Witold Szmaja

Detection limits of DLS and UV-Vis spectroscopy in characterization of polydisperse nanoparticles colloids

Dynamic light scattering is a method that depends on the interaction of light with particles. This method can be used for measurements of narrow particle size distributions especially in the range of 2-500 nm. Sample polydispersity can distort the results, and we could not see the real populations of particles because big particles presented in the sample can screen smaller ones. Although the theory and mathematical basics of DLS technique are already well known, little has been done to determine its limits experimentally. The size and size distribution of artificially prepared polydisperse silver nanoparticles (NPs) colloids were studied using dynamic light scattering (DLS) and ultraviolet-visible (UV-Vis) spectroscopy. Polydisperse colloids were prepared based on the mixture of chemically synthesized monodisperse colloids well characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), DLS, and UV-Vis spectroscopy. Analysis of the DLS results obtained for polydisperse colloids reveals that several percent of the volume content of bigger NPs could screen completely the presence of smaller ones. The presented results could be extremely important from nanoparticles metrology point of view and should help to understand experimental data especially for the one who works with DLS and/or UV-Vis only.

Domain structure of sintered SmCo5 magnets studied by magneticforce microscopy

The domain structure of sintered SmCo5 permanent magnets at the surface perpendicular to the alignment axis was investigated by magnetic force microscopy (MFM). The main domains forming a maze pattern of typically 3–5μm in width and surface reverse spikes of typically 1–2μm in size are observed. This coarse domain structure is similar to those present in sufficiently thick uniaxial crystals with strong magnetocrystalline anisotropy, reported in earlier investigations performed by Bitter pattern method or magneto-optic Kerr microscopy. In addition to the coarse domain structure, a complicated system of the fine surface domains of 10–200nm in width is observed. The thickness of the zone below the surface filled with these fine scale domains is estimated to be 100 nm and their presence is related to the reduction of the magnetostatic energy close to the specimen surface. Practically no correlation between the magnetic domain structure and the surface topography, the latter revealed by atomic force microscopy...

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.

THE THICKNESS DEPENDENCE OF THE MAGNETIC DOMAIN STRUCTURE IN COBALT MONOCRYSTALS STUDIED BY SEM

Abstract 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.

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 dependence of magnetic domain structure on the thickness of cobalt monocrystals

Abstract 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.

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

Abstract 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.

Improvements in Domain Study with the Conventional Bitter Method by Digital Image Processing System

The paper presents improvements in magnetic domain investigation with the conventional Bitter colloid method achieved thanks to the application of a digital image processing (DIP) system. The system capabilities are demonstrated referring to selected examples of the magnetic structure in a thin nickel-iron film, sintered Nd-Fe-B permanent magnets, a ferrimagnetic garnet plate, and bulk cobalt monocrystals. With the aid of our DIP system, high quality domain images, which were not possible before, could be obtained and analyzed in more detail. As a consequence, the previously reported problems related to the study of the domain configuration in soft magnetic materials and at the basal surface of bulk cobalt monocrystals by the conventional Bitter pattern technique were overcome and improvements over earlier results were made.

SEM temperature study of magnetic domain structure in cobalt monocrystals

Abstract 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.

Recent Developments in the Imaging of Magnetic Domains

Publisher Summary Magnetic phenomena have been known and used by mankind for many centuries. The earliest experiences with magnetism have involved magnetite, known also as “lodestone,” and the magnetic compass has served mariners throughout the ages. This chapter presents developments in the observation of magnetic domain structures. A ferromagnetic is divided into regions within which the atomic magnetic moments are aligned parallel to each other and the magnetization is equal to a constant value called the “saturation magnetization.” These regions are known as “magnetic domains.” Magnetic saturation of the specimen is produced by aligning the magnetization of each domain with the applied magnetic field. A material is always in a state in which its total energy is a minimum. The total energy of a ferromagnet is the sum of different energy contributions. Domains occur when a state with domain structure has a lower total energy than a uniformly magnetized state (which is also commonly called a “single-domain state”). The chapter illustrates images of the magnetic structures of cobalt monocrystals, anisotropic sintered Nd–Fe–B–based permanent magnets of different chemical composition, nanocomposite Nd2Fe14B/Fe3B permanent magnets, anisotropic sintered SmCo5 permanent magnets, thin polycrystalline permalloy and cobalt films, and,ferromagnetic garnet specimens.