Svetoslav Kolev

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.

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

Preparation and characterisation of magnetically ordered columnar structures of barium ferrite particles

In this study, we report on the columnar structures of barium ferrite particles that were prepared from water suspensions by applying a magnetic field during drying. Commercial barium ferrite (BaFe12O19) monodomain particles were used, and the surfaces of the particles were treated with an organic surfactant to reduce their agglomeration. The columnar structures were obtained by drying the water suspensions of BaFe12O19 particles on Al2O3 substrates under an applied magnetic field of 1 T. After the degradation of the organic components, the samples were sintered at 1350°C. X-ray powder diffraction, scanning electron microscopy and magnetic measurements were used to characterise the samples. When the magnetic field was applied perpendicularly to the sample plane, the as-deposited sample exhibited a higher coercivity (H c = 5434 Oe) and a higher squareness ratio (SQR = 0.76) than the sintered sample (H c = 625 Oe, SQR = 0.58). However, the sintered sample showed a higher anisotropy of the magnetic behaviour than the as-deposited sample.

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.

Influence of the preparation methods on the structure and magnetic properties of nanosized Al-substituted barium hexaferrite powders

We report studies on the correlation between the method of preparation, microstructure and magnetic properties of nanosized monodomain Al-substituted barium hexaferrite (BaAlFe11O19) powders. The powders were obtained by the co-precipitation and single microemulsion methods. The particles in the samples had a size between 80 nm and 135 nm depending on the synthesis conditions. The value of the saturation magnetization Ms measured was very high, namely, 66.12 emu/g. The hysteresis loop was very narrow, with the coercivity Hc being 163 Oe, which indicated that the particles were in a near-superparamagnetic state.

Study of quasi-monophase Y-type hexaferrite Ba2Mg2Fe12O22 powder

We present the structural and magnetic properties of a multiferroic Ba2Mg2Fe12O22 hexaferrite composite containing a small amount of MgFe2O4. The composite material was obtained by auto-combustion synthesis and, alternatively, by co-precipitation. The Ba2Mg2Fe12O22 particles obtained by co-precipitation have an almost perfect hexagonal shape in contrast with those prepared by auto-combustion. Two magnetic phase transitions, responsible for the composites multiferroic properties, were observed, namely, at 183 K and 40 K for the material produced by auto-combustion, and at 196 K and 30 K for the sample prepared by co-precipitation. No magnetic phase transitions in these temperature ranges were observed for a MgFe2O4 sample, which shows that the magnesium ferrite does not affect the multiferroic properties of this type of multiferroic metarials.

A Comparative Study of the Morphology of Y-Type Hexaferrite Powders Obtained by Sol-Gel Auto-Combustion and Ultrasonic Co-precipitation

In recent years, the Y-type hexaferrites have been the object of extensive research related to studying the magnetoelectric effect in the microwave frequency range that is typical for these materials and determines their potential use as microwave absorbers. The morphology of the investigated powders used as fillers in microwave absorbers considerably affects their properties. We present a study on the influence of the preparation technique on the morphology of Ba0.5Sr1.5Zn2Fe12O22 and Ba0.5Sr1.5Zn2Al0.08Fe11.92O22 powders. The powders were synthesized by acetic-acid sol-gel auto-combustion and ultrasonic co-precipitation. The particles of the samples obtained by ultrasonic co-precipitation had an almost perfect hexagonal shape typical for hexaferrites. Al-substitution in Ba0.5Sr1.5Zn2Fe12O22 (Ba0.5Sr1.5Zn2Al0.08Fe11.92O22) resulted in lowering the synthesis temperature.

Nanosized Ferrite Materials for Absorption of and Protection from MW Radiation

Ferrite materials suitable for mm-wave circulators should exhibit ferromagnetic resonance (FMR) in the mm range with high a saturation magnetization. In addition, the magnetic and dielectric losses and the temperature dependence of the magnetocrystalline anisotropy and of the saturation magnetization should be as low as possible. Therefore, compositionally and microstructurally homogeneous materials are required. The object of the work presented was to investigate the microwave (MW) absorbing properties of nanocomposite bulk samples. As a filler we used magnetite (Fe3O4) with different particle sizes in a silicone rubber matrix and investigated the influence of the filler concentration and particle size in the polymer matrix on the MW nonlinearity in a large frequency range (1–20 GHz). We found that the intensity and the frequency at the reflection loss minimum depend on the particle size and particle concentration of magnetite in the samples.