Search

The results of an experimental study of magnetization reversal processes of (R,Zr)(Co,Cu,Fe)Z (R = Sm, Gd) alloys taking into account micro- and nanostructure are presented. To create a highly coercive state in the samples, they were isothermally annealed at 800°C for 8-24 hours. The duration of annealing affects the formation of the nanostructure of alloys of this type. Based on magnetic measurements performed using a vibration magnetometer, magnetic hysteresis loops and graphs of the dependence of the coercive force on the chemical composition and duration of heat treatments were obtained, and the range of changes in the temperature coefficient of magnetic induction was determined depending on the relative content of Sm and Gd in the samples. Data on micro- and nanostructure were obtained using optical and scanning probe microscopy. The correlation relationships between the micro- and nanostructure parameters and the magnetic properties of the studied alloys are described. It was found that the highest values of the coercive force are achieved in an alloy in which the value is x = 0,5.

In this paper, we present the results of microhardness tests performed by Vickers indentation of germanium and silicon single crystals. It’s shown that in the investigated samples there is a dependence of microhardness on the crystallographic directions and the nature of the alloying impurity. Microhardness anisotropy coefficients are calculated: for germanium KII=1,2 and for silicon KII=1,3. The analysis of high-temperature annealing influence on microhardness value of germanium and silicon crystals is carried out. It has been established, that the microhardness of Ge(111) crystals grows on 12% after annealing at 550°С, the further thermal processing of germanium crystals at T=650°С considerably changes the structure and surface relief which contribute to a decrease in microhardness values. It is shown that the microhardness of silicon crystals increases by 10% after annealing at 750°C, further annealing to T=850°C leads to a decrease in microhardness. The surfaces of single crystals after high-temperature annealing have been studied; it has been established that thermal treatment at T≈0.6 Tm (Tm – the melting temperature of the single crystal) leads to the appearance of defects and a tenfold increase in the maximum height of the surface profile (from 10-12 nm to 100-200 nm).

This paper presents the results of complex studies of the effect of deformation obtained by the multiple isothermal forging method on the magnetocaloric properties of the Ni-Mn-Ga Heusler alloy system. Direct measurements of the adiabatic temperature change show that cycling the sample in a magnetic field μ0ΔH=1,85 T can decrease the maximum of ΔTad(T)0→H by the value corresponding to the latent heat of transition. Neglecting this transformation effect leads to an overestimation of the magnetocaloric effect of materials with the first-order transition. An analysis of the deformation treatment effect on the phase transformation temperatures and the magnitude of the magnetocaloric effect indicates that the application of multiple isothermal forging leads to a small decrease of the magnetization and the magnitude of the magnetocaloric effect. Deformation also causes a shift in the phase transition temperature towards low temperatures and decrease of the temperature hysteresis width.

This paper presents the results of studies of the effects of deformation obtained by the method of multiple isothermal forging on the microstructure and magnetic properties of the NiMnGa alloy system. It is shown that the microstructure of the initial alloy during deformation undergoes changes, grain size decreases and a two-component structure is formed. The magnetic domain structure of the original and deformed alloys was visualized by magnetic force microscopy. It is shown that the distortion of the domain shape is associated with the presence of a martensitic relief and grain boundaries, 180-degree magnetic domains are continuous within the crystallites, while they cross the flat parallel boundaries of the martensite plates. The study of temperature and field magnetization dependences of the initial and isothermal forging samples demonstrates an insignificant magnetization decrease and a shift of the phase transition temperature toward low temperatures. It’s concluded that deformation by the isothermal forging method can be considered as a way to correct the phase transition temperature in NiMnGa alloys with preservation of magnetization.

The paper describes the results of the scanning electronic and atomic force microscopy research of the surface structure of the Ni57Mn21Al22, Ni57Mn21Al19Si3, Ni40Co10Mn34Al16 rapidly quenched ribbons. The influence of the chemical composition on the size, boundary geometry and structure of the grains is considered. It is shown that all the test samples have a nano-sized martensitic relief, and its parameters are determined. It has been established that the cross section of the ribbon is represented by crystalline grains of different shapes which are due to differences in the cooling rates along the edges and in the center of the samples. Elemental mapping of the cross-sectional surface of the ribbons was carried out using an X-ray energy dispersive spectrometer, and a uniform distribution of chemical elements in the samples was established. It is shown that the cobalt doping of Ni57Mn21Al22 ribbon causes changes in the micromorphology of the surface and has a significant effect on the magnetic properties of rapidly quenched ribbons: field dependences of themagnetization and domain structure.

The results on the micro- and nanostructures and on the magnetic domain structure of Sm(Co0,45, Cu0,40, Fe0,15)5 compound are presented. It was shown that the sequential high- and low-temperature heat treatments lead to formation of a homogeneous microstructure with some nanoscale compositional heterogeneities. Such a structure provides the coercive field HcI of up to 32 kOe. The coercivity and remanent magnetization of the samples in the temperature range from 300 to 700 K linearly decrease. It is shown that the nanostructure of the alloy with the highest HcI corresponds to the fractal dimension Df-n ~ 2,3 .

A unique software and hardware system for obtaining and automatic analysis of the magnetic domain structure images in an external magnetic field for further study of the magnetization reversal process was developed. The method efficiency and possibility of its use for studying the magnetization reversal of local surface regions of magnetic materials are shown, using the experimental magnetization curves of an yttrium-iron garnet film.

This paper presents the results of complex studies, including direct experiment and computer modeling, of the magnetization reversal processes of a SmCo5 single crystal and a (GdBiLu)3(FeGa)5O12 ferrite-garnet film. The analysis of images of the domain structure in the external magnetic field of both samples allowed us to construct the field dependences of magnetization. It was found that the field dependence of the magnetization of the ferrite-garnet film coincides with the magnetometry data. For the SmCo5 single crystal the saturation field of the surface corresponds to the saturation field obtained from the measurements on a vibrating sample magnetometer, however, the view of the field dependence M(H) differs. Image analysis of the main domains of a single crystal in an external magnetic field and computer modeling have shown that the values of the domain structure disappearance fields on the surface of the film located above the single crystal are significantly less than the saturation fields according to magnetometry data. The application of an external magnetic field of 0,02 T leads to saturation of the entire volume of the ferrite-garnet film in the direction of the field, which does not allow further observation of the transformation of the main domain structure of the bulk sample and limits the applicability of the indicator film method.

The article continues a series of studies of permanent magnets with different magnetization reversal mechanisms (nucleation, domain boundary displacement). In this work, a correlation is established between magnetic characteristics of permanent magnets (Y25 and AlNiCo) and the fractal dimension of magneto-optical images of their stray fields. Bismuth-containing ferrite-garnet films were used as an indicator. It is shown that the limiting values of the fractal dimension: 1,76 for a ferrite magnet and 1,85 for an AlNiCo magnet, are consistent with the results obtained for NdFeB (grade N35) and SmCo (grade KC37) magnets, as is the behavior of the field dependence of the fractal dimension. The behavior of the field dependence of magnetization Mmo(Hrev) has similar features to a similar dependence recorded for a permanent magnet NdFeB (grade N35), but the values of the demagnetizing fields for the Y25 sample are much smaller. The paper discusses the relationship between the mechanisms of magnetization reversal of permanent magnets and the behavior of the Mmo(Hrev) dependence.

The results of experimental studies of magnetic properties and magnetization reversal processes of a series of alloys Sm-Gd-Zr-Co-Cu-Fe are presented. The studied samples were subjected to prolonged isothermal annealing for 24 hours, which made it possible to achieve the formation of a certain nanostructure that affects the hysteresis properties of the material and allows to achieve a high coercivity state. According to the magnetic measurements data obtained by the method of a vibrating magnetometer, the hysteresis loops are constructed, the dependence of the saturation magnetization, and the coercive force on chemical composition, and the values of the temperature coefficient of magnetization change depending on the relative content of Sm and Gd are determined. It is shown that the substitution of a part of samarium atoms for gadolinium atoms increases the effect of the mechanism of irreversible rotation of the spontaneous magnetization vector on the remagnetization of samples. Application of the studied samples in various devices puts serious demands on the stability of their magnetic properties at high temperatures. Our researches allow us to conclude that compounds with gadolinium concentration x=0,5 have the greatest coercive force, and compounds with gadolinium concentration in the range x=0,5-0,6 have the highest temperature stability.

The results of the study of the crystal structure, magnetic properties, as well as of micro-and domain structure of the surface of a series of compounds Tb0,16Ho0,84Fe2,00-xCox (x = 0,1; 0,2; 0,3; 0,4) are presented. It is shown that an increase in the relative content of cobalt leads to a decrease in the crystal lattice parameter a from 7,310 Å at x = 0,1 to 7,304Å at x = 0,4. At the same time, the Curie temperature increases accordingly from 581 to 614 K. It has been experimentally established that, as x increases, the specific saturation magnetization decreases linearly with increasing temperature. It has been found that the mechanical impact on the surface of sections during polishing can lead to the appearance of a regular microrelief. The results of studying the domain structure of the surface are presented. It is shown that the configuration and linear dimensions of the domains change as a result of the impact on the surface of the samples.

The simulation results on the magnetization curves I(H) and the d2I(H)/dH2 dependences are presented. For this purpose models of rotation of the magnetization vector and the Néel model of magnetic phases were used. Recommendations for implementing the SPD-method for the anisotropy field determining for polycrystalline multidomain samples of ferromagnetic compounds with easy-axis MCA are proposed.