Magnetic Field Influence on the Microwave Characteristics of Composite Samples Based on Polycrystalline Y-Type Hexaferrite.

Here, we report results on the magnetic and microwave properties of polycrystalline Y-type hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave-absorbing material. The reflection losses in the 1-20 GHz range of the Y-type hexaferrite powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in the absence and in the presence of a magnetic field. A permanent magnet was used with a strength of 1.4 T, with the magnetic force lines oriented perpendicularly to the direction of the electromagnetic wave propagation. In the case of using an external magnetic field, an extraordinary result was observed. The microwave reflection losses reached a maximum value of 35.4 dB at 5.6 GHz in the Ku-band without a magnetic field and a maximum value of 21.4 dB at 8.2 GHz with the external magnetic field applied. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the decrease of the reflected wave attenuation. At a fixed thickness, tm, of the composite, the attenuation peak frequency can be adjusted to a certain value either by changing the filling density or by applying an external magnetic field.

Structural, Magnetic and Microwave Characterization of Polycrystalline Z-Type Sr3Co2Fe24O41 Hexaferrite.

We report results on the structural and microwave properties and magnetic phase transitions in polycrystalline Sr3Co2Fe24O41 hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave absorbing material. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves revealed a change in the magnetization behavior at 293 K. The reflection losses in the 1-20 GHz range of the Sr3Co2Fe24O41 powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in both the absence and presence of a magnetic field. In the latter case, a dramatic rise in the attenuation was observed. The microwave reflection losses reached the maximum value of 32.63 dB at 17.29 GHz in the Ku-band. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the appearance of new reflection losses maxima. At a fixed thickness tm of the composite, the attenuation peak frequency can be adjusted to a certain value either by changing the filling density or by applying an external magnetic field.

Data supporting the results of the characterization of the phases and structures appearing during the synthesis process of Ba0.5Sr1.5Zn2-xNixFe12O22 by auto-combustion.

The data presented has to do with identifying the various phases arising during the synthesis of the Y-type hexaferrite series Ba0.5Sr1.5Zn2-xNixFe12O22 by auto-combustion that we deem important for their microstructural and magnetic properties. The data and the related analyses support the research paper "Ni-substitution effect on the properties of Ba0.5Sr1.5Zn2-xNixFe12O22 powders" [1]. Thus, the parameters are presented of the phases appearing after auto-combustion and after the initial annealing at 800 °C, namely, crystal cell and crystallite size. Also, additional data are provided obtained by EDS concerning the Ba:Sr:Zn:Ni:Fe ratio in Ba0.5Sr1.5Zn2-xNixFe12O22 (x = 0.8, 1, 1.5) samples synthesized at 1170 °C for 10 h. The data can be used as a reference in establishing how the phases distinguished during the initial process of auto-combustion affect the Ba0.5Sr1.5Zn2-xNixFe12O22 powders, which are candidates for room-temperature multiferroic materials. The data have not been published previously and are made available to permit critical or further analyses.

Study of the Structural and Magnetic Properties of Co-Substituted Ba2Mg2Fe12O22 Hexaferrites Synthesized by Sonochemical Co-Precipitation.

Ba2Mg0.4Co1.6Fe12O22 was prepared in powder form by sonochemical co-precipitation and examined by X-ray diffraction, Mössbauer spectroscopy and magnetization measurements. Careful XRD data analyses revealed the Y-type hexaferrite structure as an almost pure phase with a very small amount of CoFe2O4 as an impurity phase (about 1.4%). No substantial changes were observed in the unit cell parameters of Ba2Mg0.4Co1.6Fe12O22 in comparison with the unsubstituted compound. The Mössbauer parameters for Ba2Mg0.4Co1.6Fe12O22 were close to those previously found (within the limits of uncertainty) for undoped Ba2Mg2Fe12O22. Isomer shifts (0.27-0.38 mm/s) typical for high-spin Fe3+ in various environments were evaluated and no ferrous Fe2+ form was observed. However, despite the indicated lack of changes in the iron oxidation state, the cationic substitution resulted in a significant increase in the magnetization and in a modification of the thermomagnetic curves. The magnetization values at 50 kOe were 34.5 emu/g at 4.2 K and 30.5 emu/g at 300 K. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were measured in magnetic fields of 50 Oe, 100 Oe, 500 Oe and 1000 Oe, and revealed the presence of two magnetic phase transitions. Both transitions are shifted to higher temperatures compared to the undoped compound, while the ferrimagnetic arrangement at room temperature is transformed to a helical spin order at about 195 K, which is considered to be a prerequisite for the material to exhibit multiferroic properties.

Microwave properties of polymer composites containing combinations of micro- and nano-sized magnetic fillers.

We investigated the microwave absorbing properties of composite bulk samples with nanostructured and micron-sized fillers. As magnetic fillers we used magnetite powder (Fe3O4 with low magnetocrystalline anisotropy) and strontium hexaferrite (SrFe12O9 with high magnetocrystalline anisotropy). The dielectric matrix consisted of silicone rubber. The average particle size was 30 nm for the magnetite powder and 6 micro/m for the strontium hexaferrite powder. The micron-sized SrFe12O19 powder was prepared using a solid-state reaction. We investigated the influence of the filler concentration and the filler ratio (Fe3O4/SrFe12O19) in the polymer matrix on the microwave absorption in a large frequency range (1 / 18 GHz). The results obtained showed that the highly anisotropic particles become centers of clusterification and the small magnetite particles form magnetic balls with different diameter depending on the concentration. The effect of adding micron-sized SrFe12O19 to the nanosized Fe3O4 filler in composites absorbing structures has to do with the ferromagnetic resonance (FMR) shifting to the higher frequencies due to the changes in the ferrite filler's properties induced by the presence of a magnetic material with high magnetocrystalline anisotropy. The two-component filler possesses new values of the saturation magnetization and of the anisotropy constant, differing from those of both SrFe12O1919 and Fe3O4, which leads to a rise in the effective anisotropy field. The results demonstrate the possibility to vary the composite's absorption characteristics in a controlled manner by way of introducing a second magnetic material.