Vol 125, No 12 (2024)

The results of direct measurements for the adiabatic temperature change ΔT_ad in the Ni₄₇Mn₄₀Sn₁₃ alloy in cyclic magnetic fields by the magnetic field modulation method are presented. In the temperature dependence of the magnetocaloric effect (MCE), direct (ΔT_ad > 0) and inverse (ΔT_ad < 0) MCE are detected. The inverse effect value in a cyclic magnetic field depends on the temperature scanning rate. An increase in the frequency of a cyclic magnetic field with an induction of 1.2 T from 1 to 30 Hz decreases the direct effect value by more than 2 times. In a cyclic magnetic field with an induction of 1.2 T at frequencies f ≥1 Hz, complete disappearance (“collapse”) is observed for the inverse magnetocaloric effect, while ΔТ_ad during the one-time actuation of magnetic field is –0.49 K. The dependence of the inverse effect value on the temperature scanning rate, along with its strong frequency dependence, results from both the manifestation of irreversibility in the magnetostructural phase transition due to hysteresis and the presence of phase inhomogeneities influencing the phase transition kinetics.

The bulk GdTbDyHoEr magnetic high-entropy alloy is prepared by induction melting; the same alloy in the form of ribbons is prepared by rapid quenching from the melt. Peculiarities of the structure and magnetic and magnetocaloric properties of these materials are analyzed. The both states of the alloy are characterized by the hexagonal structure. The magnetic entropy change ΔS_M is determined using measured magnetic isotherms and Maxwell’s relations. The maximum ΔS_M is observed at 175 K and, for a magnetic field change of 2 T, it is 1.8 and 2.6 J/kg К for the bulk and rapidly quenched alloys, respectively. Taking into account the determined parameters of magnetocaloric effect, the alloys show promise as materials for applications in magnetic refrigeration devices.

The effect of the Mn_xGe_y buffer layer on the morphology, transport and magnetic properties of Mn₅Ge₃ thin films grown on substrates Si(111) has been studied. Using X-ray diffraction analysis and atomic force microscopy, it has been found that changing the thickness and structure of the buffer layer with a gradient Mn_xGe_y composition has made it possible to control the crystalline quality and smoothness of epitaxial films. Changes in the microstructure and surface roughness has not affected the temperature of the phase transitions revealed from the temperature dependences of the resistivity and magnetization at 75 and 300 K. It has been shown that the features of the magnetization curve shape for films with different buffer layers have been closely related to the inhomogeneity of the films in thickness and surface roughness while maintaining the micromagnetic constants and orientation of the easy magnetization axis. The value of the change in the magnetic part of entropy ΔS has been calculated to be 2.1 J kg^–1 K^–1 at 1 T, which is comparable with the value for gadolinium and exceeds that for Mn₅Ge₃(001) films grown on GaAs substrates.

Machine learning (ML) has proven to be a powerful tool, significantly speeding up and simplifying the development of new materials while enhancing their functional characteristics. In recent years, there has been an exponential growth in the number of scientific publications exploring the use of ML in materials science. Using this approach, various materials, including magnetic ones, are being actively developed and studied. This article aims to critically review research that applies ML to predict the functional characteristics of soft and hard magnetic materials. The paper is divided into three sections: the first outlines the basic principles and algorithms of machine learning, highlighting its use in addressing practical materials science challenges; the second discusses recent advances in developing magnetic functional alloys using ML; the last section provides a critical analysis of the use of machine learning methods in this area, analyzes its advantages and disadvantages, and gives recommendations for organizing such research.

In the proposed review, the structure of peculiar topological excitations of magnetically ordered media, the so-called two-dimensional magnetic vortices, is described as completely and in detail as possible. Magnetic vortices represent a distinct category of defects within the field of condensed matter physics. Accordingly, the structure of vortices in hydrodynamics and superfluids, as well as dislocations in solid-state physics, is presented at the beginning of the review. A specific section of the review is dedicated to elucidating the structural characteristics of plane vortices, instantons, spiral vortices, magnetic “targets,” vortex stripes, and their interactions employing analytical methods. A general solution of a two-dimensional isotropic ferromagnetic system is presented using methods of differential geometry. The discussion encompasses twodimensional vortices with anisotropic exchange interactions. A substantial portion of the review is devoted to helicoidal structures and vortices (skyrmions) in chiral magnets, encompassing their theoretical characterization based on a functional incorporating the DMI, as well as the outcomes of the early experiments on the detection of one-dimensional helical structures. A theoretical description of skyrmions and two-dimensional skyrmion lattices in bulk crystals is provided. It is observed that the DMI significantly alters the morphology of skyrmions with an arbitrary topological charge. Such structures can be represented as a “sack” with the shell comprised of kπ-skyrmions. The observed Archimedean spiral vortices are described, and a hexagonal lattice of Archimedean spiral is predicted to represent a new equilibrium phase.

The electrical resistivity, magnetization, and Hall effect in Co₂FeZ (Z = Al, Si, Ga, Ge, Sn, Sb) ferromagnetic Heusler alloys have been investigated. It has been demonstrated that there are a number of correlations between the electronic and magnetic characteristics of the alloys under study, which are manifested by changes in the atomic number of the Z component. For Co₂FeAl and Co₂FeSi alloys, which are HMFs, the magnetization value agrees with the Slater–Pauling rule. The electrical resistivity of Co₂FeAl, Co₂FeSi, and Co₂FeGe compounds exhibits quadratic temperature dependence at temperatures below 30 K and above 65 K. In the range of intermediate temperatures (40 to 65 K), a power-law dependence of ~T^b with an exponent of 3.5 ≤ b ≤ 4 has been revealed, which may be attributed to two-magnon scattering processes.

The effect of various heat treatments on the magnetic properties and microstructure of magnets manufactured using low-oxygen technology from the alloy (Nd, Pr)_31.9Fe_bal.(Co, Cu, Al, Ga)_1.7B_0.8 (wt%) has been studied. It has been shown that two-stage heat treatment leads to a significant increase in the coercive force of the magnets compared to the single-stage one. The obtained magnets have the properties (B_r = 13.2 kG, _MH_c = 17.9 kOe, _BH_c = 12.5 kOe, (BH)_max = 42.4 MGOe, α = –0.11 %/°С, β = –0.54 %/°С) corresponding to the properties of (Nd, Dy)–Fe–B magnets used to produce magnetic systems of wind turbines. The use of Pr and Ga makes it possible to reduce the cost of the initial alloy compared to alloys with Dy.

The structure of superconducting layers in composites with internal tin sources and distributed Nb barrier has been studied using transmission and scanning electron microscopy. It has been shown that the outer diameter of the composite (1, 0.7 and 0.5 mm) affects the morphology, grain size and composition of the superconducting Nb₃Sn phase layers formed upon reaction heat treatment in the regime 370°C for 100 h + 665°C for 40 h. A residual content of niobium has been identified in 10% of the subelements within the ∅1-mm sample, 4% within the ∅ 0.7-mm sample, and 0.8% within the ∅ 0.5-mm sample. The minimum average grain size of Nb₃Sn grains is observed in the composite with a diameter of 0.7 mm.

The evolution of the structure and properties of an Al–1.8% Mn–1.6Cu alloy under deformation via high pressure torsion at room and elevated temperatures has been studied. The sequence of mechanisms of the formation of an ultrafine-grained structure has been established, and the cycling of the phase transformations, namely, the partial dissolution and precipitation of nanosized particles, has been observed. It has been found that aging, which occurs at the accumulated strain e = 6.9, suppresses the process of the grain growth under deformation at an elevated temperature. The effect of the structural-phase transformations on the strength and ductility of the alloy has been determined. As a result of deformation, the ultimate tensile strength increases by 3 times, and the yield strength increases by 7 times. Dynamic recrystallization results in a decrease in strength and in a considerable increase in the ductility of an alloy.

The effect of equal-channel angular pressing (ECAP) on the structure and mechanical properties of experimental eutectic Al–6Ca–3Ce (wt %) alloy is studied. The ECAP of initial cast blanks is fulfilled under isothermal conditions at a temperature of 200°С using 4 passes and the BC route of extruding. As a result of ECAP, both the strength and plasticity of the alloy are found to increase by 2 and 5 to 15 times, respectively. The anisotropy of properties is found, i.e., the strength in the transverse direction is lower by 5 to 15%, whereas the relative elongation is 3 times higher than those along the lengthwise direction. The achieved combination of properties is due to the formation of ultrafine structure characterized by low density of dislocation and the refinement of eutectic particles. The higher plasticity of samples in the transverse direction is due to the lower length of boundaries of eutectic particles retarding the movement of dislocations.

Using high-spatial-resolution (~0.2 mm) neutron diffraction, we examined the residual stresses in structural alloyed steel (Cr, Si, Mn) plates, 5 mm in thickness, following shot peening. The analysis revealed that residual stresses form not only near the treated surface but throughout the entire thickness of the plate. Compressive stress zones appear near both treated and untreated surfaces, while tensile stress zones emerge in the middle region. The intensity of the shot peening affects the width of these zones and the magnitude of the maximum stresses. Neutron experiments were conducted to measure stresses near the treated surface of the plates, employing the sin²ψ method. Results obtained via the sin²ψ neutron method were consistent with those from traditional three-component strain measurement techniques. The sin²ψ neutron method proves to be advantageous for measuring stresses near the surfaces of thick samples, since it lacks the limitations of traditional measurement techniques on the thickness of the sample.