A. E. Teplykh

Magnetic state of Dy3Co

Abstract Using data on magnetization, magnetoresistance, a.c. susceptibility and specific heat, magnetic phase diagrams of the compound Dy 3 Co were obtained for the main crystallographic axis. The different antiferromagnetic (AF) structures exist below T N = 44 K: AF 1 exists at T T t = 32 K and AF 2 exists at the temperatures T t T T N . The field-induced magnetic phase transitions observed along the three main axes are associated with a complex non-collinear antiferromagnetic structure and are accompanied by a significant change in the electrical resistivity (Δ ρ/ρ up to −40%) owing to the appearance or disappearance of the energy gaps on superzone boundaries. At the phase transitions along a - and c -axes, large hysteresis loops are observed at low temperatures. At T 3 Co remains in a magnetized state after the application and removal of a magnetic field along the c -axis.

Formation of nanocrystalline structure in the amorphous Ti50Ni25Cu25 alloy upon severe thermomechanical treatment and the size effect of the thermoelastic martensitic B2 ↔ B19 transformation

The results of the comparative analysis of the Ti50Ni25Cu25-alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain-induced amorphization even with the retention, even after 5–15 rev, of a large number of highly dispersed nanocrystals less than 3–4 nm in size with a distorted B2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low-temperature nanocrystallization during subsequent annealing starting from 250–300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B2-type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2-austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.

Structural state of expanded graphite prepared from intercalation compounds

The structural state of nanocrystalline samples of expanded graphite is investigated using X-ray diffraction and neutron diffraction analyses. The expanded graphite samples are prepared by a rapid thermal decomposition of intercalation compounds of oxidized graphite based on fluorinated graphite, graphite oxide, and graphite aminofluoride. It is established that the main phase of expanded graphite belongs to the hexagonal crystal system (space group P63/mmc) and that carbon atoms in the structure occupy the 2b and 2c positions. The unit cell parameters and the unit cell volume in the structure of expanded graphite samples are larger than those in the structure of massive graphite.

The magnetic state of the Co-sublattice in Tb1−xYxCo2

Abstract The Tb 1− x Y x Co 2 pseudobinary series has been studied by means of resistivity, magnetic susceptibility and neutron diffraction measurements. According to the neutron diffraction a long-range magnetic order in this system exists only for x x c ≈0.8 a short-range magnetic order and anomalies in the transport properties: maximal value of the residual resistivity and large magnetoresistance effect (about −30%) were observed. We have found that the concentration dependence of the residual resistivity ρ res for Tb 1− x Y x Co 2 (as well as for other (R 1− x Y x )Co 2 compounds) correlates with the variation of the coefficient γ (linear term of the low-temperature specific heat). In vicinity of x c where the Co-magnetic moment is unstable the maximums of ρ res and γ can be associated with the large contribution from localized spin density fluctuations of d-electrons to both electrical resistivity and specific heat.

Size effect in nanocrystalline manganites La1−x A xMnO3 (A=Ag, Sr)

Nanocrystalline samples of the manganites La0.9Ag0.1MnO3, La0.7Ag0.3MnO3, and La0.7Sr0.3MnO3 were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.

The formation processes of oxide phases from polymer–salt complexes of ammonium molybdate and tungstate

Abstract The thermal decomposition processes of the polymer–salt solution of ammonium molybdate and tungstate are studied by small angle neutron scattering (SANS) and X-ray diffraction. The parameters of the crystal structure of the oxide phase particles WO 3 and MoO 3 , a particle size distribution function, a specific surface and fractal dimension of particle to surface, a volume part and a mean particle size have been obtained. It is shown that the best catalytic properties are reached by heating the samples up to 400°C. This state is defined by the mean particle size ∼15xa0nm, the specific surface ∼10xa0m 2 /g, the particle volume concentration ∼5×10 −2 and the fractal dimension of particles surface ∼2.5. A general mechanism for the formation of the oxide phases from different polymer–salt solution is established. The processes of the phase formation as a first-order phase transition are also shown.

Instability of the Co-magnetic moment in Tm(Co,M)2 (M=Al,Si)

Abstract Tm(Co 1− x Al x ) 2 compounds with x =0; 0.05; 0.10 and Tm(Co 0.95 Si 0.05 ) 2 have been studied by means of ac -susceptibility, magnetization, magnetostriction, electrical resistivity and neutron diffraction measurements. It is shown that the magnetic state of TmCo 2 is characterized by the absence of the Co-magnetic moment and by the presence of the localized spin density fluctuations in the d-subsystem at low temperatures and is strongly affected by an applied magnetic field. The partial substitution of Al or Si for Co in TmCo 2 leads to the appearance of the long range magnetic order and gives rise to the Co-magnetic moment up to M Co =1.25 μ B . In Tm(Co 0.9 Al 0.1 ) 2 , the field-induced phase transition in d-subsystem was observed at the critical field B c ≈17 T using high-field magnetization and magnetostriction measurements. This phase transition is associated with a collapse of the Co-magnetic moment. Instability of the Co-magnetic moment in TmCo 2 at the substitution and in an applied magnetic field is explained by the closeness of the molecular field acting on the Co-subsystem to the critical value of the Co metamagnetic transition field.

Magnetic properties of RCo2 compounds in the exchange-striction model of ferrimagnets

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo2 ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (ТС), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > ТС. For an ErСо2 sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.

Atomic and Superatomic Structures of Solid Solutions of Zirconium and Calcium Oxides

Solid solutions of the ZrO2-CaO system with 7, 9.6, and 20 mol % of calcium oxide are analyzed by X-ray and neutron diffraction and small-angle neutron scattering. Samples are synthesized by the sol-gel method and annealed at the temperature T = 600°C for 4 h. All samples have a tetragonal structure (P42/nmc space group). The size of crystallites is 10–20 nm. Small-angle neutron scattering curves are characterized by a peak near the momentum transfer q = 0.5 nm−1. The existence of the peak is caused by correlation between solid nanoparticles in the samples. Density correlation functions are numerically computed from the experimental small-angle scattering cross-sections by the inverse Fourier transform. Correlation lengths, concentrations of scattering nanoparticles, and specific surface areas of the samples are determined.

Magnetovolume effects and effect of pressure on the temperature dependence of sublattice magnetization in Ho(Co0.9Ga0.1)2 compounds

The temperature dependence of the magnetic moments of cobalt and holmium ions in the temperature range of 5–300 K at pressures of P = 0 and 5 kbar was determined based on neutron diffraction data on Ho(Co0.9Ga0.1)2 intermetallide. The temperature dependence of the lattice parameter was established, and the spontaneous bulk magnetostriction was determined. These quantities were calculated using the exchange striction model of a ferrimagnet, and good agreement with the experiment was obtained. The parameters of the exchange interaction between pairs of atoms of this compound and the magnetoelastic coupling constant were estimated. An original interpretation of the nature of the first-order magnetic phase transition in HoCo2 is proposed.