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.     

57Fe NMR spectra in domain walls of hexagonal ferrites with magnetoplumbite structure

The domain wall NMR spectra of⁵⁷Fe were measured on polycrystalline samples of BaFe₁₂O₁₉ and SrFe₁₂O₁₉ at 4·2 K. We have calculated the anisotropy of the hyperfine field dipolar component. The measured NMR spectra were interpreted supposing that the hyperfine field anisotropy is caused only by the dipolar field anisotropy.     

The relation between crystal structure and luminescence in β-alumina and magnetoplumbite phases

Crystallographic data of aluminate and gallate β-aluminas and magnetoplumbites are reviewed. For the XAl₁₁O₁₇ and XGa₁₁O₁₇ compounds c/a increases with increasing r_X+. The magnetoplumbites have c/a values ranging between 3.915 and 3.954 with the exception of BaAl₁₂O₁₉ and BaGa₁₂O₁₉. These compounds seem to have strongly distorted magnetoplumbite-derived structures. Upon activation of the MeAl₁₂O₁₉(Me = Ca, SrorBa) lattices with Eu²⁺ and Ce³⁺, the luminescence properties of Ba containing compounds deviate from the other compounds. This seems to be due to different co-ordinations of the activator ions. Substitution of Mg²⁺ for Al³⁺ in magnetoplumbite phases results in the case of SrAl₂O₁₉ in changes of the c/a ratio, of diffraction intensities and of the luminescence properties. The various data suggest a close similarity of this type of phases and BaAl₁₂O₁₉. In the MeGa₁₂O₁₉ series (Me = SrorBa), the quantum efficiency of BaGa₁₂O₁₉: Mn²⁺ was found to be anomalously low.     

The description of the domain structure in thin ferromagnetic films with thickness near the critical value

The magnetization distribution is considered in a thin ferromagnetic film of a thickness near the critical value, for which the continuous phase transition from the homogeneous magnetization state to the domain structure appears. The easy axis of the magnetization is perpendicular to the film plane and the anisotropy constant β < 4π. On the basis of the self-consistent theory the vortex-like magnetization distribution in the interdomain wall was obtained. The value of the critical thickness, the period of the domain structure and the amplitudes of the magnetization vector components as dependent on the film parameters were derived.     

Influence of thickness on the domain structure of barium-strontium-titanate films on MgO substrates

Ba_0.8Sr_0.2TiO₃/MgO(001) heterostructures are studied using atomic-force spectroscopy in several modes (the contact and semicontact modes and that of piezoelectric response) to obtain information on the relief and the distribution of the electric potential on the surface. Based on data obtained for films with different thicknesses, the correlation between the domain sizes and orientations and the film thickness and the surface relief is established. It is shown that the films with thicknesses less than 36 nm contain only aa domains whose dimensions decrease with increasing film thickness. The films with thicknesses greater than 36 nm contain only c domains whose dimensions increase with increasing film thickness.     

NMR spectra of57Fe in hexagonal ferrites with magnetoplumbite structure

The NMR spectra of⁵⁷Fe in domains of MFe₁₂O₁₉ (M=Ba, Sr, Pb) were measured by spin echo technique at 4.2 K. The change of the heavy ion causes frequency shifts of lines corresponding to Fe³⁺ ions in 2b and 4f₂ sites while leaving other lines essentially unchanged; the significant role of different Fe³⁺−M²⁺ bonding was found. The dipolar broadening of lines in BaFe₁₂O₁₉ caused by random and static displacement of bipyramidal Fe³⁺ ions from the mirror plane is calculated and the results are compared with the experiment.     

Sintering of magnetoplumbite

A limiting density was reached at each sintering temperature. It was difficult to fix the operating mechanism during the initial stage of sintering. More than one mechanism overlap. At low temperatures surface diffusion predominates and at higher temperatures or firing periods a shift occurs to volume, then to grain boundary diffusion.During the intermetiate sintering stage, a straight line relationship was obtained on plotting the density or shrinkage against log time.The sintering rate was found to be inversely proportional to the particle size and increased in compositions deficit in iron oxide and at lower oxygen partial pressures.     

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.     

Electronic structure of three-dimensional topological insulators: thickness dependence and warping effect

We theoretically study the electronic structure of a three-dimensional topological insulator using a 3D lattice model with a warping term included. Thickness dependence of the electronic structure and the shape of constant-energy contour at different energy are both investigated in this model. We demonstrate that an energy gap opens in the surface state when the thickness is below six quintuple layers and disappears for thicker topological insulators. With the increasing of energy, the shape of constant-energy contour of the surface state band gradually evolves from a point at Dirac point to a circle with increasing volume, then to a hexagon, and finally to a hexagram. We clarify that the critical thickness of six quintuple is not modified by the warping term and the appearance of a hexagram is not influenced by the finite size effect. These phenomena agree well with recent angle-resolved photoemission spectroscopy experiments.     

Temperature dependence of the Stranski-Krastanov layer thickness

Thin films grow on a substrate in one of three possible growth modes: Laminar (Frank-van der Merwe mode), as islands (Volmer-Weber mode) or as islands on top of a few laminar layers (Stranski-Krastanov (SK) mode). The thickness of the laminar layer in SK-mode, Θ_SK, is a materials constant, dependent only on the physical properties of the substrate-film interface. We show, using scanning Auger/electron microscopy, that Θ_SK has a specific temperature dependence, in excellent quantitative agreement with theoretical prediction (M. Zinke-Allmang et al., Surf. Sci. 200 (1988) L427). For Si(100)/Ge Θ_SK increases about 3 × 10⁻⁴ monolayers/K with increasing temperature in the range 400–900 K. The change is completely reversible and due to the existence of the islands on top of the SK-layer.     

The thickness dependence of the permittivity of barium titanate single crystals

A thickness dependence of the permittivity ofc-domain barium titanate single crystals was found. The experimental results were explained by means of a model of a dielectric with surface layers. The thickness of the layer and the permittivity of the surface layer and bulk of the crystal were determined.

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iO₃

iO₃, -. . , .

     

Composition and magnetic properties of hexagonal Ca,La ferrite with magnetoplumbite structure

A polycrystalline crystal-oriented sample containing 97% of Ca_0,814²⁺La_0.341³⁺Fe_0.228²⁺Fe_11.617³⁺O₁₉ and 3% non-magnetic impurities was prepared. The magnetization and anisotropy field are very similar to those of BaFe₁₂O₁₉, except for the anisotropy field at 4K, which is lower. The value σ_s = 75.7 gausscm³g^?1 measured at 290 K is considerably higher than values reported earlier in the literature.     

On the dependence of surface undulation on film thickness

This paper investigates the dependence of surface undulation on a film thickness considerably greater than the critical value of a thin film system. It considers that surface tension and residual stress are the main cause of surface undulation. The study found that there is a critical undulation wavelength that minimizes the free energy of a thin film system, that this critical wavelength depends on the film thickness, and the effect of undulation amplitude is insignificant. The research also found that the surface undulation has a negligible influence on the residual stresses in the thin film system.     

The structure and the energy of the cross-tie domain walls

The paper gives the model of the cross-tie walls in thin ferromagnetic films. The distance between side walls, in our model, increases for increasing thickness D of the film, which is in accordance to the experimental data.