S.A. Gudkova

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.

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.