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Holmium (Ho 3+ ) substituted W-type hexagonal ferrites (BaCo 2 Fe 16 O 27 ) were prepared by sol gel auto-combustion technique. The precursors were prepared by Ba 2+ , Fe 3+ nitrates solutions with citric acid as chelating agent. The prepared powder was sintered at 950 °C for 8 h to produce a single phase of W-type hexa ferrite with corresponding crystallite size ranging from 34 nm to 44 nm. The thermal decomposition of nitrate-citrate gel of as-prepared powder was investigated by TGA and DTA. The FTIR spectral study reveals the presence of two absorption bands which are characteristics of hexaferrites. DC electrical resistivity increases from 3.01×10 9 to 5.69×10 10 (ohm-cm). The real and imaginary parts of dielectric constant (ɛʹ, ϵ״) and dielectric loss tangent (tan δ) are determined in the frequency range of 20 Hz–1 MHz. It was observed that both real and imaginary parts of the dielectric constant and tanδ decrease with the increase of frequency on the basis of Wagner and Koop's theory. [ABSTRACT FROM AUTHOR]

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]

Highlights: [•] Sr-substituted Y-type hexaferrites synthesized by sol–gel method have been investigated. [•] Platelet grains with well defined hexagonal shape are suitable for microwave absorbers. [•] Saturation magnetization values were calculated by the law of approach to saturation. [•] Coercivity of a few hundred oersteds found for all samples is suitable for EM materials. [ABSTRACT FROM AUTHOR]

Highlights: [•] Al-substituted W-type hexaferrites with formula BaCo2Al x Fe16−xO27 were investigated. [•] DC resistivity increased with increasing Al contents due to unavailability of Fe3+ ions. [•] The increased value of resistivity is favorable for microwave absorption. [•] The coercivity (Hc ) values for all the samples lie in the range of a few hundred oersteds. [•] The low value of Hc is suitable for EM materials, switching and sensing applications. [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: A series of single phased Y-type hexagonal ferrites Sr2Ni2−x Mg x Fe12O22 (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by the sol–gel auto combustion method. The effects on structural, magnetic and electrical properties have been investigated by substituting Mg2+ at Ni2+ sites. The X-ray diffraction (XRD) patterns confirm single phase Y-type hexaferrite and various parameters such as lattice constants, cell volume, X-ray density, bulk density and porosity have been calculated from XRD data. The Fourier transform infrared (FTIR) spectra show the characteristics absorption ferrite peaks of the sintered sample. The microstructure was studied by scanning electron microscopy (SEM). All the ferrites show a hexagonal platelet-like shape which is a most suitable shape for microwave absorption. The dielectric constant followed the Maxwell–Wagner interfacial polarization and relaxation peaks were observed in the dielectric loss properties. The room temperature dc electrical resistivity and activation energy were found to decrease for samples x=0.1, 0.2 and increase for the rest of samples hence making these materials suitable for multilayer chip inductors (MLCIs). A soft magnetic behavior was revealed by M–H loops. Saturation magnetization (M s), retentivity (M r), coercivity (H c) and magnetic moment (n B) were found to decrease as the Mg2+ contents increased. [Copyright &y& Elsevier]

Abstract: A series of Al-substituted Co2W hexaferrites BaCo2Al x Fe16 − x O27 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) synthesized by sol-gel autocombustion method have been investigated. The samples were characterized by DTA/TGA analysis, FTIR spectroscopy, X-ray Diffractometer and field emission gun scanning electron microscopy (FEGSEM). Magnetic behavior of a selected sample was examined up to a magnetic field of 9T between a temperature range of 4.2 K–400 K. The values of saturation magnetization were calculated by the law of approach to saturation. The coercivity of the investigated sample has a value of about hundred oersteds at room temperature which is favorable for EM materials. The complex permittivity and complex permeability of a composite sample have been measured in the frequency range of 0.5–13 GHz. Based on the theoretical calculation of reflection loss; the microwave absorption properties have been investigated. The results indicate that the attenuation peak frequency of these compounds can be tuned for the desired frequency by varying the thickness of the absorber. [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: Nano-sized and single phase W-type hexaferrite (BaCo2Fe16O27) powders synthesized by sol–gel autocombustion method have been investigated. The samples were sintered in a temperature range of 1000–1200°C for 5h. The thermal decomposition behavior of as-prepared powder was studied by thermogravimetry (TG), differential thermal analysis (DTA) and infrared spectra (IR). The structural and magnetic properties of powders were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The grain size calculated by Scherer equation was found in the range of 36–47nm which is small enough to obtain a suitable signal-to-noise ratio in high density recording media. Hysteresis loop measurements show that the coercivity values lie in the range of 210.61–1602.6Oe with increasing sintering temperature. Magnetization studies show a significant increase in the values from 15 to 22emu/g. The dc electrical resistivity is observed to decrease up to a certain value as the temperature increases and then rises at higher temperature. [Copyright &y& Elsevier]

Abstract: Lanthanum doped W-type hexaferrites BaZn2La x Fe16−x O27 (x =0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized by co-precipitation and sintered at 1320°C. The X-ray diffraction reveals W-type hexagonal structure with few traces of secondary phase. The decrease in grain size as a function of La-concentration is attributed to the fact that La acts as a grain inhibitor. The saturation magnetization and remanance decrease due to spin canting on B-sites. The increase in coercivity follows 1/r behavior where r is the radius of grain. The DC resistivity was observed to increase from 0.59×107 to 8.42×107 Ωcm with increasing La-contents due to the unavailability of Fe3+ ions. This enhancement in resistivity makes these materials promising candidates for use at high frequencies in order to reduce eddy current losses. [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]

Abstract: We have prepared BaCd2−x Sr x Fe16O27 (x =0, 0.5, 1, 1.5 and 2.0) W-type hexagonal ferrites by standard ceramic method. In this work, the structural, dielectric and magnetic properties have been studied of the prepared samples. The XRD analysis of the samples reveals single phase behavior sintered at 1400°C for 6h. The saturation magnetization (M s ) shows increasing behavior with the increasing concentration of Sr2+. While the coercivity (H c ) decreases rapidly up to 409G for x =1.5 and then increases for (x >1.5) due to the preference of Cd2+ for tetrahedral sites and the number of spin-down magnetic ions. The real and imaginary parts of the dielectric constant (ɛ′,ɛ″) and dielectric loss tangent (tan δ) are determined in the frequency range 0.1–107 Hz. It is observed that both the real and imaginary parts of the dielectric constant and tan δ decrease with the increasing concentration of Sr2+, which is due to the contribution of Cd2+ ions in addition to Fe3+ and Fe2+ ions to interfacial polarization. [Copyright &y& Elsevier]

Abstract: CoZn ferrite–SiO2 composites having general formula (1− x)Co0.5Zn0.5Fe2O4 + xSiO2 with x =0.0–0.8 were prepared by co-precipitation technique. The X-ray diffraction analysis of the composites reveals that they are bi-phase. Room temperature resistivity increases from 105 to 109 (Ωcm) from x =0.0–0.8. This drastic increase in resistivity may be attributed to the presence of pores and the segregation of Si at grain boundaries. The Arrhenius plots of these samples show that resistivity decreases as the temperature increases indicating their semi conducting behavior. Arrhenius plots show a change of slope at particular temperature (except for x =0.8) that may be attributed to their Curie temperature. It is observed that the activation energies are small in Para-region as compared to Ferri-region and is an indication of the hopping conduction mechanism. The variation of thermopower with temperature reveals that these samples are degenerate type semiconductors. The values of activation energies calculated from log μ d vs. 1000/T are slightly lower than the values of activation energies obtained from Arrhenius plots. This suggests that the conduction phenomenon is due to polaron hopping. [Copyright &y& Elsevier]