Formation and microwave absorption of barium and strontium ferrite prepared by sol-gel technique

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Abstract
Ba and Sr ferrites are prepared by sol‐gel technique with different Fe/Ba(Sr) ratios in the starting materials. Magnetization, coercive, and anisotropy field strength are determined depending on the heat treatment of the gel and the iron/barium(strontium) ratio in the starting material. A two‐step heat treatment is used to prepare single‐domain powders with high magnetization. These powders prepared by sol‐gel technique show single‐domain behavior with specific magnetization σ S =649 A cm2/g and coercive field strength H cM =402 kA/m in the case of Ba ferrite and σ S =695 A cm2/g and H cM =416 kA/m for Sr the ferrite. Al‐substituted ferrites with high anisotropy field strengths are prepared additionally. Ferromagnetic resonance absorption is used to determine the anisotropy field strength and to investigate the formation process of the hexaferrite phase during the heat treatment. The beginning of hexaferrite formation occurs at annealing temperatures below 700 °C.

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