V. N. Varyukhin

Phase transitions in TbMnO3 manganites

X-ray diffraction and magnetic measurements of polycrystalline and nanosize TbMnO3 manganites were performed. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature. Nanosize manganites were synthesized via the sol-gel method at different (800 and 900 °C) temperatures. The average size of the synthesized nanoparticles (from 45 to 70 nm) was estimated by x-ray diffraction and low-temperature adsorption of argon. Information about the evolution of properties of TbMnO3 with changing grain size, temperature, and magnetic field was obtained. Crystal structure parameters of nano-samples change slightly with changing nanoparticle size. Peculiarities of magnetic ordering in polycrystalline and nanosize TbMnO3 were compared. Magnetization and the Neel temperature corresponding to antiferromagnetic ordering of the Tb3+ sublattice decrease as the particle size is reduced. Inverse magnetic susceptibility of the nanoparticle samples deviates from the Curie–We...

Conductivity of La0.7Sr0.3MnO3 − δ films with small deviations from stoichiometry with respect to oxygen

We have studied the dynamics of the transport and optical characteristics of epitaxial La0.7Sr0.3MnO3 − δ (LSMO) films with oxygen deficiency varied in the course of a multistage annealing. A hierarchical scheme for evaluating the defectness of the film material is proposed, which is based on the results of measurements of the minimum dc conductivity (σdc(T)) and the range of its variation with the temperature. It is established that the optical conductivity (σopt) at a radiation quantum energy of 2 eV corresponds to the limiting value of minimum σdc(T) in LSMO films without oxygen deficiency. This result does not contradict previously reported optical data and is consistent with the existing notions about the mechanisms of conductivity operative in the system under consideration.

Magnetic properties and thermal modification of nanostructured films of nickel nitrides

Nanostructured films of nickel nitrides (from Ni to Ni2N) have been synthesized by magnetron sputtering of a nickel target in an argon atmosphere with the addition of 0.2–70 vol % nitrogen. Magnetization saturation 4πMS and Curie temperature TC of the solid solution of nitrogen in nickel and nitride Ni4N have been determined. With an increase in nitrogen concentration, 4πMS and TC decrease, respectively, to 800 G and 490 K. When heated in air, nickel nitride films undergo transformation with nitrogen loss up to phase transitions with structural changes. In particular, heating to 600 K leads to transformation of paramagnetic nitride Ni2N into the ferromagnetic Ni phase in the form of solid solution of nitrogen in nickel.

Synthesis and magnetic properties of nanocolumnar nickel films deposited in argon-nitrogen atmosphere

Ferromagnetic nickel films with a structure comprising nanocolumns grown perpendicular to a substrate have been obtained by magnetron sputtering of a nickel target in argon atmosphere containing 2 vol % nitrogen. Measurements of X-ray diffraction and investigation of magnetic properties showed that the obtained films consist of a solid solution of nitrogen in nickel. The saturation magnetization of the films is 1.5–2 times smaller and their Curie temperature is 30 K lower than the corresponding values for pure nickel. The film material exhibits a dimensional effect: as the grain size decreases below 10 nm, the magnetization sharply drops; at a grain size of 0.5 nm, the film possesses no magnetic order.

Magnetocaloric effect in (La0.6Ca0.4)0.9Mn1.1O3

The results of investigations of the magnetization, susceptibility, and magnetic-field-induced changes in the entropy of polycrystalline manganite (La0.6Ca0.4)0.9Mn1.1O3 near the magnetic phase transition have been presented. Magnetic measurements have been carried out at temperatures in the range from 210 to 310 K in magnetic fields of up to 9 T. The magnetocaloric effect has been revealed by measuring the magnetic-field dependences of magnetization. The magnitude of the magnetocaloric effect is compared with similar results obtained for other manganites.

Coercive force of nanocrystalline manganites

Samples of La0.7Mn1.3O3±Δ and (La0.65Sr0.35)0.8Mn1.2O3±Δ with particle sizes ranging from 6to200nm are obtained using cold isostatic pressing. The coercive force of the experimental samples is determined from the field dependences of the resistance and dynamic magnetic susceptibility. It is determined by two methods that the contribution of the surface layer to the magnetic characteristics of manganites is composition dependent. It is shown experimentally for the first time that the coercive force in manganites reaches its highest values with particle size of the order of 70nm for both compositions and vanishes completely for lanthanum manganite with ∼6nm particles as a result of reaching a superparamagnetic state.

Structure and magnetic properties of Cu-Fe fiber composites obtained using packet hydrostatic extrusion

The structure and magnetic properties of Cu-Fe fiber composites prepared by packet hydrostatic extrusion are studied. The number of armco-iron fibers in the copper matrix attained n ∼ 8 × 1010, and rated diameter d of fibers varied from 2 mm to 3 nm. A correlation is observed between the magnetization hysteresis curves and low-frequency magnetic susceptibility obtained on samples with various values of d. It is shown that the magnetic properties of composites in the submicrometer range of d values are satisfactorily described in the magnetization reversal theory for small ferromagnetic particles.

Influence of screw extrusion on the atomic order in constructional steel

The atomic order in low-carbon steel strained by screw extrusion making the microstructure finer and changing the phase ratio in the material is studied by X-ray diffraction analysis. The steel is found to consist of multiscale structural fractions: a finely crystalline (300–600 Å) α-Fe matrix, nanodimensional (180–250 Å) Fe3C and Fe3Si or Fe5Si3 phases, and a fraction with randomly arranged atoms. A second-order order-disorder phase transition is revealed with order prevailing in the longitudinal (‖) section of the samples and disorder prevailing in their cross (⊥) section. Sets of “extended” and “compressed” planes are found to coexist in the structure for the first time, with extended planes prevailing in the (‖) section and compressed planes prevailing in the (⊥) one.

Crystallite size and magnetic properties of La0.7Mn1.3O3 ± Δ

The results of investigation into nanocrystalline lanthanum manganites La0.7Mn1.3O3 ± Δ produced by repeated cold isostatic pressing of a charge material are reported. A powder compact with a crystallite size of 5–7 nm exhibits no magnetic properties, unlike a coarse-grained (20 nm) powder compact.

Multiscale structural changes of atomic order in severely deformed industrial aluminum

The regularities of multiscale structural changes in the atomic order of the aluminum alloy AD-1 after a severe cold plastic deformation by conventional rolling in smooth rolls or in rolls with relief recesses favorable for shear deformation have been investigated. It has been found that there are four types of structural fractions that differ in scale and perfection of atomic order: crystallographic planes with a long-range order; nanoscale fragments of the planes (D = 100–300 Å) with an incipient long-range order; smaller groups of atoms (D = 20–30 Å) of amorphized structure; and the least ordered structural fraction of intercluster medium, keeping only a short-range atomic order (2–3 interatomic distances, 10 Å). The presence of diffuse halo bands in the region of intense Debye lines indicates phase transitions of the order → disorder type with the formation of one to three groups of amorphous clusters with the dominance, in the nanometer scale, of the atomic order characteristic of the family of planes (111), (220), and (311) of crystalline aluminum. We have found a dynamic phase transition with the changing crystallographic order of aluminum, with the matrix structure of a face-centered cubic (FCC) lattice, in the form of nanosized local groups of atoms, that is, the deformation clusters of aluminum with a simple cubic K6 lattice. In the case of conventional rolling, the development of large clusters 50–500 Å in size is observed; however, in the use of rolls with relief recesses, the difference in the sizes of the clusters is one half as much: 50–250 Å. Based on the analysis of the integrated intensity of incoherent X-ray scattering by the samples, we have elucidated the nature of the lowest measured density for the sample subjected to conventional rolling, which consists in the volume concentration of disorderly arranged atoms, the highest of the compared structures, which indicates the formation therein of the greatest amount of fluctuation “voids.”