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Abstract: A series of nano-sized single phase W-type SrGa x Zn2Fe16−x O27 (x = 0.0, 0.1, 0.2, 0.3, 0.4) hexaferrites prepared by sol gel technique and sintered at 950 °C have been investigated. The thermal decomposition behavior of nitrate–citrate gel of as prepared powder was investigated by means of DTA/TGA analysis. The sintered powders were characterized by FTIR, XRD, SEM, VSM and vector network analyzer (VNA). X-ray diffraction patterns for pure and substituted W-type hexaferrites show the single phase structure with no impurity phase. The lattice parameters (a and c) decrease with the increase of Ga contents (x). The grain size estimated from SEM images is in the range of 139–76 nm which confirms the nanocrystalline nature of the investigated samples. The saturation magnetization (M s) decreases whereas coercivity (H c) increases with the increase of Ga contents (x). The values of H c for all of the investigated samples lie in the range of few hundred oersteds which is one of the necessary conditions for EM materials. The microwave absorption property is enhanced in the frequency (with respect to −20 dB) from 0.5 to 13 GHz, and the bandwidth reaches 0.899 GHz. The attenuation peak value is −32 dB at the matching thickness of 3.4 mm. [Copyright &y& Elsevier]

The effect of rare earth elements substitution in Sr 1.96 RE 0.04 Co 2 Fe 27.80 Mn 0.2 O 46 (RE=Ce, Gd, Nd, La and Sm) X-type hexagonal ferrites prepared by using sol gel autocombustion method was studied. The XRD and FTIR analysis show the single phase of the prepared material. The lattice constants a (Å) and c (Å) varies with the additives. The particle size measured by Scherer formula for all the samples varies in the range of 54–100 nm and confirmed by the TEM analysis. The average grain size measured by SEM analysis lies in the range of 0.672–1.01 µm for all the samples. The Gd-substituted ferrite has higher value of coercivity (526.06 G) among all the samples which could be a good material for longitudinal recording media. The results also indicate that the Gd-substituted sample has maximum reflection loss of −25.2 dB at 11.878 GHz, can exhibit the best microwave absorption properties among all the substituted samples. Furthermore, the minimum value of reflection loss shifts towards the lower and higher frequencies with the substitution of rare earth elements which confirms that the microwave absorption properties can be tuned with the substitution of rare earth elements in pure ferrites. The peak value of attenuation constant at higher frequency agrees well the reflection loss data. [ABSTRACT FROM AUTHOR]

In order to understand the substitutional effect of rare earth element Sm 3+ and divalent Mn 2+ on structural, magnetic and microwave absorption properties of hexagonal ferrites, a series of Sr 2− x Sm x Ni 2 Fe 28−y Mn y O 46 X-type hexagonal ferrites with concentration ( x =0.00, 0.02, 0.04, 0.06, 0.08, 0.10 and y =0, 0.1, 0.2, 0.3, 0.4, 0.5) was synthesized by the sol–gel method. The XRD analysis shows that the material crystallized into single X-type hexagonal phase. The absorption bands at low wave number in FTIR curves are the characteristics of the X-type hexagonal ferrites. Decreasing trend in the magnetic properties with the substitution of Sm–Mn contents was also observed, which may be attributed to the oxidation of Mn 2+ ions into Mn 3+ ions and spin canting effect of rare earth element Sm 3+ . The reflection loss peak shifted towards the low frequency and microwave absorption properties of the material enhanced with the substitution of Sm–Mn contents which reflects its applications in super high frequency (SHF) devices. The attenuation constant curves are in good agreement with the reflection loss peak. [ABSTRACT FROM AUTHOR]

Highlights: [•] A series of X-type hexagonal ferrites were prepared by sol–gel method. [•] The XRD analysis showed that the X-type hexagonal structure. [•] The c/a ratio of these samples falls in the range of X-type hexagonal ferrites. [•] The magnetic properties enhances with the increase of Nd–Co contents. [•] The samples show the reflection loss at smaller frequencies. [Copyright &y& Elsevier]

Abstract: The magnetic and microwave characterization of single phase hexaferrites of entirely new composition Ba1-x Sr x Co2AlFe15O27 (x=0.2–1.0) for application in a microwave absorber, have been reported. The samples synthesized by sol–gel method were investigated by differential thermal analyzer, Fourier transform infrared spectroscope, X-ray diffractometer, field emission gun scanning electron microscope, vibrating sample magnetometer and vector network analyzer. Platelet grains exhibit well defined hexagonal shape which is a better shape for microwave absorption. M–H loops for a selected sample were measured for a temperature range of 4.2–400K. Moreover M–H loops for all Sr-substituted samples were also measured at room temperature up to a maximum applied field of 9T. Saturation magnetization values were calculated by the law of approach to saturation. The room temperature coercivity for all the samples is found to be a few hundred oersteds which is necessary for electromagnetic materials and makes these ferrites ideal for microwave devices, security, switching and sensing applications. The complex permittivity, permeability and reflection losses of a selected ferrite–epoxy composite were also investigated over a frequency range of 0.5–13GHz. [Copyright &y& Elsevier]

Abstract: A series of single phase W-type Sr3−x Ce x Fe16O27 (x=0, 0.02, 0.04, 0.06, 0.08, 0.10) hexagonal ferrites prepared by the Sol–Gel method was sintered at 1050°C for 5h. The X-ray diffraction analysis reveals that all the samples belong to the family of W-type hexagonal ferrites. The c/a ratio falls in the range of W-type hexagonal ferrites. The grain size was measured by SEM varies from 0.7684 to 0.4366μm which shows that the Ce3+ substituted samples have smaller grain size than pure ferrite Sr3Fe16O27 which results from the difference in ionic radii of Ce3+ (1.034Å) and Sr2+ (1.12Å). The room temperature resistivity of the present samples varies from 6.5×108 to 272×108 Ω-cm. The coercivity increases from 1370 to 1993Oe which is consistent with the decrease in grain size. The coercivity values indicate that the present samples fall in the range of hard ferrites. The large value of H c may be due to domain wall pinning at the grain boundaries. [Copyright &y& Elsevier]

Abstract: A series of BaHo x Fe16−x O27 (x =0.0, 0.2, 0.4, 0.6, 0.8, 1.0) W-type hexagonal ferrites were prepared by co-precipitation technique at high annealing temperature of 1320°C. XRD reveals single W-type hexagonal phase in these ferrites. The grain size is measured by SEM analysis using line intercept method. Saturation magnetization, retentivity and coercivity were measured from MH-loops taken on VSM. It was observed that magnetization increases with the increase of Ho content due to difference in ionic radii of Ho3+ (0.901Å) and Fe3+ (0.67Å) ions. Room temperature dc resistivity increases as a function of Ho3+ that may be due to separation between grains. The dc electrical resistivity decreases as a function of temperature which indicates the semi-conducting behavior of the samples. [Copyright &y& Elsevier]