V. S. Gaviko

Magnetic properties and crystal structure of novel high anisotropic compounds based on the RFeV system (RY, Nd, Sm, Gd)

Abstract We have investigated the crystal structure and magnetic properties of novel ternary compounds of the composition R 2 (Fe 0.91 V 0.09 ) 17 (R≡Y, Nd, Sm, Gd). X-ray diffraction showed that their crystal structure is a superstructure of a new type based on the CaCu 6 structure, but with lattice parameters five times larger than those of CaCu 5 . The Curie temperature and saturation magnetization of the compounds studied fall into the ranges 440–520 K and 15–30 μ B (f.u.) −1 at 4.2 K respectively. The influence of dissolved cobalt, carbon and nitrogen on the magnetic properties of the compounds R 2 (Fe 0.91 V 0.09 ) 17 has been studied.

Kinetics of interaction of Mg-based mechanically activated alloys with hydrogen

Parameters of interaction of hydrogen with magnesium powders and structure of powder magnesium alloys alloyed with different metals and oxides (such as Fe, Ni, Al, Cu, Ti, Pd, NiPd, V2O5, and VH2) prepared by mechanical activation under either argon or hydrogen atmosphere in a vibration mill have been studied. The mechanically activated alloys absorb to 7 wt% hydrogen at 300°C for 20 min. For most of the additions used, the effect of the grain size and type of addition on the rate of hydrogen absorption was found to manifest itself only at the stage of the formation of the MgH2 phase upon mechanical activation in the hydrogen atmosphere; virtually no effect is observed upon subsequent hydrogenation. The temperature of the hydrogen desorption also depends only slightly on the addition kind. The increase in the hydrogenation rate of the Mg-based alloys resulting from the mechanical activation was shown to be due to the formation of a specific structural state of the particle surface, which exhibits a high catalytic activity for the hydrogen sorption. A study of the mechanically activated alloys by proton nuclear magnetic resonance showed a substantial increase in the rate of proton spin-lattice relaxation as compared to that observed for MgH2 produced by direct hydrogenation. This can be due to the interaction of protons with paramagnetic centers formed at the imperfect surface of mechanically activated Mg particles.

The structure and magnetic properties of rapidly quenched and annealed multi-phase nanocrystalline Nd9Fe91−xBx ribbons

Abstract Melt-spun and annealed Nd 9 Fe 91− x B x ribbons with 0 ≤ x ≤ 9 have been studied by means of X-ray diffraction, thermomagnetic analysis and magnetic measurements. The metastable phase diagram up to 780°C has been plotted. A metastable phase of the TbCu 2 structure has been identified in the ribbons of binary composition and those containing up to 7 at.% B. The Curie temperature of this phase was found to be strongly affected by both boron content and annealing temperature. The structure of exchange coupled nanocrystallines of Nd 2 Fe 14 B. α-Fe and Fe 3 B was realized by means of short-time annealing. The annealed isotropic Nd 9 Fe 84 B, ribbons had a remanence of 100.5 emu g −1 , an intrinsic coercivity of 4.5 kOe, and an energy product of about 13 MG Oe. The details of structure formation, as well as the criteria and conditions for the effective exchange coupling, are discussed.

Specific features of luminescence properties of nanostructured aluminum oxide

Aluminum oxide nanopowders are prepared by the gas phase method and characterized according to the particle sizes and the phase composition. Samples of the nanostructured ceramic material are produced by pressing and annealing in air. The photoluminescence and cathodoluminescence spectra of the Al2O3 nanostructured ceramic material and α-Al2O3 anion-defect single crystals are investigated under comparable conditions. The luminescence bands of centers formed by oxygen vacancies are revealed in the spectra of two types of samples. The nanostructured ceramic material is characterized by the appearance of a new luminescence band at 3.4 eV and a decrease in the luminescence decay time. The inference is made that the characteristic features of the luminescence of the nanostructured ceramic material can be associated with the presence of non-equilibrium phases and the specific features of relaxation processes.

Anomalies in the optical properties of nanocrystalline copper oxides CuO and Cu2O near the fundamental absorption edge

A 0.1–0.15-eV displacement of the fundamental absorption edge in the optical absorption spectra of nanocrystalline oxide n-CuO (relative to the position of the fundamental absorption edge in the spectra of CuO single crystals) towards lower energies (red shift) is observed against the background of strong blurring. Nanocrystalline n-Cu2O exhibits a displacement of the fundamental absorption edge towards higher energies (blue shift) by approximately 0.35 eV. The size of crystallites in n-CuO and n-Cu2O ranges from 10 to 90 nm. The blue shift of the fundamental absorption edge of n-Cu2O is typical of classical wide-gap semiconductors and can be explained by size quantization upon a change in the particle size. The anomalous red shift of the fundamental absorption edge of the strongly correlated nanocrystalline oxide n-CuO can be attributed to the highly defective structure of n-CuO, anomalies in the electronic structure of strongly correlated compounds based on 3d metals, and their tendency to electronic phase separation with the formation of metal-like inclusions.

Structure evolution and changes in magnetic properties of severe plastic deformed Nd(Pr)–Fe–B alloys during annealing

Abstract Structural changes and hysteresis properties of Nd(Pr)–Fe–B alloys of various compositions were studied after severe torsion straining under high pressure and following annealing. It was shown that in all the alloys studied, severe plastic deformation leads to formation of ultrafine-grained non-equilibrium structure and, at extremely large strains, even to formation of an amorphous structure. Annealing resulted in crystallization of alloys, formation of stable magnetic and non-magnetic phases and in marked improvements in hard magnetic properties. Homogenization of alloys is responsible for an increase in coercivity of the deformed samples.

Synthesis, structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe3O4 oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.

Investigation of phase composition and remanence enhancement in rapidly quenched Nd9(Fe, Co)85B6 alloys

Melt-spun and annealed Nd9(Fe1−xCox)85B6 alloys with x = 0–0.4 have been studied by means of X-ray diffraction, transmission electron microscopy, thermomagnetic analysis and magnetic measurements. Both quenching of the alloys and crystallization of the amorphous phase are accompanied by the formation of a metastable phase of ThCu7 type. When 0 ⩽ x ⩽ 0.3, short-time annealing at about 680°C results in coexistence of nanocrystalline exchange coupled Nd2(Fe,Co)14B and b.c.c. FeCo phases. Because of the stabilizing effect of Co on the 1:7 phase, the annealing that is optimal with respect to the sizes of the α grains is found to be insufficient for complete disappearance of the 1:7 phase in Co substituted alloys.

Magnetic and magnetoelastic properties of amorphous and crystalline TbFe2Hx hydrides

Abstract The crystal structure, magnetic and magnetoelastic properties of TbFe2Hx hydrides (x ⩽ 4.2), amorphous TbFe2H2.5 hydride and amorphous TbFe2 obtained by mechanical grinding have been investigated. It is established that the structure of atomic short-range order in the amorphous hydride is quite different from that in the mechanically ground sample. Both the introduction of hydrogen and amorphization is shown to cause a decrease in the magnetic moment and the magnetostriction, and a weakening of Tb-Fe and Fe-Fe exchange interactions. The changes in the magnetic properties of the samples are interpreted in terms of a supposition of random anisotropy and non-collinear magnetic structure in the Tb sublattice.

The synthesis, structure, and properties of carbon-containing nanocomposites based on nickel, palladium, and iron

Nickel, iron, palladium, and bimetallic nickel-palladium nanoparticles encapsulated in carbon were synthesized by contactless levitation fusion of metals in a magnetic field in a flow of an inert gas containing a hydrocarbon. The products were characterized by X-ray diffraction, differential thermal analysis, thermogravimetry, high-resolution transmission electron microscopy, and adsorption. A layered carbon shell preventing agglomeration and oxidation formed on the surface of nickel- and iron-containing particles. The size of particles depended on preparation conditions and could be of 5–15 nm.