RESUMEN

We prepared an organically templated magnet, (enH2)0.5VPO4OH (enH2 = diprotonated ethylenediamine), hydrothermally and characterized its crystal structure by powder X-ray diffraction and Fourier-transform infrared spectroscopy, and its physical properties by magnetization, specific heat and nuclear magnetic resonance measurements and density functional theory calculations. (enH2)0.5VPO4OH consists of uniform chains of V3+ (d2, S = 1) ions and exhibits Haldane magnetism with spin gap Δ = 59.3 K from the magnetic susceptibility χ(T) at µ0H = 0.1 T, which is reduced to 48.4 K at µ0H = 9 T according to the 31P shift. The NMR data evidence the formation of a spin-glass state of unpaired S = 1/2 spins at TS-G ≈ 3 K and indicate that the Haldane S = 1 spin chain segments are much longer in the organically templated magnet (enH2)0.5VPO4OH than in the ammonium counterpart NH4VPO4OH. The single-ion anisotropy D and the interchain exchange J' in (enH2)0.5VPO4OH and NH4VPO4OH were estimated in density functional calculations to find them very weak compared to the intrachain exchange J.

RESUMEN

The spatial spin modulated structure (SSMS) of the cycloid type present in bulk BiFeO3 prevents the linear magnetoelectric effect. One way to influence this structure is to reduce the crystal size to the nanoscale. Various opinions are circulating in the literature about the effect of nanocrystal size on SSMS, and to investigate this issue, we used a number of methods, with zero-field NMR (ZF NMR) spectroscopy at the forefront. ZF NMR spectroscopy enables the direct observation of the distribution profile of local fields on iron atoms and defines the SSMS presence and its properties. We also examined the synthesized samples using XRD, TEM, and magnetometry. We conclude that SSMS persists as the nanocrystal size decreases to the cycloid period and less, becoming more harmonic. This is accompanied by the change of the anisotropy type from an "easy axis" to an "easy plane". Magnetic measurements show a significant increase in the saturation magnetization, remanent magnetization, coercivity, and exchange bias of nanocrystals with sizes close to the cycloid period, which is probably associated with incomplete spin compensation in the case of an incomplete cycloid period. Despite the fact that SSMS is retained in the samples with decreased size, the magnetic properties experience a sharp increase up to applicable values.

RESUMEN

We studied dependences of T2 relaxation time on magnetic field and concentration of nanoparticles. It was found that nanocontrast media are effective under the influence of the magnetic fields in the range 0.3-7 T. Data of electron paramagnetic resonance confirm the assumption on aggregation of nanoparticles not coated with proteins in high magnetic fields.

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RESUMEN

NMR spectra and relaxation times T1 and T2 for 31P in membranes of Rhodobacter sphaeroides were investigated at different relative humidity levels. The results are compared to the hydration curves, fatty acid composition and the structure-dynamic and functional characteristics of the membranes of photosynthetic bacteria Rb. sphaeroides, Rhodospirillum rubrum and Ectothiorhodospira shaposhnikovii. The differences in the state of lipid phase of these membranes are revealed under low humidity, and this is conducive to variability of their structural dynamic and functional characteristics during the hydration process. Based on the results obtained and the data on model systems, four stages of hydration process are distinguished with different effects on the structure and dynamics of membrane components. These stages are: hydration of a portion of polar groups, involvement of water molecules in the hydrogen bonds within macromolecules and the lipid phase, hydration of all polar groups with the appearance of water with high dielectric constant thus making possible the lateral diffusion within the membrane and realization, through water participation, of conditions within organelles and cells required for the process regulation at these levels. The mechanism of water action on various membrane components and their dynamics at each stage are discussed, as well as the effect of different types of motion on the efficiency and regulation of electron transport in the photosynthetic chain of the membranes studied.

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