I. Nedkov

Superparamagnetic relaxation in CuxFe3-xO4 (x = 0.5 and x = 1) nanoparticles

Abstract The scope of this article is to report very detailed results of the measurements of magnetic relaxation phenomena in the new Cu 0.5 Fe 2.5 O 4 nanoparticles and known CuFe 2 O 4 nanoparticles. The size of synthesized particles is (6.5±1.5) nm. Both samples show the superparamagnetic behaviour, with the well-defined phenomena of the blocking of magnetic moment. This includes the splitting of zero-field-cooled and field-cooled magnetic moment curves, dynamical hysteresis, slow quasi-logarithmic relaxation of magnetic moment below blocking temperature. The scaling of the magnetic moment relaxation data at different temperatures confirms the applicability of the simple thermal relaxation model. The two copper-ferrites with similar structures show significantly different magnetic anisotropy density and other magnetic properties. Investigated systems exhibit the consistency of all obtained results.

Microstructure and Magnetic Behaviour of Nanosized Fe3O4 Powders and Polycrystalline Films

Summary. The object of investigation were the magnetic interactions in nanostructured Fe3O4 assemblies of two kinds (powder and film) where particles of similar size present nearly uniform domains in a close to planar arrangement with spacings sufficient for magnetic interactions. We discuss the use of the soft-chemistry method, i.e. the modified ‘ferrite plating’ (MFP) technique, for the synthesis of polycrystalline films of magnetite with nanosized crystallites.

Structural and Magnetic Properties of Nanosized Barium Hexaferrite Powders Obtained by Microemulsion Technique

Thin hexagonal barium hexaferrite particles synthesized using the microemulsion technique were studied. A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and an aqueous phase were used. The microstructural and magnetic properties were investigated. The particles obtained were mono-domain with average particle size 280 nm. The magnetic properties of the powder were investigated at 4.2 K and at room temperature. The saturation magnetization was 48.86 emu/g and the coercivity, 2.4 x 105 A/m at room temperature. The anisotropy field Ha and magneto-crystalline anisotropy K1 were 1.4 x 106 A/m and 2.37 x 105 J/m3, respectively.

Investigation of iron-containing products from natural and laboratory cultivated Sphaerotilus-Leptothrix bacteria

Bacterial biomass collected from sheath-forming bacteria of the genera Sphaerotilus and Leptothrix was collected from a high-mountain natural stream water source. The elemental constitution and oxide phases of the products after selective cultivation of the bacteria on two different elective media using neutron activation analysis (NAA), electron microscopy (SEM, TEM), and X-ray diffraction (XRD) were studied. A high enrichment level of iron was revealed by the NAA technique in cultivated isolates as compared to the reference sample from nature. Three types of iron oxide compounds were established after cultivation in Adler’s medium: lepidocrocite (γ-FeOOH), magnetite (Fe3O4), and goethite (α-FeOOH). The cultivation in the Isolation medium yielded a single phase, that of goethite, excluding one sample with a distinguishable amount of lepidocrocite. XRD and EM investigations show that the biogenic oxides are nanosized. Our study exemplifies the possibilities of the biotechnology approach for obtaining, under artificial conditions, large quantities of iron-containing by-products that could be of further used in appropriate nano- and biotechnologies.

The Influence of the Polycrystalline State and Partial Dy Substitution on the Superconducting Properties of YBCO

The influence of the quasi-equal partial substitution of non-magnetic (Y 3+ )/magnetic (Dy 3+ ) ions in YBa 2 Cu 3 O 7-y on the superconducting properties of the material has been investigated. To minimize the influence of the stress field induced by lattice mismatch, Dy-substituted YBa 2 Cu 3 O 7-y was studied because Y 3+ and Dy 3+ ions have similar ionic radii. The investigation was performed on polycrystalline and powder samples with general formula Dy x Y 1-x Ba 2 Cu 3 O 7-y (x = 0.4-0.6). Higher magnetic losses and intragranular critical current density J c were observed for partially substituted samples in comparison with DyBCO. The strongest influence of magnetic substitution on the electrical resistivity properties was observed in the mixed state.

Biogenic oxides from neutrophilic iron bacteria and possibilities for application in the nanotechnology

The aim of this study is to obtain and characterize the ferric oxides/(oxy)hydroxides formed after cultivation of bacteria under laboratory conditions. The pure cultures of these bacteria isolated from natural habitats are identified by the methods of classical and molecular taxonomy as strains of the Leptothrix genus. Adler (AM) and Silicon iron glucose peptone (SIGP) media are the most appropriate ones for obtaining the iron oxides. The characterization of the oxides and sheaths is performed by different physical methods. The sheaths are formed in a SIGP medium. Light micrograph images and SEM revealed the average size and diameter of the sheaths. The XRD measurements showed the composition of the oxides obtained, as well as the average size of the iron particles (up to 30 nm). The TEM micrographs showed the shape of the biogenic nanoparticles, while the magnetic measurements demonstrated the superparamagnetic character of the magnetic part of the biomaterials. The new biogenic materials are promising for application in magneto electronic for building biosensors.

Magnetic properties of nanosized MgFe2O4 powders prepared by auto-combustion

Targets were prepared to be used for magnetron sputtering and laser ablation and their microstructural and magnetic properties were investigated. The base material was nanosized MgFe2O4 powder produced by citrate auto-combustion synthesis. The auto-combusted powders were annealed at temperatures in the range 600 - 1000 o

Structural and Magnetic Properties and Preparation Techniques of Nanosized M-type Hexaferrite Powders

In recent years, the scientific efforts of a large number of research teams have been concentrating on developing, exploring and applying nanosized magnetic ferroxides. In this review, we consider the fundamental structural and magnetic characteristics of nanosized particles of barium hexaferrite. We discuss in some detail the most common techniques for preparation of nanosized ferroxide powders. Finally, we present original results on applying a promising chemical technique, namely, the single microemulsion technique, for the synthesis of barium hexaferrite powders consisting of homogeneous in shape and size particles.

Nanosized Barium Hexaferrite Powders Obtained by a Single Microemulsion Technique

Barium hexaferrite (BaFe12O19) powders of particle size of 130 and 180 nm were synthesized by a single microemulsion technique. The influence of the concentration of Ba2+ and Fe3+ metallic ions in the aqueous phase in the microemulsion system on the particle size distribution, crystallinity and magnetic properties of BaFe12O19 was studied. The coercive force and saturation magnetization of the sample obtained at a lower concentration of metallic cations in the aqueous phase were higher than those of the sample obtained at higher concentration.

Magnetoelastic and total anisotropy effects on the domain-wall dynamics in the polycrystalline state

This work investigates the domain-wall (DW) mobility due to magnetoelastic interaction. Polycrystalline samples with high grain-size homogeneity were prepared based on the general formula Y3Fe5-xMnxO12 (0 < x < 0.28). A series of samples was especially selected where the longitudinal magnetostriction changed from −2.14 × 10−6 to +7.65 × 10−6 at 300 K, while the single-crystal crystalline anisotropy constant K1 remained negative. The effects of the magnetostriction and the total anisotropy K on the complex magnetic permeability spectrum μ∗(f) were investigated in a broad frequency range (0.3–3 GHz). Two characteristic regions of variation of the real (μ′) and the imaginary (μ″) parts of the complex permeability as functions of λs were established. Near λs = 0, a resonance-type spectrum was observed. The model of ‘diametrical spherical DW’ is inapplicable for λs ⪢ 0 - the DW dynamics is then influenced exclusively by the stress anisotropy and by the strong isotropic exchange stress arising from the anisotropic magnetostrictive deformation constrained by the neighboring grains. The significant decrease of the initial permeability in this region can be explained by the rise in the concentration of DW not having a 180° orientation.