Volume 126, Nº 7 (2025)

The features of temperature and field behavior of the ferromagnetic state in the two-spin Kugel–Chomsky model describing the relationship between the ferromagnetic spin and antiferromagnetic pseudospin orbital orderings are investigated. The case is analyzed when the critical temperature of the antiferromagnetic orbital ordering Tl is higher than the critical Curie temperature of the spin ferromagnetic order Ts. It is
shown that in this case the existence of an exchange coupling of the ferromagnetic spin subsystem with the orbitally ordered subsystem leads at finite temperatures to a decrease in both the spontaneous and induced magnetization of the ferromagnetic state compared to similar characteristics of the ferromagnetic state in a media without orbital ordering.

The paper presents the results of measurements of the current-voltage characteristics (IVC’s) in the electron-doped layered superconductor Nd_2–xCe_xCuO₄ and the results of the scaling analysis of the U(I) dependence for a large set of layered HTSC’s. It is shown that the IVC’s of hole-doped compounds indicate a monotonic behavior of the d-wave symmetry order parameter, while the IVC’s of electron-doped compounds Nd_2–xCe_xCuO₄ with x = 0.15 and Sm_2–xCe_xCuO₄ exhibit a non-monotonic nature of the d-wave symmetry order parameter associated with the coexistence of superconductivity and fluctuations of antiferromagnetic spin excitations.

Magnetic ordering of the sandwich structure of the Fe₃GeTe₂ compound is investigated. Two cases are considered. In the first case, the interaction between the layers containing iron atoms is neglected. In the second case, such interaction is considered small. It is shown that in the first case, a phase transition to a ferromagnetic state of the Ising type with a one-component order parameter takes place in two-dimensional layers containing only iron atoms. The magnetic moment is oriented along the z axis. The assumption is used that in the Fe/Ge layer, the exchange interaction of iron atoms is indirect through germanium atoms and antiferromagnetic. Long-range magnetic order is absent in this plane, but antiferromagnetic fluctuations of the transverse spin wave type are present. When the interaction between the layers is taken into account, a long-range ferromagnetic order will appear in the system, realized on the layers containing only iron atoms. Antiferromagnetic fluctuations will coexist with ferromagnetism.

The fractal thermodynamics model was applied for the first time to study the process of temperature-induced transformation of the magnetic domain structure (DS) on the basal plane of an Nd₂Fe₁₄B single crystal within the temperature range of 20–285 K. This range encompasses a second-order spin-reorientation transition (SRT) from the “easy axis” type of magnetocrystalline anisotropy (MCA) to the “easy cone” type of MCA. A high degree of similarity between the patterns of the Nd₂Fe₁₄B single crystal's domain structure images and fractals was demonstrated across the entire temperature interval. Specifically, the values of the parameter δ, characterizing the relative deviation of the studied DS from fractals, fall within the range of 1.16⋅10⁻⁶ to 1.72⋅10⁻². A notable difference was observed in the character of the temperature dependencies of the fractal parameters D(T), S_f(T), and T_f(T) at temperatures below and above the SRT temperature T_SRT = 135 K. Furthermore, the temperature behavior of the fundamental constants of the Nd₂Fe₁₄B compound and the fractal parameters of the DS within the “easy axis” MCA region suggests a possible correlation between them in this temperature range.

The structure and phase composition of the lap joint of aluminum alloy AMg6 obtained by friction stir welding are studied. The formation of complex macroscopic defects in the welded joint is detected. The conditions for the formation of macroscopic defects in aluminum joints obtained by friction stir welding and methods for preventing the formation of defects are discussed.

A model to explain the causes of anomalous carbon diffusion in tungsten carbide during spark plasma sintering (SPS) of α-WC + W₂C powders has been proposed. The phenomenon of anomalously deep carbon penetration into tungsten carbide ceramics during SPS of a powder billet in graphite tooling, as discovered in previous studies by the authors, is characterized by the formation of an anomalously deep layer (∼ 50 mm in thickness) of pure tungsten monocarbide (a-WC) on the ceramic surface. The formation of this layer is due to the reduction of tungsten semicarbide (W₂C) during its interaction with carbon diffusing from the tooling surface. The thickness of the reduced layer (∼ 50 mm) is several orders of magnitude greater than the values that follow from calculations of the depth of carbon diffusion, based on tabulated values of the activation energy of carbon diffusion in tungsten carbide. The model is based on the assumption that to facilitate the phase transformation of W₂C → α-WC, during which, due to the restructuring of the atomic crystal structure, a change in the volume of the unit cell occurs, it is necessary to ensure not only the flow of additional carbon to the surface of the W₂C particles, but also the formation of additional volume, the value of which is proportional to the change in density during this phase transformation (∼ 10%). The creation of additional volume is provided by the flow of nonequilibrium vacancies from the sample surface. The concentration of such vacancies is proportional to the value of the change in density during the phase
transformation of W₂C→α-WC and the volume fraction of the W₂C particles, and in this case exceeds the equilibrium concentration of vacancies in tungsten carbide by several orders of magnitude. The presence of these vacancies facilitates the accelerated diffusion of carbon in tungsten carbide. The model allows for the estimation of changes in the diffusion coefficient and enables the calculation of the depth of carbon penetration into the sintered material.

The phase composition of ingots and hot-rolled sheets of alloys in the Al–Cu–Mn–Ni system, containing 6–8% Cu, 2% Mn, and up to 4% Ni (weight percent), has been analyzed. A structure for the phase diagram in the aluminum region has been proposed, which suggests that in the solid state, there are three fourphase regions that involve a solid solution based on aluminum and different intermetallic compounds. The possibility of creating heat-resistant deformable aluminum alloys, whose structure consists of an aluminum matrix with Al₂₀Cu₂Mn₃ dispersoids and Al₃(Cu,Ni)₂ eutectic phases, has been established.