Pb substituted Ba,Sr-hexaferrite nanoparticles as high quality microwave absorbers

Abstract

The effect of Pb2+ substitution on structural, magnetic and microwave properties of Ba0.5Sr0.5Fe12O19 was investigated. Samples were synthesized using solid state reaction route with 1 wt% B2O3 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, Pb2+ 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)Fb(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.