of its applications and potential is given. A simple account of electron scattering, presented initially, serves as a basis to explain the physical origin and characteristics of the two principal modes of magnetic contrast. Simple models of the imaging process are discussed as well as the instrumental factors necessary to optimize contrast and resolution.The SEM has been used to investigate the domain structures of recording tape, cobalt and many magnetic oxides, including bubble materials. It also yields quantitative information about field distributions such as those found in recording heads. Domain and domain wall images may also be observed in cubic materials and this is particularly useful for studying the behaviour of transformer grain oriented silicon-iron, especially at high accelerating voltages where the coating can be penetrated to reveal the magnetic structure beneath.When allied with its versatility in other fields of operation it is concluded that the SEM constitutes a valuable tool for domain observation and will find increasing use as such.     

Domain Investigation by the Conventional Bitter Pattern Technique with Digital Image Processing

In the cases of soft magnetic materials or complex domain configurations, investigations of the domain structure by the conventional Bitter pattern technique are generally difficult. It is demonstrated in this paper, referring to the examples of domain images of thin permalloy films and the basal surface of bulk cobalt single crystals, that this problem can be overcome by the application of digital image processing (DIP) system. In particular, the visibility limit in domain observation was expanded by an order of magnitude and high quality domain images could be obtained. Improvements over earlier results were achieved.     

Estimation of SEM imaging resolution of the domain structure of pyroprobe-heated ferroelectrics

The specific contrast of scanning electron microscope (SEM) images of ferroelectric domains observed in the pyrocurrent (pyroprobe) mode is analyzed. Calculations taking into account both the nonuniform heating of domains and the heat diffusion via the domain walls and the crystal boundaries are performed. It is established that the heat diffusion smears the domain images of small domains. Along with the probe diameter, the scan rate is shown to be an important factor determining the character of the SEM image contrast. A decrease in the scan rate may appreciably lower the resolution even in case of a fairly thin probe.     

Investigation of the domain structure of sintered Nd-Fe-B permanent magnets by Bitter-pattern method

The conventional Bitter-pattern technique and the colloid-scanning electron microscopy (colloid-SEM) method were used to study the domain structure of polycrystalline sintered Nd-Fe-B permanent magnets. In the thermally demagnetized state most of the grains are multidomain and the domain structures resemble those observed in bulk uniaxial crystals with strong magnetocrystalline anisotropy. Investigations of the magnetic microstructure during magnetizing cycle showed that the domain walls can easily be moved within the grains and that the magnetization reversal in sintered Nd-Fe-B magnets occurs predominantly by the nucleation and expansion of reverse domains at structural imperfections near the grain boundaries. It is also shown that the colloid-SEM method is more surface sensitive and reveals the domain structure with better resolution than the conventional Bitter technique. Thanks to the application of digital image processing systems, clear and high contrast domain images were obtained. The work was supported by the Lódź University within Research Grant 505/694 (2004).     

Understanding magnetic force microscopy

Magnetic force microscopy is a new method for imaging ferromagnetic domains with a high lateral resolution (10 nm). In this paper we give the basic tip parameters that have to be taken into account to achieve a quantitative image interpretation. For the electrochemically otched polycrystalline iron, nickel and cobalt wires, the tip-apex domain is found to be oriented along the tip axis, because of shape anisotropy. The stray field emerging from the tip apex is comparable to the size of the tip saturation field. The effective domain lengthL determines the image formation: the force due to magnetization patterns of scales which are large compared toL follow the point-dipole approximation. In the opposite case, a single-pole model is more appropriate. While a cobalt tip can be treated as an isolated domain, for nickel and iron a net polarization in the tip wire induced by the front apex-domain has to be considered. A new analytical theory provides an overall understanding of the image formation and allows the determination of the magnetic field vector and the estimation of its magnitude from measurements.     

图像复原处理方法在显微测量LiNbO_3表面缺陷中的应用

研究了图像复原处理在显微测量中的应用。在实测系统点扩展函数的基础上,首先利用图像复原方法对显微图像进行复原处理,然后用二维处理方法实现对缝宽的精确显微测量。实现了在普通光学显微镜下铌酸锂(LiN- bO_3)极化畴结构样品缺陷大小的便捷测量。     

Off-axis electron holography of hetero-interfaces

Off-axis electron holography is one of the new emerging methods for high spatial resolution characterization of interfaces in materials. It enables recording and retrieval of both amplitude and phase of an electron wavefunction scattered by a specimen. Phase changes introduced by magnetic and electrostatic fields have been studied in the first applications of electron holography to domain walls in ferromagnetic and ferroelectric materials and to p-n junctions. Planar defects in single crystals, such as stacking faults, have been observed with strong phase contrast due to dynamical diffraction.Applications to heterogeneous interfaces have only started. High phase contrast due to mean inner potential differences is found for interfaces between high and low atomic number materials. Dynamical contributions to the phase of the transmitted beam are important for epitaxial interfaces in strongly diffracting orientations. Numerical hologram reconstructions yield quantitative amplitude and phase images of an interface which are energy filtered and are in perfect registry. Both are function of specimen thickness. The thickness dependence can be eliminated by division of the phase image with a logarithm of the amplitude image. This ratio maps the product of the mean inner potential and the mean free path for inelastic scattering across a hetero-interface in weekly diffracting orientations.Resolution enhancement through aberration correction has not been demonstrated for interfaces as yet. Holography of interfaces in plan view is unexplored.     

Magnetic contrast from domain walls in BaFe12O19 by tunneling stabilized magnetic force microscopy

The magnetic structure of BaFe₁₂O₁₉ is imaged with a scanning tunneling microscope having a flexible, magnetic tip. We find that Fe thin films evaporated on a silicon tip, integrated with a cantilever, behave as magnetically soft tips. Therefore, we are able to image domain walls with high lateral resolution. A different contrast along the domain wall due to surface magnetic charges is observed. We explain the data using previously established models for wavy domain walls. The obtained images are the first experimental evidence of magnetic charges induced on the wavy domain walls in BaFe₁₂O₁₉.     

单晶石墨、多晶石墨电导行为的差异

报道了单晶石墨、多晶石墨电导行为的差异.在8.15T磁场下，4.5K处单、多晶石墨的正磁电阻效应分别为69400%，170%，同时在0T，8T磁场作用下，多晶石墨的电导行为呈现类绝缘体型性质，但在单晶石墨中我们发现了与磁场相关的类半金属-绝缘体型转变，通过分析，我们认为：单、多晶石墨电导行为存在较大差异的原因可能来源于库仑相互作用在高定向热解石墨中变得不可忽视，而多晶石墨样品却存在晶界散射. 关键词： 半金属石墨 类半金属-绝缘体型转变     

Hyperfine interaction in amorphous ferromagnetic cobalt-phosphorous-alloys measured by inelastic scattering of neutrons

The hyperfine interaction at the cobalt nucleus in amorphous ferromagnetic cobalt-phosphorous-alloys CoP _x has been investigated by measuring the inelastic incoherent spin-flip scattering of neutrons. A neutron spectrometer of an extremely high energy resolution of 67 MHz full width at half maximum (FWHM) was used. It was found that the magnetic hyperfine splitting, in first order, is proportional to the magnetic moment of the cobalt atom and that there is a distribution of hyperfine fields of about 50 MHz around a mean value which depends on the phosphorous concentration.

     

Width determination of SiO2-films in Si-based devices using low-loss EFTEM: image contrast as a function of sample thickness

Energy filtering transmission electron microscopy (EFTEM) has become one of the most efficient tools for specimen characterization at nanometer length scales. EFTEM imaging is most often carried out in the core-loss region but image intensity becomes more and more a limiting factor with decreasing feature size. Alternatively, it is possible to record EFTEM images in the low-loss region, where intensities are essentially higher and where in many cases the images contain material specific contrasts. In this paper we investigate the influence of the important parameters on the material contrast between silicon and silicon dioxide, e.g. specimen thickness, specimen orientation, energy-loss and energy selecting slit width. We show that sample thickness plays an important role and present two methods to calculate material contrast as a function of energy-loss and sample thicknesses. The first method uses spectra taken from both materials at different sample thickness by electron energy-loss spectroscopy, the second calculates contrast directly from a series of energy filtered images. From the results we determine the ideal acquisition parameters for the Si/SiO(2) system and demonstrate imaging at sufficient resolution below 2nm with a test sample of thin SiO(2) layers on Si.     

X-ray magnetic circular dichroism imaging with hard X-rays

X-ray polarization-contrast images resulting from X-ray magnetic circular dichroism (XMCD) in the hard X-ray region have been successfully recorded for the first time. The apparatus used consisted of an X-ray polarizer, double X-ray phase retarders, and a high-spatial-resolution X-ray charge-coupled-device detector. The sample used was a hexagonal-close-packed cobalt polycrystal foil having a thickness of about 4 microns. The X-ray polarization-contrast image resulting from XMCD was observed at a photon energy of 10 eV above the cobalt K-absorption edge (7709 eV). The observed contrast in the image was reversed by inversion of the magnetic field. Furthermore, the contrast was reversed again at a photon energy of 32 eV above the cobalt K-absorption edge.     

Auger electron microscopy: An overview

The aim of Auger electron imaging is to obtain quantitative surface elemental distribution maps at high spatial resolution. The realization of this goal is complicated by many instrumental effects and by spurious data processing contributions giving rise to an image contrast unrelated to the specimen surface composition. The critical properties of scanning Auger microscopy that may cause such a false information or imaging artefacts are reviewed. Instrumental or data processing related effects appear in the case of the beam current variation, of the background slope effect, and of the use of a combined peak to background ratio. The second set of artefacts are mainly due to the significant differences between the penetration depth of the exciting primary electrons and the escape depth of the Auger electron signal. In this case the net effect is a surface elemental contrast which is dominated by the substrate or by the overlayer rather than by the surface under investigation. In addition, there are also topographical effects of the specimen under test which normally affect the Auger yield and hence the contrast in the image. Methods for the successful suppression of some of these artefacts are outlined. They are based on the creation of reference images from complementary signals acquired by additional detection channels in parallel with the Auger signal of interest.Invited lecture presented at the Seminar on Secondary Electrons in Electron Spectroscopy, Microscopy, and Microanalysis, Chlum (The Czech Republic), 21–24 September 1993.     

Imaging excitations in magnetic thin film microstructures

We show how a photoemission electron microscope (PEEM) installed at a synchrotron can be used to image magnetic objects with very high spatial and temporal resolution. Sub-nanosecond magnetic field pulses are used to excite the fundamental magnetic modes of micron sized permalloy particles. The time evolution of the magnetization is imaged using a pump-probe technique where the magnetic contrast is given by X-ray magnetic circular dichroism (XMCD). Depending on the shape and size of the magnetic object we can observe modes related to either the homogenously magnetized domains, to the domain walls or to the vortex. All of these can be analyzed quantitatively yielding their frequencies, amplitudes and damping time constants. For objects with controlled defects we show that the magnetic vortex can be switched between defects using magnetic field pulses.     

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc. These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, X-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution X-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.     

基于光场一阶关联的时域成像

与通常利用二阶强度关联测量实现时域鬼成像不同,本文利用时域热光源借助干涉仪通过一阶关联实现时域成像.基于空域光束的近轴衍射和时域窄带脉冲在色散介质中色散之间的空间-时间二象性,在时域脉冲响应函数的基础上得到了表征一阶关联时域成像的强度表达式,分析研究了光源脉冲宽度和相干时间对成像可见度和分辨率的影响.结果一方面表明基于热光场一阶关联的时域成像在不需要额外色散补偿或消除条件下可以实现时域物体信号的再现,另一方面表明当光源脉冲宽度一定时,成像可见度随光源脉冲相干时间的增加而增加,但是成像分辨率逐渐降低,其中当光源脉冲宽度约为100 ps,相干时间约为0.5 ps时,间隔为20 ps,宽度为8 ps的时域矩形波型物体的成像质量(兼顾可见度和分辨率)较好.该结果对于基于热光一阶关联的时域成像在时序信号测量中的应用具有重要意义.     

Separation of image-distortion sources and magnetic-field measurement in scanning electron microscope (SEM)

The electron-microscope image distortion generated by electromagnetic interference (EMI) is an important problem for accurate imaging in scanning electron microscopy (SEM). Available commercial solutions to this problem utilize sophisticated hardware for EMI detection and compensation. Their efficiency depends on the complexity of distortions influence on SEM system. Selection of a proper method for reduction of the distortions is crucial. The current investigations allowed for a separation of the distortions impact on several components of SEM system. A sum of signals from distortion sources causes wavy deformations of specimen shapes in SEM images. The separation of various reasons of the distortion is based on measurements of the periodic deformations of the images for different electron beam energies and working distances between the microscope final aperture and the specimen. Using the SEM images, a direct influence of alternating magnetic field on the electron beam was distinguished. Distortions of electric signals in the scanning block of SEM were also separated. The presented method separates the direct magnetic field influence on the electron beam below the SEM final aperture (in the chamber) from its influence above this aperture (in the electron column). It also allows for the measurement of magnetic field present inside the SEM chamber. The current investigations gave practical guidelines for selecting the most efficient solution for reduction of the distortions.     

优化X射线同轴相位衬度成像的理论研究

采用同轴相位衬度成像方法，基于PGW理论获得的频域相位衬度传递函数，对于理想光源和多色X射线源，用计算机模拟分析了如何选择物像距离和空间频率以获取最佳相位衬度像。实际应用中要综合考虑具体成像实验装置对于物像距离的可调范围以及成像分辨率的要求。对于具有一定谱宽的X射线源，应选择谱宽尽量小的源，以获得含信息量丰富、分辨率高的图像。