2016

Sm_1−xLa_xFe₃(BO₃)₄ (x = 0, 0.75) single crystals were synthesized using the flux method. The crystal nucleation procedure and flux parameters are described. The prepared single crystals were studied by Raman spectroscopy. Raman spectra anomalies related to the magnetic ordering established in both compounds were found. The main changes occur in the low-wavenumber region (up to 100 cm⁻¹), where the modes corresponding to magnon scattering arise.

The Kulunda steppe is part of the greatest conversion areas of the world where 420,000 km² grassland have been converted into cropland between 1954 and 1963. However, little is known about the recent and future impacts of land-use change (LUC) on soil organic carbon (OC) dynamics in Siberian steppe soils under various climatic conditions. By investigating grassland vs. cropland soils along a climatic gradient from forest to typical to dry steppe types of the Kulunda steppe, our study aimed to (i) quantify the change of OC stocks (0–60 cm) after LUC from grassland to cropland as function of climate, (ii) elucidate the concurrent effects on aggregate stability and different functional soil organic matter (OM) fractions (particulate vs. mineral-bound OM), and (iii) assess climate- and LUC-induced changes in the microbial community composition and the contribution of fungi to aggregate stability based on phospholipid fatty acid (PLFA) profiles. Soil OC stocks decreased from the forest steppe (grassland: 218 ± 17 Mg ha⁻¹) over the typical steppe (153 ± 10 Mg ha⁻¹) to the dry steppe (134 ± 11 Mg ha⁻¹). Across all climatic regimes, LUC caused similar OC losses of 31% (95% confidence interval: 17–43%) in 0–25 cm depth and a concurrent decline in aggregate stability, which was not related to the amount of fungal PLFA. Density fractionation revealed that the largest part of soil OM (>90% of total OC) was associated with minerals and <10% of C existed in particulate OM. While LUC induced smaller relative losses of mineral-associated OC than particulate OC, the absolute decline in total OC stocks was largely due to losses of OM bound to minerals. This result together with the high ¹⁴C ages of mineral-bound OM in croplands (500–2900 yrs B.P.) suggests that mineral-bound OM comprises, in addition to stable OC, also management-susceptible labile OC. The steppe type had a larger impact on microbial communities than LUC, with a larger relative abundance of gram-positive bacteria and less fungi under dry conditions. Our results imply that future drier climate conditions in the Siberian steppes will (i) result in smaller OC stocks on a biome scale but (ii) not alter the effect of LUC on soil OC, and (iii) change the microbial community composition more than the conversion from grassland to cropland.

For the system of strongly correlated electrons on a triangular lattice, the possibility of coexisting superconductivity with the chiral order parameter and the 120°-type noncollinear spin ordering is demonstrated. The integral self-consistency equation for the superconducting order parameter is derived using the diagram technique for Hubbard operators taking into account the spin structure, exchange interaction within two coordination spheres, and intersite Coulomb repulsion.

The article investigates the phase composition of (Al₂O₃–20 wt % ZrO₂)–ZrW₂O₈ ceramic composites obtained by cold-pressing and sintering processes. Using X-ray analysis it has been shown that composites mainly have monoclinic modification of zirconium dioxide and orthorhombic phase of aluminum oxide. After adding zirconium tungstate the phase composition of sintered ceramics changes, followed by the formation of tungsten-aluminates spinel such as Al_x(WO_y)_z. It has been shown that thermal expansion coefficient of material decreases approximatly by 30%, as compared with initial ceramics.

Polarized Raman spectra of single crystals of lead diborate, PbB₄O₇ (PBO), are studied in detail at 300 K. The TO-, LO-, and IO-phonon lines of the A₁, A₂, B₁, and B₂ symmetries in the Raman spectra of this compound are assigned. Changes in the Raman spectra of the internal vibrations of boron–oxygen complexes upon transition from the crystalline to the glassy and the molten states of PBO are observed. On the basis of the obtained results, the regularities in the formation of boron–oxygen complexes in glasses, melts, and crystals of the PbO · 2B₂O₃, SrO · 2B₂O₃, and Li₂O · 2B₂O₃. diborate compositions are analyzed.

Noncollinear second-harmonic generation from two ultrashort pulses intersecting in a nonlinear medium is studied in spectral and time domains. We derive analytical expressions for the second-harmonic (SH) amplitude in crystals of finite thickness neglecting diffraction. The contribution from characteristics of the fundamental radiation and interaction geometry to the process is analyzed. In addition, refined phase-matching conditions are obtained. We find that the spectral bandwidth is determined by the intersection angle and can be enlarged. The SH pulse duration can be optimized by varying the fundamental beam size and the intersection angle. It is found that the SH beam excited by a Gaussian fundamental beam becomes elliptical. It is shown that the fundamental pulse duration can be readily characterized with single pulses by means of measuring the ellipticity of the SH beam profile. The approach developed can potentially be used to calculate parametric interactions of fundamental pulses with an arbitrary spectrum.

The temperatures for saturation of the MBO₃ (M=Fe, Ga, In, Sc, Lu) compounds in the M₂O₃–B₂O₃– (70 PbO–30 PbF₂, wt%) solvents are determined. The growth rates of FeBO₃ and GaBO₃ crystal faces as functions of the flux supercooling are obtained. It is demonstrated that the bulk FeBO₃ and GaBO₃ crystals can be grown in a narrow flux supercooling range using a controlled seeding technique. The MBO₃ (M=In, Sc, Lu) crystals in the form of (111) plates are synthesized by spontaneous crystallization.

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The magnetic properties of ferrihydrite nanoparticles, which are products of vital functions of Klebsiella oxitoca bacteria, have been studied. The initial powder containing the nanoparticles in an organic shell was subjected to low-temperature (T=160 °C) heat treatment for up to 240 h. The bacterial ferrihydrite particles exhibit a superparamagnetic behavior. Their characteristic blocking temperature increases from 26 to 80 K with the heat treatment. Analysis of the magnetization curves with regard to the magnetic moment distribution function and antiferromagnetic contribution shows that the low-temperature heat treatment enhances the average magnetic moment of a particle; i.e., the nanoparticles coarsen, probably due to their partial agglomeration during heat treatment. It was established that the blocking temperature nonlinearly depends on the particle volume. Therefore, a model was proposed that takes into account both the bulk and surface magnetic anisotropy. Using this model, the bulk and surface magnetic anisotropy constants K_V≈1.7×10⁵ erg/cm³ and K_S≈0.055 erg/cm² have been determined. The effect of the surface magnetic anisotropy of ferrihydrite nanoparticles on the observed magnetic hysteresis loops is discussed.

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We report the results of investigations of the effect of cooling in an external magnetic field starting from the temperature over superparamagnetic blocking temperature TB on the shift of magnetic hysteresis loops in systems of ferrihydrite nanoparticles from ∼2.5 to ∼5 nm in size with different TB values. In virtue of high anisotropy fields of ferrihydrite nanoparticles and open hysteresis loops in the range of experimentally attainable magnetic fields, the shape of hysteresis loops of such objects in the field-cooling mode is influenced by the minor hysteresis loop effect. A technique is proposed for distinguishing the exchange bias effect among the effects related to the minor hysteresis loops caused by high anisotropy fields of ferrihydrite particles. The exchange bias in ferrihydrite is stably observed for particles not less than 3 nm in size or with TB over 40 K, and its characteristic value increases with the particle size.

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