A. V. Lashkul

Isomerization of α-Pinene Oxide Over Iron-Modified Zeolites

Fe-modified mordenite, ferrierite, Y, ZSM-5, ZSM-12 and beta zeolite catalysts were prepared by solid state ion-exchange and conventional liquid phase ion-exchange methods from aqueous solutions. Sn- modified H-beta-300 zeolite catalyst was prepared by the later method. The characterization of proton form, Fe and Sn modified zeolites was carried out using X-ray powder diffraction, scanning electron microscopy, Mössbauer spectroscopy with magnetic measurements, transmission electron microscopy, nitrogen adsorption, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma spectroscopy, thermo-gravimetric analysis and FTIR spectroscopy using pyridine as a probe molecule. Isomerization of α-pinene oxide over the Fe and Sn modified zeolite catalysts was carried out in the liquid phase using a batch-wise glass reactor. Formation of campholenic aldehyde and fencholenic aldehyde were observed to be influenced by the structure, acidity of zeolite and contents of Fe and Sn, reaction temperature and the catalysts pretreatment.

Hopping conductivity of Ni-doped p-CdSb

Mechanisms of the resistivity, ρ, of single crystal samples oriented along the [100] (No 1), [010] (No 2) and [001] (No 3) axes of anisotropic semiconductor p-CdSb doped with 2 at.% of Ni are investigated. In zero magnetic field the Mott type variable-range hopping (VRH) conductivity is observed in No 2 and the Shklovskii‐Efros type in No 1 and No 3 at T 2. 5K . The magnetoresistance (MR) of the samples obeys the law ln ρ ∼ B 2 up to B ∼ 6T . However, the temperature dependence of MR gives evidence for the Mott-VRH conductivity in No 1 at T 4.2 K and the nearest-neighbor hopping conductivity in No 2 between T = 3 and 4.2 K and in No 3 between 1.5 and 4.2 K. From the experimental data the values of the localization radius and dielectric permittivity and details of their critical behavior near the metal‐insulator transition, as well as the widths and the values of the density of the localized states, the acceptor energies, their concentrations and the anisotropy coefficients, are obtained. (Some figures in this article are in colour only in the electronic version)

Analysis of the lateral resolution of electrostatic force gradient microscopy

Signal measured by electrostatic force gradient microscopy (EFGM) is the z-gradient of the electrostatic force acting between the probe of an atomic force microscope (AFM) and the surface under study. A model is presented for calculating the z-gradient of the electrostatic interaction of the AFM probe with local charges in a dielectric layer at the surface. In the EFGM mode, the interaction of charges with only the probe tip apex should be taken into account. In this approach, a simplified expression can be derived for calculating the z-gradient of the electrostatic force. The model makes it possible to estimate the lateral resolution limit for EFGM imaging of individual charges and to simulate experimental EFGM images as a function of the tip-surface distance and the tip radius. The adequacy of the model was confirmed by quantitative simulation of the experimental EFGM images of local charges injected from the AFM probe into a planar array of Si nanocrystals in a thin SiO2 layer on a Si substrate.

Synthesis and magnetic properties of Mn-doped Cd0.1Zn0.9GeAs2 solid solutions

Homogeneous crystals of diluted magnetic semiconductor solid solution Zn0.9Cd0.1GeAs2 doped with 0, 1.13 and 2.65 mass% of Mn were synthesized. The grown crystals were characterized by atomic absorption and synchrotron x-ray powder diffraction analyses showing that the lattice parameter of the sample crystals are closely related to GaAs. Measurements of magnetic and transport properties were performed in the temperature range of 4–400 K using steady and pulsed magnetic fields up to B = 50 and 150 kG, respectively. The ferromagnetic Curie temperature of the sample crystals was found to be about 350 K. The magnetic ordering is attributed to the presence of MnAs clusters with mean size ~3.8 nm in the ferromagnetic phase. Influence of paramagnetic Mn2+ ions on magnetic properties of the crystals was observed only at low temperatures and was explained by the p–d interaction between charge-carrying holes and localized Mn moments.

The influence of Ni-rich nanoclusters on the anisotropic magnetic properties of CdSb doped with Ni

The magnetic properties of oriented CdSb single crystals doped with 2 at% of Ni are investigated. From measurements of magnetic irreversibility defined by deviation of the zero-field-cooled (ZFC) susceptibility from the field-cooled (FC) susceptibility, the value of the mean anisotropy field BK ~ 4 kG is obtained. The ZFC susceptibility displays a broad maximum at a blocking temperature, Tb, depending on B according to the law [Tb(B)/Tb(0)]1/2 = 1 ? B/BK with Tb(0) ~ 100 K. The field dependence of the magnetization exhibits saturation above ~20?30 kG with values of Ms different for B along the [1?0?0], [0?1?0] and [0?0?1] axes. The temperature dependence of Ms is weak, increasing slightly upon cooling the sample below ~100 K. The temperature dependence of the coercive field, Bc(T), is weak above Tb but is enhanced strongly with decreasing temperature below Tb. The anisotropy of Bc is inverted with respect to the anisotropy of Ms. Such behaviour can be attributed to spheroidal Ni-rich Ni1?xSbx nanoparticles with a high aspect ratio, broad size distribution and distribution of the orientation of the major axis around a preferred direction. The relation Bc BK and the anisotropies of Ms and Bc are consistent with reversal of the magnetization by the curling mode, whereas the Tb(B) dependence is typical of the coherent rotation mode. This difference is connected to the proximity of the average transversal cluster radius to a critical value for transition between the two magnetization reversal modes within a wide crossover interval, due to broad distribution of the cluster sizes.

Magnetic freezing near 200 K in the semimagnetic semiconductor (Cd1−xMnx)3As2

Abstract It is shown that below T f ≈ 200 K the d.c. susceptibility of the semimagnetic semiconductor (Cd 1- x Mn x ) 3 As 2 cooled in zero magnetic field differs from that observed after cooling the sample in a field. This phenomenon was found only in the range of H ≤ 300 Oe. We attribute it to freezing of magnetic clusters according to local anisotropy fields in the sample. The high value of the freezing temperature indicates relatively strong intracluster magnetic interactions.

Dilute magnetic semiconductor: Magnesium-doped Zn0.9Cd0.1GeAs2

Dilute magnetic semiconductors based on manganese-doped Zn0.9Cd0.1GeAs2 solid solution with various doping levels were synthesized. Their Curie point in 5-T magnetic field was 349 K. Ferromagnetic ordering in these semiconductors was due to MnAs nanoclusters, whose sizes were 3.7–3.8 nm.

The Hall effect and electron energy spectrum near the conduction band edge of n-CdSb in magnetic fields up to 25 T

The Hall effect in the II–V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to B = 25 T and temperatures between T = 2 and 77 K for samples oriented along the [0 1 0] and [0 0 1] crystallographic axes. The Hall coefficient, R (B, T), exhibits a sequence of an almost flat region followed by a descending interval and an upturn when B is increased. The decrease of R (B, T) is interpreted by the presence of two groups of electrons with concentrations n2(T) > n1(T) and mobilities μ2(T) < μ1(T). Analysis of n1(T) and n2(T) demonstrates that the high-mobility carriers n1 are the conduction band (CB) electrons, whereas the low-mobility carriers n2 are itinerant electrons of a lower resonant impurity band (IB), having at low T energies of Ei ~ 3–4 meV above the CB edge. In addition, near the CB edge lies a higher IB containing only localized electron states. The IBs are split by spin to states differing by an energy ΔEi ≈ 0.9 meV. The upturn of R (B, T) in the high-field region is explained by the redistribution of the electrons between the IBs due to the decrease of ΔEi when B is increased. The mobility of the CB electrons is determined presumably by strong anisotropic scattering on neutral impurity centres, accompanied at high T by isotropic scattering on acoustic phonons, whereas scattering from ionized impurities is small.

The Hall effect in Ni-doped p-CdSb in a strong magnetic field

The Hall effect in single crystals of the group II–V semiconductor p-CdSb doped with 2 at% of Ni is investigated between T = 1.5 and 300 K in pulsed magnetic fields up to B = 25 T. The Hall resistivity, ρH, exhibits a nonlinear dependence on B, which is strongly pronounced below ~10 K but is still observed even up to 300 K. The analysis of ρH(B) gives evidence for the presence of a positive normal and a negative anomalous contribution, ρN = R0B and ρA, respectively. The temperature dependence of the (normal) Hall coefficient R0 is determined by the activation of holes into the valence band with a small contribution of the itinerant holes from the acceptor band at lowest T. The dependence of ρA on T is quite different within two temperature intervals, being weak between ~50 and 300 K and very strong below ~50 K, the latter resembling that of the resistivity, ρ. Analysis of ρA below ~77 K demonstrates that it scales approximately as ρn with n = 1.6 ± 0.1 within four decades of ρA and more than two decades of ρ. The anomalous Hall effect in p-CdSb:Ni is attributable to the presence of magnetic Ni-rich nanoclusters, whose properties have previously been investigated by the analysis of the magnetization data.

Influence of the magnetic field on the conductivity within the Coulomb gap of n-ZnSe single crystals doped with Ag

The conductivity and magnetoresistance of undoped and Ag-doped n-ZnSe single crystals are investigated in pulsed magnetic fields up to 20 T at temperatures between 1.5 and 300 K. In fields below 10 T, the resistivity is found to obey the temperature dependence ρ(T) = ρ0exp{T0/T}1/2 indicating variable range hopping (VRH) conductivity over the states of the Coulomb gap in the vicinity of the Fermi level. In low fields, a region of negative magnetoresistivity (NMR) is observed and attributed to the increasing density of states around the Fermi level in a magnetic field. As the magnetic field increases, NMR is gradually cancelled by exponentially increasing the contribution of positive magnetoresistivity (PMR) caused by shrinking of the impurity electron wavefunctions. Values of the localization radius of the electron wavefunction, a, and the dielectric constant, κ, are obtained from analysis of the PMR data. Both a and κ increase when approaching the metal–insulator (MI) transition, at which the ratio of the critical indices νκ/νa = 3.5 is found.