Nikolina Novosel

Temperature induced magnetic bistability in a crystal of tetrachlorosemiquinone anion radical

The anion radicals with nonaromatic π–systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

Temperature induced reversible structural and magnetic changes in a crystal of tetrachlorosemiquinone anion radical

The anion radicals with nonaromatic π-systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

Effect of magnetic NiCoB nanoparticles on superconductivity in MgB2 wires

A systematic study of the influence of doping MgB2 with single domain magnetic nanoparticles of NiCoB alloy, uncoated and coated with SiO2, has been performed. Electrical resistivity, transport critical current density, Jc(B,T), and magnetization of well characterized undoped and doped with 1.38 and 2.67 wt% of NiCoB particles (both uncoated and coated) MgB2 wires have been investigated in the temperature interval 2–300 K and in magnetic field B ≤ 16T. The superconducting transition temperature, Tc, decreases approximately linearly with the amount of dopand and the intergranular connectivity (the active cross-sectional area fraction, AF) is also reduced upon doping. Reduction of critical fields (irreversibility field, Birr, and upper critical field, Bc2) of doped wires was observed in the whole temperature interval, but an enhancement of Jc of doped wires with respect to the undoped one was observed at low temperature (5 K). Common scaling of Jc(B,T) curves, Birr(T) and volume pinning force, Fp, for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples.

NADH oxidation in a microreactor with an oscillating magnetic field

In this study, magnetic nanoparticles (MNPs) of maghemite (γ-Fe2O3) were synthesized and characterized. The method of multifactor experimental design and evolutionary operation (EVOP) was used to optimize immobilization of the alcohol dehydrogenase (ADH) enzyme on MNPs. Optimal operating conditions for the immobilization process were determined (γADH = 0.08 mg/mL, 2% glutaraldehyde for surface activation, t = 28 h), and in such conditions, a specific activity of S.A. = 118 ± 6 U/mg and immobilization efficiency of η = 84.97 ± 3.67% were achieved. Compared to the native enzyme, ADH immobilized on MNPs retained 66.45 ± 3.66% of the initial activity. ADH immobilized on MNPs at optimal conditions was used as a biocatalyst for model reaction—NADH oxidation. NADH oxidation was performed in two different magnetic microreactor configurations: (1) microreactor equipped with permanent square magnets and (2) microreactor equipped with an electromagnet and an oscillating magnetic field that enables magnetic particles movement in the microreactor. In the system with the oscillating magnetic field, equal conversion (X = 100%) was achieved in 2-fold shorter residence time.

Enhancement of critical fields and current of MgB2 by co-doping

The electromagnetic properties of well-characterized iron-sheathed MgB2 wires, undoped and doped with dextrin-coated magnetite nanospheres and nanorods, have been studied in the temperature range 5?300?K and a magnetic field up to 16?T. Doping hardly affected the superconducting transition temperature and the active cross-sectional area of the wires, and increased the low temperature upper critical field, Bc2. Wire doped with nanospheres also showed enhanced irreversibility field, Birr, and low temperature (T???15?K) critical current density. Magnetization measurements generally confirm the transport results and indicate an enhancement of flux pinning in nanosphere doped wire for T???20?K. Annealing of wire doped with nanorods at higher temperature (750?? C) enhanced its critical fields, as expected for co-doping.

Flux pinning and inhomogeneity in magnetic nanoparticle doped MgB2/Fe wires

The effects of magnetic nanoparticle doping on superconductivity of MgB2/Fe wires have been investigated. Fe2B and SiO2-coated Fe2B particles with average diameters 80 and 150 nm, respectively, were used as dopands. MgB2 wires with different nanoparticle contents (0, 3, 7.5, 12 wt.%) were sintered at temperature 750°C. The magnetoresistivity and critical current density Jc of wires were measured in the temperature range 2–40 K in magnetic field B ≤ 16 T. Both transport and magnetic Jc were determined. Superconducting transition temperature Tc of doped wires decreases quite rapidly with doping level (~ 0.5 K per wt.%). This results in the reduction of the irreversibility fields Birr(T) and critical current densities Jc(B,T) in doped samples (both at low (5 K) and high temperatures (20 K)). Common scaling of Jc(B,T) curves for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples. Rather curved Kramers plots for Jc of doped wires imply considerable inhomogeneity.

Magnetic properties of Fe2−xAlxCoO4 (0 ≤ x ≤ 1) nanoparticles

Nanocrystalline particles of the Fe2−xAlxCoO4 (0 ≤ x ≤ 1) ferrites with cubic spinel structure have been synthesized by the sol-gel method. Size of the particles decreases from 39 nm to 6 nm as the Al content x increases. In the present work magnetic hysteresis curves and ZFC-FC curves have been studied. The magnetization of the Fe2CoO4 nanoparticles is lower than the magnetization of the bulk sample due to the surface and finite size effects. Saturation magnetization and remanent magnetization decrease as the x increases because of substitution of magnetic Fe ions with non-magnetic Al ions and possibly due to the spin canting and changes in ferrimagnetic structure. Coercive field changes with x in a complex way. Anisotropy barrier is lowered by reducing the size of the particle, but it can be increased by changing the distribution of Fe and Al ions in the spinel structure. The observed splitting between ZFC-FC curves in a rather high field indicates the presence of particles with the large anisotropy barrier for all samples.

Magnetic properties of nickel nanoparticles embedded in amorphous Al2O3 matrix

Multilayer thin films consisting of 10 bilayers of (Al2O3+Ni)/Al2O3 were deposited by magnetron sputtering onto Si(100) substrate. Nickel nanoparticles are formed inside amorphous alumina matrix by self-assembly growth process. It was determined by GISAXS measurements that nickel particles are spheroidal with diameter < 3 nm and that they form paracrystal-like body-centered tetragonal lattice. Magnetic properties of the prepared thin films were studied. Due to the nanometer size of nickel particles, their magnetic structure is single domain and they show superparamagnetic behaviour. Anisotropy of magnetic properties was observed when magnetic field is applied parallel or perpendicular to the thin film surface and it was attributed to dipole-dipole interactions between particles. This was confirmed using simulations of the M(H) curve of the 2D superlattice of the identical superparamagnetic particles, which was performed using Monte Carlo method and Metropolis algorithm.

Temperature effect on microstructure and electromagnetic performance of polycarbosilane and sugar-doped MgB2 wires

The effect of processing temperature on structural and superconducting properties of 10 wt.% sugar- and 10 wt.% PCS-doped MgB2 wires is systematically investigated. It is demonstrated that these dopants significantly enhance the electromagnetic performance of Fe-clad MgB2 superconductor and increase its potential for practical application. The enhancement of in-field critical current density (Jc(Ba)) and upper critical field (Bc2) is due to formation of a large amount of lattice defects caused by impurities and C substitution into the MgB2 crystal lattice. High temperature sintering of sugar-doped sample results in as high Bc2 value as 37 T (at 5 K), which correlates with higher level of C substitution into MgB2 crystal lattice in this sample. In contrast, for PCS doped MgB2 wire higher Bc2 value (32 T at 5 K) is observed at lower sintering temperatures. In spite of the fact that the level of C in the crystal lattice and Bc2 value are higher in the sugar doped MgB2 sample, this sample has lower Jc(Ba) when compared to the sample with PCS addition. We speculate that it is due to a higher level of MgO impurities in the sugar doped sample (18.6 wt.% compared to 9.15 wt.% in the PCS doped sample), which results in the dissipation of supercurrent flowing through this sample.