I. Zakharchuk

Magnetic Properties of Mn Substituted Barium Hexaferrite Single Crystals

This paper presents magnetic properties of manganese substituted barium hexaferrite BaFe12-xMnxO19 single crystals. Crystals of BaFe12–xMnxO19 with x up to 1.5 and sizes up to 8 mm were observed. The influence of Fe substitution by Mn on the magnetic properties was investigated. For the BaFe10.5Mn1.5O19 single crystal samples saturation magnetization reduced from 72 to 63.5 emu/g at room temperature, and Curie temperature decreased from 455 to 380 °C.

Mechanisms of charge transfer and electronic properties of Cu 2 ZnGeS 4 from investigations of the high-field magnetotransport

Recent development of the thin film solar cells, based on quaternary compounds, has been focused on the Ge contain compounds and their solid solutions. However, for effective utilization of Cu2ZnGeS4, deeper investigations of its transport properties are required. In the present manuscript, we investigate resistivity, ρ (T), magnetoresistance and Hall effect in p-type Cu2ZnGeS4 single crystals in pulsed magnetic fields up to 20 T. The dependence of ρ (T) in zero magnetic field is described by the Mott type of the variable-range hopping (VRH) charge transfer mechanism within a broad temperature interval of ~100–200 K. Magnetoresistance contains the positive and negative components, which are interpreted by the common reasons of doped semiconductors. On the other hand, a joint analysis of the resistivity and magnetoresistance data has yielded series of important electronic parameters and permitted specification of the Cu2ZnGeS4 conductivity mechanisms outside the temperature intervals of the Mott VRH conduction. The Hall coefficient is negative, exhibiting an exponential dependence on temperature, which is quite close to that of ρ(T). This is typical of the Hall effect in the domain of the VRH charge transfer.

Magnetic and Microstructural Properties of Cobalt Substituted NiZn Ferrite Powders

This paper presents the results of magnetic and microstructural study of cobalt-substituted NiZn ferrite powders, Ni0.5Zn0.3Co0.2Fe2O4, synthesized with the solid-state reaction method. The lattice parameter decreased with Co substitution, as the ionic radius of cobalt is less than that of zinc. Raman spectroscopy showed clear peaks of the A1g, Eg, and F2g modes, revealing cubic spinel structure and good crystallinity of the samples. The saturation magnetization reduced in the substituted sample to 50.28 emu/g at room temperature. In comparison to the bulk ferrite, the powder sample showed pronounced coercivity even at room temperature. The temperature dependence of the magnetization had superparamagnetic curvature, which yielded the effective anisotropy constant equal to 8.832 kJ/m3.

Paramagnetic anatase titania/carbon nanocomposites

Abstract. The nanocomposites comprising of anatase titania nanoparticles uniformly distributed inside the porous carbon matrix were synthesized using furfuryl alcohol, tetrabutyltitanate, and nonionic surfactant. The characterization of the nanocomposite by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray analysis, x-ray diffractometry, and Raman spectroscopy revealed the formation of anatase titania nanoparticles of average sizes 10 to 20 nm. The observed broadening and blueshift of the main Raman peak of titanium dioxide nanoparticles were explained using the phonon confinement effect. The nanocomposite exhibited strong paramagnetism at low temperatures and was diamagnetic at higher temperatures, as well as a small contamination by magnetic impurities was detected. The paramagnetism originated from the amorphous defect-rich carbon and oxygen vacancies in titania.

Magnetic and Microstructure Study of Thin Films of FeCuNbMoSiB FINEMET Alloy

Thin films of FeCuNbMoSiB have been sputtered on Corning glass substrates with thicknesses varying from 10 to 200 nm with post annealing at 450 °C and 550 °C. Annealing in the presence of the magnetic field applied along the plane of a substrate develops an uniaxial magnetic anisotropy with the in-plane easy axis. Estimation of the effective anisotropy constant from the magnetization measurements gave Keff = 3.23 kJ/m3. Structure and surface of the films were investigated with the X-ray powder diffraction (XRD), resistivity measurements, and Raman spectroscopy. XRD and resistivity analyses show that thermal annealing at 550 °C improves the crystalline fraction and Fe-Si grain size. Raman spectra identified hematite, goethite, magnetite, as well as graphite contamination of film surfaces.

Magnetic moment of single vortices in YBCO nano-superconducting particle: Eilenberger approach

Temperature dependence of single vortex magnetic moment in nanosize superconducting particles is investigated in the framework of quasiclassical Eilenberger approach. Such nanoparticles can be used for preparation of high-quality superconducting thin films with high critical current density. In contrast to bulk materials where the vortex magnetic moment is totally determined by flux quantum, in nano-sized specimens (with characteristic size, D, much less than effective penetration depth, λeff) the quantization rule is violated and magnetic moment is proportional to D2/λ2eff(T). Due to strong repulsion between vortices in nanoparticles only a single vortex can be trapped in them. Because of small size of particles the screening current of the vortex is located near the vortex core where the current is quite high and comparable to depairing currents. Therefore, the superconducting electron density, ns, depends on the current value and the distance from the vortex core. This effect is especially important for superconductors having gap nodes, such as YBCO. The current dependence of ns in nanoparticles is analogous to the Volovik effect in flux-line lattice in bulk samples. The magnitude of the effect can be obtained by comparing the temperature dependence of magnetic moment in the vortex and in the Meissner states. In the last case the value of screening current is small and superconducting response to the external field is determined by London penetration depth. Because of importance of nonlinear and nonlocal effects, the quantum mechanical Eilenberger approach is applied for description of the vortex in nanoparticles. The flattening of 1/λ2eff(T) dependence has been found. A comparison of the theoretical results with experimental magnetization data in Meissner and mixed states of YBCO nanopowders has been done. The presence of nonlinear and nonlocal effects in vortex current distribution is clearly visible. The obtained results are important for the description of pining in nanostructured high-Tc thin films.

IMPACT OF THE ORDER PARAMETER SYMMETRIES ON THE VORTEX CORE STRUCTURE IN IRON-BASED SUPERCONDUCTORS

Effects of the order parameter symmetries on the cutoff parameter ξh (determining the magnetic field distribution) in the mixed state are investigated in the framework of quasiclassical Eilenberger theory for isotropic s±, s++ and anisotropic dx2-y2-wave superconducting pairings. These symmetries are proposed for the pairing state of the Fe-pnictides. In s± pairing symmetry, the gap function has opposite sign at the electron and hole pockets of the Fermi surface, it is connected with interband antiferromagnetic spin fluctuations. In s++ pairing symmetry, the gap function has the same sign at the Fermi surface, it is mediated by moderate electron–phonon interaction due to Fe-ion oscillation and the critical orbital fluctuation. The dx2-y2 pairing symmetry can rise from intraband antiferromagnetic spin fluctuation in strongly hole overdoped iron pnictide KFe2As2 and ternary chalcogenides. The s± pairing symmetry results in different effects of intraband (Γ0) and interband (Γπ) impurity scattering on ξh. It is found that ξh/ξc2 value decreases with Γ0 leading to the values much less than those predicted by the analytical Ginzburg–Landau (AGL) theory for high Γ0. At very high Γ0, the interband scattering suppresses ξh/ξc2 considerably below one in the whole field range making it flat for both s± and s++ pairing symmetries. Scaling of the cutoff parameter with the electromagnetic coherence length shows the importance of the nonlocal effects in mixed state. The small values of ξh/ξc2 were observed in μSR measurements of Co-doped BaFe2As2. If Γ0 and Γπ are small and equal than the ξh/ξc2(B/Bc2) dependence for s± symmetry behaves like that of the AGL model and shows a minimum with value much more than that obtained for s++ superconductors. With high Γπ, the ξh/ξc2(B/Bc2) dependence resides above the AGL curve for s± pairing symmetry, as observed in SANS measurements of stoichiometrical LiFeAs compound. In d-wave superconductors, ξh/ξc2 always increases with Γ similar to the s± symmetry case with Γ0 = Γπ.

Cutoff parameter versus Ginzburg-Landau coherence length in the mixed state of high-κ superconductors with impurities: quasiclassical approach

The influence of the impurities on the ratio of the cutoff parameter, ξh and the Ginzburg-Landau coherence length, ξc2, in the mixed state of high-κ s-wave superconductors is investigated in framework of the quasiclassical nonlocal Eilenberger theory. Quasiparticle scattering by impurities and lowering of the temperature reduce the value of ξh to values much less than ξc2. This is different from the prediction of the local Ginzburg-Landau theory where ξh is scaled by ξc2. Detailed comparison with the behavior of the order parameter coherence length ξ1 is done. It is found that impurities influence by different way on ξh and ξ1. The curve ξh/ξc2(B/Bc2) shifts downward with increasing of impurity scattering rate while ξ1/ξc2(B/Bc2) curve shifts upward in this case.

Effects of the order parameter symmetry on the vortex core structure in the iron pnictides

Effects of the order parameter symmetry on the cutoff parameter ξh (determining the magnetic field distribution) in the mixed state are investigated in framework of quasiclassical Eilenberger theory for isotropic s± and for s++ pairing symmetries of superconductors using computational methods. In s± pairing symmetry the gap function has opposite sign and equal absolute values of the electron and hole pockets of the Fermi surface and in s++ pairing symmetry the gap function has the same sign of the electron and hole pockets of the Fermi surfaces. The s± pairing symmetry results in different effects of intraband (Γ0) and interband (Γπ) impurity scattering on ξh. It is found that ξh/ξc2 decreases with the Γ0 leading to values much less than those predicted by the analytical Ginzburg-Landau (AGL) theory for high Γ0. At very high Γ0 the interband scattering suppresses ξh/ξc2 considerably less than the one in the whole field range making it flat. If Γ0 and Γπ are small and equal then the ξh/ξc2(B/Bc2) dependence behaves like that of the AGL model and shows a minimum with value much more than that obtained for s++ superconductors. With high Γπ the dependence of ξh/ξc2(B/Bc2) resides above the AGL curve. Such behavior is quite different from that in s++ pairing symmetry where intraband and interband scattering rates act in a similar way and ξh/ξc2 decreases monotonously with impurity scattering and resides below the AGL curve.