D.A. Vinnik

Growing Doped Barium Ferrite Single Crystals Using the Flux Method

Substances exhibiting ferromagnetic properties areused in inductors, devices for recording and reproduction of sound, and information storage devices, as wellas in electronic and electrical appliances.For solving many technical problems, ferromagnets with rather high Curie temperatures (350°С) areof crucial importance. This property is inherent in barium ferrite (BaFe

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe12–xMnxO19

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe12-xMnxO19 was observed by soft X-ray absorption spectroscopy at the Mn-L2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron(III) compound BaFe12O19 is made possible by the fact that the charge disproportionation of Mn3+ into Mn2+ and Mn4+ requires less energy than that of Fe3+ into Fe2+ and Fe4+, related to the smaller effective Coulomb interaction of Mn3+ (d4) compared to Fe3+ (d5). The different chemical environments determine the location of the differently charged ions: with Mn3+ occupying positions with (distorted) octahedral local symmetry, Mn4+ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn2+ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L2,3 XAS spectra of two different samples with x = 1.5 and x = 1.7 led to Mn2+:Mn3+:Mn4+ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.

Anomalies in growth of electrodeposited Ni–Fe nanogranular films

Thin Ni–Fe films were produced via electrodeposition onto silicon substrates using direct current and pulse current (with different pulse durations) regimes. The correlation between the technological regimes, chemical composition and microstructure of Ni–Fe nanogranular films was investigated. Analysis of the microstructure evolution revealed that the mechanism of film growth changes from Volmer–Weber (island film growth) to Stranski–Krastanov (layer by layer growth) with the shortening of the pulse duration below 10 μs. This anomalous behavior was explained by the increase of the binding energy between the initial film atoms and the substrate surface. The results showed that using electrodeposition regimes, which ensure growth with an average crystal size less than the critical value of 10 nm, provides less roughness and defectiveness and better film uniformity in thickness.

Growth of Lead and Aluminum Substituted Barium Hexaferrite Single Crystals from Lead Oxide Flux

Lead and aluminum substituted barium hexaferrite (Ba,Pb)Fe12–хAlхO19 single crystals were grown from lead oxide flux at 1260 °C as hexagonal plates with sizes of about 2 mm. A maximum substitution level of x = 4.82 was achieved for the first time for bulk single crystals. The variation of the unit cell parameters with Al content is in good agreement with literature data on exclusively Al substituted barium hexaferrite, while the substitution of Ba by Pb has hardly any influence. Similarly, Pb has only a negligible influence on the magnetic properties, while Al substitution significantly reduces the saturation magnetization in a very similar manner as known from Pb-free barium hexaferrite.

Magnetic and Structural Properties of Barium Hexaferrite BaFe12O19 from Various Growth Techniques

Barium hexaferrite powder samples with grains in the μm-range were obtained from solid-state sintering, and crystals with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes, respectively. Carbonate and borate fluxes provide the largest and structurally best crystals at significantly lower growth temperatures of 1533 K compared to flux-free synthesis (1623 K). The maximum synthesis temperature can be further reduced by the application of PbO-containing fluxes (down to 1223 K upon use of 80 at % PbO), however, Pb-substituted crystals Ba1–xPbxFe12O19 with Pb contents in the range of 0.23(2) ≤ x ≤ 0.80(2) form, depending on growth temperature and flux PbO content. The degree of Pb-substitution has only a minor influence on unit cell and magnetic parameters, although the values for Curie temperature, saturation magnetization, as well as the coercivity of these samples are significantly reduced in comparison with those from samples obtained from the other fluxes. Due to the lowest level of impurities, the samples from carbonate flux show superior quality compared to materials obtained using other methods.

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.

Structure and adsorption properties of microporous glassy-carbon materials

Products of the calcination of polymer based on furan resin modified by surface-active agent and diluter are studied. Microporous glassy-carbon materials with specific areas of up to 1350 m2/g are obtained. The materials are studied by means of electron microscopy, and their specific surface areas are compared to their structures. The range of solution compositions in which bicontinual structures are formed as three-dimensional networks from polymer drops is determined; the drop size varies from 0.3 to 1.5 μm.

Distribution of dopant metals between PbTiO3 crystals and PbO-B2O3 flux

Single crystals of lead titanate PbTiO3 doped with silicon, calcium, chromium, manganese, cobalt, nickel, copper, zinc, and cadmium were grown. The compositions and crystallographic parameters of the crystals were studied. The lowest distribution coefficients of dopants between PbTiO3 crystals and flux were observed with Mn+2 and Co+2 and the highest, with Ca+2. Doping with niobium leads to the formation of solid solutions with the pyrochlore structure A2B2O7 and even higher distribution coefficient. A correlation was found between dopant concentrations and crystal cell parameters.

Lead Germanate Pb5Ge3O11 Crystal Growth

Single crystals of lead germanate are obtained by the Czochralski technique. The dimensions of hexagonally shaped crystals are up to 25 × 25 × 5 mm3. According to X-ray diffraction measurements the grown crystals respond to lead germanate Pb5Ge3O11 structure. Curie temperature was also measured.

The Thermal Expansion of Solid State BaFe12O19 and Flux Ba0.8Pb0.2Fe12O19 Pellets

Solid state sintered pellets of barium hexaferrite were performed. The Curie temperature of BaFe12O19 pellets were measured with differential scanning colorimetry (DSC). The coefficient of thermal expansion (CTE) of pellets was defined above and below the Curie temperature. The Curie temperature calculated basedon the dilatometry is are in good agreement with the DSC measurements.