Solid State Phenomena Vol. 190

Abstract: We study the critical behavior of three-dimensional antiferromagnet Heisenberg model with nearest-neighbor (J) and next-nearest-neighbor (J₁) interactions by the Monte Carlo method using a high-effective replica exchange algorithm. Here is calculated a full set of main static critical exponents for values R =J₁/J= 0.0; 0.025; 0.05; 0.075; 0.1; 0.115 using the finite-size scaling theory. A phase diagram of dependency of the critical temperature on a relation between nearest-neighbor and next-nearest-neighbor R is plotted.

Abstract: We report the results of a detailed study of EuRh₄B₄ compound synthesized by an un-common method under high pressure temperature conditions for the first time. Synthesized sample has been initially characterized by X-ray diffraction, macroscopic electrical resistivity and ac-susceptibility methods. The ferromagnetic and superconducting phases were found below 83K and 5.8K respectively. Element-and shell-selective XANES and XMCD techniques have been further applied to check the possible valence transition of Eu ions and their magnetic states. XANES spectra recorded at the Eu L_2,3 absorption edges clearly indicate the presence of either magnetic Eu²⁺ or non-magnetic Eu³⁺ ions in the sample. The XMCD studies have also shown that local magnetic moments of Eu²⁺ ions exist in the superconducting EuRh₄B₄, at least above the transition.

Abstract: We consider the Josephson effect in a three-layer symmetric superconductor-ferromagnet-superconductor (SFS) system. In the framework of the proximity effect theory we calculate the Josephson current as a function of the ferromagnet thickness, taking into account the Umklapp processes at the internal boundary and the critical temperature dependence of the order parameter phase difference. The results of the calculations are in good agreement with experimental data.

Abstract: The possibility of the Cooper instability in transition metal compounds has been established on the bases of an idea about a strong interaction in a unit cell. The phase diagram of the superconducting ordering depending on a number of holes (h_d) of the 3d-shells has been obtained. The non-phonon BCS constant has been calculated.

Abstract: The study of Fe/SiO₂ nanocomposites magnetic properties and structure relationships was performed in dependence on Fe to SiO₂ relative concentration and type of precursors preparation (mechanical mixing or ultrasonic homogenization). Thermal metallization method in hot hydrogen of initial reagents [хFeOOH and (100-x)SiO₂] was applied for nanocermets production. It was determined that formation of air-tight shell on the particles surface affects the nanocomposites magnetization values and their air temperature stability.

Abstract: Interlayer interactions in FeNi/Bi/FeNi trilayers have been experimentally studied. Based on the data of SQUID magnetometry and magnetic resonance investigations, it is shown that the interlayer interaction is determined by a bismuth spacer thickness and temperature. The giant magnetoresistive effect in the trilayer films has been discovered.

Abstract: The magnetic properties of small 4d, 5d metal nanoparticles of Pd, Pt (clusters) are attracting a great attention because these materials in bulk are paramagnetic. In this work we report the ferromagnetic-like behavior of the small Pt nanoparticles prepared by chemical method. Highly dispersed Pt clusters have been synthesized on the surfaces of a porous spherical γ-Al₂O₃ particles. The process of the chemical deposition of metalorganic fluid with employment of the supercritical fluid was used. The samples of the Pt/γ-Al₂O₃ nanoparticles have been prepared in INEOS RAS. The nanoparticles size distribution was determined by small-angle X-rays scattering (SAXS). It was found that the Pt clusters have a bimodal particle size distribution with two peaks: R1max=20 Å and R2max=40 Å. The magnetic properties of the clusters have been investigated, using VSM magnetometer, in magnetic field up to ±3 kOe and at a temperature range from 80 to 400 K. It was observed that Pt/γ-Al₂O₃ nanoparticles show the ferromagnetic-like behavior in whole specified temperature range, the value of coercivity decreases gradually from 130 Oe to 80 Oe. The origin of ferromagnetic-like behavior of the Pt/γ-Al₂O₃ nanoparticles is discussed.

Abstract: Goethite nanopowder was prepared in the process of chemical precipitation of iron salt (FCl₃) and alkali (NaOH) with addition of various surface active substances (SAS): cation-active cetylpyridinium chloride (CPC), anion-active sodium dodecyl sulphate (SDS) and complexon EDTA. The concentrations of SAS were 0.3% and 1%. It could be deduced from TEM data that these SASes unambiguously influence sizes of prepared goethite particles, compared with samples, obtained without SAS: in case of adding 1% SDS and EDTA significant crystal growth takes place. To explain this situation the study of magnetic characteristics and phase composition of obtained nanoparticles was carried out using Mossbauer spectroscopy and thermomagnetic analysis (TMA). It was suggested, that different organic Fe³⁺- complexes formed on goethite surface under SAS influence take part in the goethite crystallization process.

Abstract: Fe19Ni81/Tb₃₅Co₆₅ magnetic films having unidirectional anisotropy were investigated with focus on the influence of the Fe19Ni81 layer and Ti spacer thicknesses on magnetic and magnetoresistive properties of the multilayered structures. Principles of the optimization of the properties of magnetoresistive multilayers with unidirectional anisotropy were proposed.

Abstract: Film composites based on magnetite Fe₃O₄ nanoparticles in the polyvinyl alcohol (PVA) matrix and the dried ferrofluid ARA-250 (Chemicell GmbH, Germany) were studied at temperatures 78-300 K by Mössbauer spectroscopy and magnetic measurements in magnetic fields H < 5 kOe. The Mössbauer spectra taken in magnetic fields show essential restoration of the magnetic hyperfine structure characteristic for the nanoparticles. The experimental Mössbauer spectra have been analyzed within the recently developed three-level [ and multi-level [ relaxation models of magnetic dynamics in an ensemble of single-domain particles. These models take into account the precession and diffusion of particles uniform magnetization as well as the particles size distribution and interparticle interactions.