type-I Magnetic Contrast Research Articles

SEM investigation of the magnetic domain structure of sintered SmCo 5 permanent magnets

The paper presents scanning electron microscopy (SEM) study of the magnetic microstructure of anisotropic sintered SmCo 5 permanent magnets. Observations were made in the thermally demagnetized state of the magnets at the surfaces both perpendicular and parallel to the alignment axis. Magnetic domains were revealed using the technique of type-I magnetic contrast (for the first time) and the colloid-SEM method. The domain structure consists of main domains (which extend through the whole grain thickness) and surface domains of reverse magnetization (reverse spikes). The main domains form a maze pattern near the surface perpendicular to the alignment axis. The reason for the presence of the maze domain structure and reverse spikes at the surface perpendicular to the alignment axis is the reduction in the magnetostatic energy at the cost of a larger total Bloch wall area. Investigations carried out on the surface parallel to the alignment axis allowed to obtain much better insight into the orientation of grains.

Comparison of different methods for studying magnetic domains in Ni–Mn–Ga martensites

The relationship between magnetic domains and martensite twin structure was studied in selected Ni–Mn–Ga alloys. Optical microscopy, scanning electron microscopy (SEM) and magnetic force microscopy were used for studying the magnetic domains and the morphology of the martensitic microstructure. SEM Type-I magnetic domain contrast was consistent with the contrast obtained from magneto-optical indicator, as both contrasts originate from the stray field above the sample surface. SEM Type-II magnetic contrast in the backscattered electron (BE) image can display, apart from the martensitic twinning structure, both the magnetic domain contrast and the domain wall contrast. Therefore, the coupling of magnetic and twin structures can be evaluated from the BE image. Surprisingly, optical non-polarizing microscopy can reveal the domain contrast, too. This contrast is due to the surface relief associated with the magnetic domains.

Imaging magnetic microstructures with the use of electrons

AbstractThe paper presents investigations of magnetic microstructures by techniques which make use of electrons. In this context, we refer to the examples of magnetic microstructure images of cobalt monocrystals, Nd–Fe–B permanent magnet and a thin permalloy film, while the methods used are the type‐I magnetic contrast technique of scanning electron microscopy (SEM), the colloid‐SEM method and the Fresnel mode of transmission electron microscopy (TEM). It is shown that the SEM type‐I magnetic contrast and the colloid‐SEM method have quite different probing depths and consequently provide useful complementary information on the magnetic microstructure at the surface of bulk materials which exhibit an out‐of‐plane component of magnetization. It is also demonstrated that TEM is a powerful tool for investigating the magnetic microstructure of thin magnetic films. Some improvements over previous results are presented. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigations of the domain structure of anisotropic sintered Nd–Fe–B-based permanent magnets

The magnetic structure of anisotropic sintered Nd–Fe–B-based permanent magnets of different chemical compositions has been studied by the conventional Bitter pattern technique, the colloid-scanning electron microscopy (SEM) method, magnetic force microscopy (MFM) and the type-I magnetic contrast of SEM. The methods used have been digitally enhanced. Improvements over earlier results have been achieved. In the thermally demagnetized state, practically each grain consists of magnetic domains. At the surface perpendicular to the alignment axis, the main domains forming a maze pattern of typically 1–2 μm in width and surface reverse spikes of typically 0.5–1 μm in diameter are present. This coarse domain structure resembles those for sufficiently thick uniaxial crystals with strong magnetocrystalline anisotropy, commonly reported in earlier studies. Nevertheless, besides the coarse domain structure, fine surface domains having widths in the range 20–250 nm are observed by MFM. The fine scale domains are found to be magnetized perpendicular to the specimen surface and their existence is attributed to the reduction of the magnetostatic energy near the surface. The main and surface domain widths, the domain wall energies, the domain wall thicknesses, the single-domain particle diameters and the exchange constants for the investigated magnets were determined. The influence of Dy addition on the magnetic domain structure was estimated. As predicted by the theory, the main and surface domain widths were observed to increase with increasing Dy concentration. Studies of the magnetic structure during magnetizing cycle, carried out on the surface parallel to the alignment axis, show that the domain walls are easily moved within the grains and that the magnetization reversal occurs predominantly by the nucleation and expansion of reverse domains at structural imperfections near the grain boundaries. Some aspects related to magnetic domain observation by the methods used are also discussed.

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.

Improvements and actual problems in domain imaging by type-I magnetic contrast in SEM

Improvements in domain observation by the scanning electron microscope (SEM) type-I magnetic contrast have been achieved thanks to the use of digital image processing (DIP) system. Domain images with considerably enhanced contrast and reduced noise can be obtained. By combining image alignment with image subtraction and summation techniques, it is possible at present to separate superimposed magnetic and non-magnetic features in the images taken with a single secondary electron detector. As a consequence, detailed studies of domain structures in the cases of very weak type-I contrast and complex domain patterns, which were not possible before, can now be made. The actual problems related to the complicated mechanism of type-I magnetic contrast and its relatively low resolution are also discussed.

Studies of the surface domain structure of cobalt monocrystals by the SEM type-I magnetic contrast and Bitter colloid method

The type-I magnetic contrast in a scanning electron microscope (SEM), the conventional Bitter pattern method and the colloid-SEM method have been applied for investigating the magnetization configuration at the (0 0 0 1) surface of thick cobalt monocrystals. All the methods used have been digitally enhanced. In particular, thanks to this, detailed studies by the conventional Bitter technique in zero or weak external magnetic fields, which were not possible before, could be made. The magnetic images observed are analyzed in terms of the domain structure. Type-I magnetic contrast observations show that the surface domain structure of bulk cobalt (at room temperature) is substantially determined by its uniaxial magnetocrystalline anisotropy perpendicular to the (0 0 0 1) surface. From the behavior of the Bitter patterns under the influence of external magnetic field it can be concluded that regions with different in-plane magnetization components are also present on the crystal surface. As a consequence, the magnetization direction at the (0 0 0 1) surface of bulk cobalt monocrystals is generally tilted away from the surface and changes from one domain region to another. Some aspects related to the character of the domain structure observed are discussed in more detail. The results obtained by the methods used are compared with each other as well as with those reported in the literature and obtained with other domain observation methods.

Digitally enhanced type-I magnetic contrast in SEM as a method of domain investigation

In this paper, the type-I magnetic contrast for studying domain structures in a scanning electron microscope (SEM) is reviewed. The application of digital image processing (DIP) system, which has become common in recent years, brings some new possibilities. Owing to this, it is now possible to perform detailed study of domain behavior in the cases of very low magnetic contrast and complicated domain patterns. The features of type-I magnetic contrast are illustrated by the examples of domain images at the (0 0 0 1) surface of cobalt monocrystals. The main advantages and limitations of the method are discussed in more detail.

Digital image processing system for magnetic domain observation in SEM

A digital image processing (DIP) system and its application for domain observation in cobalt monocrystals by the type-I magnetic contrast in a scanning electron microscope (SEM) is presented. The system capabilities shown include contrast enhancement, noise reduction, removal of tilted illumination and undesirable periodic patterns from the image, Fourier analysis and determination of the magnetic domain width. In particular, it is demonstrated for the first time that even as the SEM possesses a single secondary electron detector it is possible to obtain images with type-I magnetic contrast only (i.e. without topographic contrast), owing to combining image alignment with image subtraction. Other advantages of using DIP systems are also mentioned.

Estimation of the Surface Magnetic Structure of Materials by Scanning Electron Microscopy

As a visualization technique of the surface magnetic structure, the type-I magnetic contrast in a scanning electron microscope is proposed. Trajectories of secondary electrons emitted from the specimen surface are deflected by the magnetic flux that leaks from the surface magnetic domain. The magnetic flux and an electric field in the specimen chamber are calculated numerically for a given magnetic structure of the specimen, and the spatial deviation of the electrons at the detector is obtained. By an analysis of the relationship between the structure and the deviation, it is expected that the magnetic structure can be estimated by the deviation, as the inverse problem. In the present paper, this relationship is examined under various conditions, and rules are derived to interpret the results into the magnetic structure of the specimen.

On Temperature Dependence of Domain Structure in Cobalt

The type-I magnetic contrast in a scanning electron microscope is used to study the domain behaviour on the basal planes of cobalt monocryslals of different thicknesses during a heating cycle. Digital image processing is applied to the original scanning electron microscope images for their restora tion, enhancement and analysis. The main reasons for the application of digital image processing are: low level of type-I magnetic contrast, particularly used at the higher temperatures, and the complex character of the magnetic domain structure. The changes in both domain structures and type-I magnetic contrast are due to the strong temperature dependence of magnetocrystalline anisotropy energy for the hcp phase of cobalt, which implies the rotation of magnetic easy axis from the c-axis to the basal plane. The temperature of the magnetic phase transition between an open-flux and a closed-flux domain configuration was found to be dependent on the specimen thickness. The changes in domain structure during the heating cycle were reversible under the condition that the specimen was not carried through the hcp-to-fcc phase transition. Otherwise, they were partially or completely irreversible and were caused by crystal imperfections originating from the transition. The paper shows the great advantage of using digital image processing system for data restoration, enhancement and analysis.

The thickness dependence of the magnetic domain structure in cobalt monocrystals studied by SEM

The type-I magnetic contrast in a scanning electron microscope (SEM) was used to observe the magnetic domain structure on the basal planes of cobalt monocrystals. The study of the thickness dependence of the domain structure was carried out on a series of cobalt single crystals in the thickness range 1.9 mm down to 10 μm. Because of the problems in imaging magnetic domains for the smaller thicknesses used, and the complex character of the domain structure in thick crystals, digital image processing was applied to the original SEM images for their restoration, enhancement and analysis. The limit of resolution of type-I magnetic contrast in cobalt monocrystals has been evaluated, and the statistical distributions of the magnetic domain widths have been calculated.

Observation Technique of Surface Magnetic Structure Using Type-I Magnetic Contrast in the Scanning Electron Microscope

The type-I magnetic contrast in the scanning electron microscope is simulated. The magnetic flux that leaked from the surface magnetic domain is calculated based on the Maxwell equation. Trajectories of secondary electrons emitted from the surface are traced considering this magnetic field and the electric field generated by the secondary electron detector. On the basis of the characteristic variation of the spatial deviation of electrons arriving at the detector, the original domain structure at the specimen surface is estimated.

SEM temperature study of magnetic domain structure in cobalt monocrystals

A scanning electron microscope (SEM) has been used to observe the magnetic domain structure of a cobalt monocrystal. The temperature dependence of the domain structure is investigated. The changes in domain structure are found to be due to the strong temperature dependence of the magnetocrystalline anisotropy of cobalt. Digital image processing is used on the images obtained directly from the SEM. The main reasons for the application of digital image processing are: low level of type-I magnetic contrast, particularly at the higher temperatures used, and the complex character of the magnetic domain structure. Statistical distributions of magnetic domain width are presented.

The temperature dependence of magnetic domain structure in cobalt monocrystals studied by SEM

The domain behaviour on the basal planes of cobalt monocrystals was investigated during a heating cycle. The study was carried out for specimen thicknesses in the range 0.8-1.9 mm using the type-I magnetic contrast in a scanning electron microscope (SEM). Because of the problems in imaging magnetic domains at the higher temperatures used and the complex character of the domain structure, digital image processing was applied to the original SEM images for their restoration, enhancement and analysis. The changes in both domain structure and type-I magnetic contrast are caused by the reduction in magnetocrystalline anisotropy energy with increasing temperature. The temperature of the phase transition between an open flux and a closed-flux domain configuration was found to be slightly dependent on the specimen thickness.

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.

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.