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The effect of Pb 2+ substitution on structural, magnetic and microwave properties of Ba 0.5 Sr 0.5 Fe 12 O 19 was investigated. Samples were synthesized using solid state reaction route with 1 wt% B 2 O 3 addition which was used as a catalyst for better crystal growth. Both lattice parameters (a and c) and crystallite sizes increased initially with Pb substitution and then they started to decrease according to the calculations based on Rietveld Refined XRD patterns. Magnetic properties of Pb substituted samples degraded compared to Pb-free hexaferrite, for instance, coercivity decreased substantially from 3.35 to 1.4 kOe. On the other hand, Pb 2+ substitution enhanced microwave absorption properties of the samples, for example the reflection loss increased from −40 dB to −57.5 dB in a 2 mm-thick Ba 0.3 Sr 0.4 Pb 0.3 Fe 12 O 19 sample. The mechanisms of microwave absorption appeared to be natural resonance of magnetic moments and relaxation of electrical dipoles for the reflection loss observed at high frequencies (>10 GHz) and quarter wave cancellation at matching thickness at frequencies ≤ 10 GHz. There is a very good agreement between experimentally measured and calculated matching frequency values for thicknesses between 2 and 5 mm. Finally, it was observed that matching thickness decreases exponentially with matching frequency in the 2–10 mm thickness range. [ABSTRACT FROM AUTHOR]

Mg-Ti substituted Ni-Zn spinel ferrites (Ni 0.5 Zn 0.5 Mg x Ti x Fe 2−2x O 4, where 0.05≤x≤0.25) have been prepared by conventional solid state reaction method. 1 wt% B 2 O 3 was added to increase the crystal growth at low temperatures. The structural characterization of samples was performed by X-ray powder diffractometry, scanning electron microscopy and Mössbauer spectroscopy. Magnetic parameters of samples were measured by vibrating sample magnetometer at room temperature. Microwave characteristics were determined by near field measurements using a network analyzer in reflection/transmission mode. Mössbauer spectra of all the samples consist of 3 sextets and one doublet. Variation of the line width, isomer shift, quadrupole splitting and hyperfine magnetic field values have been obtained from the 57 Fe Mössbauer spectroscopy data for Ni-Zn spinels having different Mg 2+ and Ti 4+ concentrations. The saturation magnetization of the samples decreases as the concentration of Mg-Ti ions increases, which can be explained by site preferences of the substituted cations. The electrical properties of Mg-Ti substituted Ni-Zn spinel ferrites were examined extensively as a function of temperature, frequency and Mg-Ti amount. It was found that Mg-Ti substitution has a strong effect on the ac conductivity, dielectric constant and dielectric loss mechanisms. The activation energy levels can be regulated by varying Mg-Ti concentration. There is one reflection minimum observed in the microwave characterization of the samples in the 2–18 GHz range located around 10 GHz in X-band, the mechanism of which is the quarter wave cancellation due to the matching thickness. The Ni 0.5 Zn 0.5 Mg 0.2 Ti 0.2 Fe 1.6 O 4 sample has the best microwave properties with the reflection loss of −29 dB at 10.4 GHz for a thickness of 3 mm and better than −35 dB at 16 GHz when thickness is decreased to 2 mm. As a result, it was concluded that this material can be used as microwave absorber both in X and Ku-bands. [ABSTRACT FROM AUTHOR]