Chavdar Ghelev

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.

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.

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.

Hexaferrite multiferroics: From bulk to thick films

We report studies of the structural and microstructural properties of Sr3Co2Fe24O41 in bulk form and as thick films. The precursor powders for the bulk form were prepared following the sol-gel auto-combustion method. The prepared pellets were synthesized at 1200 °C to produce Sr3Co2Fe24O41. The XRD spectra of the bulks showed the characteristic peaks corresponding to the Z-type hexaferrite structure as a main phase and second phases of CoFe2O4 and Sr3Fe2O7-x. The microstructure analysis of the cross-section of the bulk pellets revealed a hexagonal sheet structure. Large areas were observed of packages of hexagonal sheets where the separate hexagonal particles were ordered along the c axis. Sr3Co2Fe24O41 thick films were deposited from a suspension containing the Sr3Co2Fe24O41 powder. The microstructural analysis of the thick films showed that the particles had the perfect hexagonal shape typical for hexaferrites.

Characterization of Y-type hexaferrite Ba 2 Mg 2 Fe 12 O 22 powders

Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf> is a Y-type hexagonal ferrite known for a relatively high temperature (∼200 K) of the magnetic transition to a spiral spin arrangement and an easy magnetization axis lying in a plane perpendicular to the c crystal axis. The multiferroicity exists in the absence of an external magnetic field H; a longitudinal-conical spin arrangement sets below 50 K; and the direction of the electric polarization P can be governed by relatively low magnetic fields (<0.02 T). The structural, magnetic and ferroelectric parameters have been mostly studied on monocrystals and, in spite of the attractive possibilities of designing promising composite multiferroic structures, the information on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders is scarce. We report on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders studied by X-ray and neutron powder diffraction, and magnetometry. The samples were prepared by two different techniques: sonochemical co-precipitation and sol-gel auto-combustion. The structures retained the rhombohedral crystal symmetry, but mixed occupancies of the 14 (Mg, Fe) cation sites was a new feature. The magnetizations at a magnetic field of 60 kOe at 300 K were 22.78 emu/g for the sample prepared by sol-gel auto-combustion, and 24.95 and 25.06 emu/g for the samples obtained by sonochemical co-precipitation and annealed at 1170 °C and 1200 °C, respectively.