Papers by Author: S.A. Nikitin

Abstract: The melt-spun Y₂(Fe,Mn)₁₇ ribbons were synthesized by rapid quenching from the melt. The phase composition, crystal structure and magnetocaloric effect (MCE) in low-cost iron-based pseudo-binary Y₂(Fe,Mn)₁₇ ribbons were investigated respectively by using x-ray diffraction and direct measurements of MCE. It was found that crystal structure of the as-spun ribbons retains hexagonal Th₂Ni₁₇-type because of the weak glass-forming ability. The Y₂Fe₁₄Mn₃ and Y₂Fe₁₃Mn₄ nanostructured ribbons demonstrate higher MCE compare to polycrystalline alloys.

Abstract: The paper presents the investigation of GdNiH_3.2 and TbNiH_3.4 hydrides magnetic transitions and magnetocaloric properties. The isothermal magnetization data in the fields up to 5T are obtained for GdNi and TbNi compounds and their hydrides and the values of magnetic entropy change are calculated. The maximum values of magnetic entropy change ΔS_M in GdNiH_3.2 and TbNiH_3.4 are extremely large. It is shown that the hydrogenation shifts ΔS_M(T) maximum to lower temperatures.

Abstract: The direct measurements of the magnetocaloric effect (MCE) and the magnetization for R₂Fe₁₇ (R = Y, Tb, Dy) compounds are reported. The maximal values of the MCE for different R₂Fe₁₇ compounds are almost the same at the Curie temperature and are equal to 0.8 – 0.85 K at ΛH = 13.5 kOe. The field dependencies of the magnetization and MCE and the dependence of MCE via magnetization deviates from the classical regularities and describes by the thermodynamic theory attracting the higher terms. Obviously the occurrence of the mixed exchange interactions in these compounds leads to the requirement of consideration both thermal and local fluctuations of the exchange integrals near the magnetic transition temperature.

Abstract: The purpose of this work was the complex investigation of magnetic, magnetocaloric and magnetoelastic properties of compounds based on Gd₅Si₂Ge₂ with small In substitutions in p-sublattice. The conducted measurements revealed that both the magnetocaloric effect and the volume magnetostriction upon cooling reach the higher values than upon heating. Indium substitution leads to the appearance of the second maximum on the temperature dependence of the magnetocaloric effect resulting in the increase of the refrigerant capacity.

Abstract: The MCE in MnAs and doped Mn (As,Sb) compounds with low concentration of Sb has been studied by direct investigation. It was shown that for MnAs the maximum value of MCE is ΔT = 0.28 K on heating (at 308 K) and ΔT = 0.88 K on cooling (at T = 306 K) in magnetic filed 12.5 kOe. In doped Mn (As,Sb) compounds the temperature dependence of MCE is similar to MnAs. All investigated compounds demonstrate a strong temperature and field hysteresis of magnetic properties in magnetic field less than 40 kOe. It was established that a small concentration of Sb leads to decrease of MCE hysteresis and preserves the phase transition at room temperature region.

Abstract: The direct measurements of the magnetocaloric effect (MCE) and the magnetization have been conducted for a large number of compounds based on RCo₂ (R = Tb, Ho) with substitutions by nonmagnetic elements (Y, Al, Ga) in rare earth and 3d- sublattices. The concentration dependencies of the Curie temperature and the value of MCE are discussed. The linear dependencies of the value of the MCE from the second power of the magnetization were find out at the region of the phase transition of the second type .

Abstract: The effect of micro-and nanocrystalline structural state on magnetic properties and magnetic entropy (ΔS_M) of Gd is investigated. The marked influence of severe plastic deformation on ΔS_M is demonstrated.

Abstract: Magnetic properties of the hydrides of an intermetallic compound Ho₂Fe₁₄B are studied in the temperature range 78 700 K. The temperature dependencies of magnetization σ (Т) are obtained under continuous control of the hydrogen content in the samples investigated. The dependence of the Curie (T_C) and the spin-reorientation transition (T_SR) temperatures on hydrogen pressure (P) and its concentration (x) are determined. The dependence of hydrogen concentration on pressure at constant temperatures (isotherms x (P)) in the vicinity of T_C and the temperature variation of hydrogen concentration at a constant pressure (isobars x (T)) are investigated.

Abstract: An X-ray phase analysis and the metallographic examination were employed in the investigation of crystal structure of the Tb4Sb3 compound. Magnetic properties have been studied by means of magnetometric measurements, including a magnetocaloric effect, electrical resistivity and magnetoresistivity, and specific heat in broad temperature and the magnetic field ranges on a polycrystalline sample. It was confirmed that the Tb4Sb3 compound is an antiferromagnet with the temperature of the magnetic transition, TN = 111.4 K (specific heat) or 114 K (magnetization) . The results are discussed in the term of a crystal field splitting scheme.

Abstract: Direct measurements of the magnetocaloric effect (MCE) in RCo5 single crystals in the region of spin-reorientation transitions (SRT) were carried out. It was shown that the values of MCE in the region of SRTs are comparable to the values of MCE in the vicinity of the Curie temperature. The anisotropy constants of the compounds were measured by the torque method. The anisotropic MCE calculation on the basis of anisotropy constants showed a good accordance with the experimental data.