New Publications

Nickel nanoparticles were synthesized by the reduction of nickel chloride with hydrazine hydrate in a polyol medium in the presence of sodium polyacrylates (Na-PA) having molecular weights (M_w) of 1200, 5100 and 8000. The size and morphology of the resulting nickel nanoparticles were characterized by X-ray diffraction, scanning and transmission electron microscopy. Polymers having lower M_w values were found to be more efficient in reducing the nickel particle size. A decrease in the polymer concentrations yielded the smaller particles. Magnetic measurements showed that the as-prepared powders are ferromagnetic and their saturation magnetization and coercivity are size-dependent. Compared with bulk nickel, the nanoparticles exhibit an enhanced coercivity which is due to their small size and a decreased saturation magnetization resulted from the surface oxidation of the powder. The synthesis procedure offers a simple approach to preparing nickel nanopowders on a large scale which could be used as magnetic recording materials, including high-density memory storage devices.

Nanoparticles of antiferromagnetic materials acquire the magnetic moment due to the surface effects and structural defects. According to the Neel hypothesis, magnetic moment μ _P of a particle containing N magnetically active atoms with magnetic moment J can be estimated as μ _P ∼ J · N ⁿ or μ _P ∼ V ⁿ , where V is the particle volume. Numerous studies of the magnetic properties of ferrihydrite 5Fe₂O₃⋅9H₂O and ferritin revealed a value of n ≈ 1/2 for this material, in which Fe atoms have the octahedral surrounding of anions. We investigate the effect of low-temperature annealing of cobalt-doped ferrihydrite nanoparticles on their average size and magnetic properties. Using the Mössbauer spectroscopy study, we demonstrate that doping with Co makes Fe atoms enter the anion tetrahedra which leads to an increase in the exponent n〉1/2 in the expression μ _P ∼ J · N ⁿ .

Evolution of the local environment of Fe³⁺ ions in deposited Fe₂O₃/SiO₂ nanoparticles formed in samples with different iron contents was investigated in order to establish the conditions for obtaining the stable ε-Fe₂O₃/SiO₂ samples without impurities of other iron oxide polymorphs. Microstructure of the samples with an iron content of up to 16% is studied by high-resolution transmission electron microscopy, X-ray diffraction analysis, and Mössbauer spectroscopy, and their magnetic properties are examined. At iron concentrations below 6%, calcinations of iron-containing precursor nanoparticles in a silica gel matrix lead to the formation of the ε-Fe₂O₃ iron oxide polymorphic modification without foreign phase impurities, while at the iron concentration in the range of 6–12%, the hematite phase forms in the sample in the fraction of no more than 5%. It is concluded on the basis of the data obtained that the spatial stabilization of iron-containing particles is one of the main factors facilitating the formation of the ε-Fe₂O₃ phase in a silica gel matrix without other iron oxide polymorphs. It is demonstrated that the increase in the iron content leads to the formation of larger particles in the sample and gradual changes of the Fe³⁺ ion local environment during the phase transition ε-Fe₂O₃ → α-Fe₂O₃.

Dissipation in granular high-temperature superconductors (HTSs) during the passage of macroscopic transport current j is mainly determined by carrier tunneling through intergrain boundaries (Josephson junctions). In the presence of external magnetic field H, it is necessary to take into account the significant magnetic flux compression, which can lead to the situation when the effective field B_eff in the intergrain boundaries exceeds the external field by an order of magnitude. This is observed as a wide hysteresis of the field dependence of magnetoresistance R(H). In this study, we investigate the R(H) hysteresis evolution in granular 1–2-3 HTSs in different j–H orientations. The magnetic flux compression significantly affects the magnetoresistance and its hysteresis for both perpendicular (H ⊥ j) and parallel (H ǁ j) orientations. The obtained experimental data on the R(H) hysteresis at the arbitrary angles θ = ∠H, j are explained using the approach developed for describing the magnetoresistance hysteresis in granular HTSs with regard to the magnetic flux compression and the model representations proposed by Daghero et al. [Phys. Rev. B 66(13), 11478 (2002)]. A concept of the effective field in the intergrain medium explains the well-known anisotropy of the magnetotransport properties of granular HTSs.

We report on the effect of interparticle magnetic interactions in an ensemble of superparamagnetic magnetite particles with an average size of ~8.4 nm dispersed in the diamagnetic matrix on the blocking of this ensemble in external magnetic field. The two limit cases are investigated: the case of strongly interacting particles, when the value of magnetic dipole-dipole interaction between particles is comparable with the energy of other interactions in the ensemble (the interparticle distance is similar to the nanoparticle diameter) and the case of almost noninteracting particles distant from each other by about ten particle diameters. We demonstrate that the experimental dependence of the blocking temperature on external field is described well within the model [1], in which the density of particles in a nonmagnetic medium is taken into account and the correlation value depends on external magnetic field. The model for describing the magnetic properties of a disperse nanoparticle ensemble is proposed, which makes corrections related to the particle size and mean dipole-dipole interaction energy for the anisotropy constant. The surface magnetic anisotropy of Fe₃O₄ particles and parameters of the interparticle coupling are estimated.

Y_0.75Nd_0.25Ba₂Cu₃O_7−d superconductor has the peak effect with pronounced fishtail feature on magnetization hysteresis. The magnetic field of the peak effect maximum is in ~4 times higher than one of NdBa₂Cu₃O_7−d. The magnetization hysteresis is tilted anticlockwise due to paramagnetic Nd³⁺ ions occurred on the surface of grains and in the vortex cores.

Polycrystalline Y_1−xNd_xBa₂Cu₃O_7−δ (x=0.02, 0.11, and 0.25) superconductors are synthesized. Nd atoms are uniformly distributed over grains. The magnetization loops of the samples have a pronounced second peak in a wide temperature range. The magnetization data are analyzed using the extended critical state model. It is found that the order-disorder transition of the vortex lattice is affected by doping with neodymium and temperature; the second-peak field and width decrease monotonically with increasing x value. The undoped polycrystalline YBa₂Cu₃O_7−δ compound is assumed to exhibit the peak effect in higher magnetic fields.