SEM investigation of the dependence of magnetic domain structure on the thickness of cobalt monocrystals

The magnetic domain structure of 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 magnetic domain width on the specimen is thickness 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 complex character of magnetic domains. The resolution limit of type-I magnetic contrast in cobalt monocrystal is evaluated. 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.     

Observation of magnetic domain structure in Terfenol-D by scanning electron acoustic microscopy

The magnetic domain structure in oriented Tb_0.3Dy_0.7Fe_1.92 (Terfenol-D) is investigated by scanning electron acoustic microscopy (SEAM) in a wide frequency range from 75 to 530 kHz. Both secondary electron image and electron acoustic image can be obtained in situ simultaneously. By changing the modulation frequencies, the SEAM can be used as an effective nondestructive method to observe not only the surface topography and domain structure but also the subsurface domain structure and defects. The magnetic domain structure is verified by magnetic force microscopy (MFM). Furthermore, magnetic domains can be observed in both linear and nonlinear imaging modes by SEAM. The contributions to the image contrast are related to the signal generation through the piezomagnetic coupling mechanism, magnetostrictive coupling mechanism, and thermal-wave coupling mechanism.     

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).     

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.     

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.     

Extracting domain wall patterns from SEM magnetic contrast images

The magnetic contrast images of a soft magnetic metallic glass Fe₇₉Si₆B₁₄Cu₁ subjected to a periodic magnetic field were recorded with a scanning electron microscope by using a stroboscopic technique. An image processing method for the extraction of domain patterns from these images is presented. By this technique, delicate details of the dynamic magnetization process can be investigated and differences in the local magnetisation and domain wall movement can be analysed.     

Photoemission electron microscopy of neodymium-iron-boron (Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B)

The magnetic domain structure of a neodymium-iron-boron single crystal (Nd₂Fe₁₄B) was investigated in a photoemission electron microscope equipped with an aperture for partial restriction of the electron beam. As a result of the influence of magnetic microfields, electron trajectories are deflected in such a way that some of them are stopped by the aperture in the electron optical path. As a result, the contrast caused by the stray fields of the magnetic domains is significantly enhanced. The distribution of the local magnetic fields at the surface is reconstructed from the image by means of the proposed theory on the contrast mechanism. The size of the stray field close to the sample surface under study was 0.5–0.7 T. PACS 68.37.Xy; 75.50.Bb; 75.70-i; 75.70.Kw     

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.     

Demonstration of ultra-high-resolution MFM images using Co90Fe10-coated CNT probes

We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic storage media using carbon nanotube probes coated by ferromagnetic Co₉₀Fe₁₀ films (20, 30, 40, and 50 nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic domain image with an 40 nm-Co₉₀Fe₁₀-coated tip (50 nm tip diameter) about a lateral detect density of 1200 k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM imaging reported for this material system and structure. The observed dependence of tip dimension on signal contrast and image resolution was successfully explained by a theoretical analysis indicating that the signal contrast, along with the physical probe-tip dimension, should be taken into account to design magnetic probes tips for high-resolution magnetic force microscopy.     

Induced magnetic superstructure in the FeBO3: Mg weak ferromagnet

The domain structure of a FeBO₃: Mg single crystal was studied with a polarizing microscope. It was found that application of a magnetic field along the hard axis in the basal plane of this weak ferromagnet gives rise, within a certain field-strength interval, to a magnetic superstructure observed against the background of the macrodomain structure of the sample. The magnetic superstructure is visually represented as a quasi-periodic system of bands oriented perpendicular to the applied field, with an alternating magneto-optic image contrast along an axis coinciding with the magnetic-field direction. The absence of sharp changes in the contrast of the magnetic superstructure image along this axis is explained as being due to the smooth variation of the sublattice magnetic-moment azimuth with spatial coordinates. The results obtained are discussed within the parameters of the instability of a uniform magnetic state of a system in the random field induced by a magnetic field.     

利用扫描透射X射线显微镜观测磁涡旋结构

利用上海光源软X射线谱学显微光束线站(STXM)并结合X射线的磁圆二色效应, 我们对方形、圆形和三角形的Ni₈₀Fe₂₀薄膜微结构中的磁涡旋结构进行了定量实验观测, 并利用同步辐射光源的元素分辨特性, 分别在Fe和Ni的L₃吸收边对涡旋磁结构进行了观测. 我们还对磁涡旋中磁矩的分布进行了定量分析, 发现实验结果与微磁学模拟结果完全符合.     

结合小波域变换和空间域变换的图像增强方法

提出了一种结合了小波域和空间域处理方法的图像增强算法. 该算法首先对小波域中的高频系数进行修正,使图像具有更好的局部对比度和更丰富的细节,由于双树复小波变换（Dual-tree Complex Wavelet Transform,DT CWT）具有更好的方向选择性,在小波变换的过程中选用了这一方法;然后,通过空间域中的非线性变换,调整图像的整体对比度. 该算法可根据图像本身的特性实现参数的自动选择. 经过本文方法的处理所得的图像,无论在视觉效果上还是在统计上,都优于前人工作的结果.     

Origin and tailoring of the antiferromagnetic domain structure in α-Fe2O3 thin films unraveled by statistical analysis of dichroic spectromicroscopy (x-ray photoemission electron microscopy) images

The magnetic microstructure and domain wall distribution of antiferromagnetic α-Fe2O3 epitaxial layers is determined by statistical image analyses. Using dichroic spectromicroscopy images, we demonstrate that the domain structure is statistically invariant with thickness and that the antiferromagnetic domain structure of the thin films is inherited from the ferrimagnetic precursor layer one, even after complete transformation into antiferromagnetic α-Fe2O3. We show that modifying the magnetic domain structure of the precursor layer is a genuine way to tune the magnetic domain structure and domain walls of the antiferromagnetic layers.     

Magnetooptic setup for investigating the dynamic properties of domain walls in thin ferromagnetic films

A magnetooptic Kerr setup is designed for investigating the dynamic properties of domain walls in thin ferromagnetic films subjected to an external magnetic field in the temperature range 20–150°C. With this setup, the method of interrupted magnetization is implemented and the magnetic (domain) structure is visualized based on the meridional Kerr effect. The domain structure is displayed on a PC monitor using a Nikon DXM 1200 high-resolution digital video camera. A dedicated software makes it possible to automatize measurements and data processing.     

Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

不同线宽结构杂散光Kirk模型分析

采用Kirk测量法的杂散光模型研究了杂散光在不同线宽结构上杂散光的光强变化,通过图像对比度分析了杂散光对不同线宽结构的影响。基于Matlab软件仿真分析表明:线宽一定时,线条越稀疏,图像对比度越低,杂散光对成像图形分辨力的影响越大;线条线间比一定时,线宽尺寸越小,图像对比度越低,杂散光对成像图形分辨力的影响也越大。所以杂散光对线宽较小并且线条稀疏空间结构所成的图形造成的影响较大。光刻; Kirk测量法; 杂散光; 点扩散函数; 图像对比度     

Magnetic ordering in uranium monophosphide

The magnetic ordering in uranium monophosphide (UP) has been studied by neutron diffraction from a single crystal in a magnetic field. UP orders at T_N ? 122 ± 0.1 K with the type-I antiferromagnetic structure (+-+-), the ordering taking place in a first-order transition. At T₀ = 22.5 K the ordered magnetic moment jumps from 1.7 μ_B to 1.9 μ_B. With a magnetic field H = 25 kOe applied along the [11&#x0304;10] direction, it is found that UP has the collinear single-$\text{K}$ type-I structure above T₀ and undergoes a first-order transition to the planar double-$\text{K}$ type-I structure, accompanied by a “moment jump” due to the change in the moment direction from <001> to <110>.     

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.