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

TLDR

Ba and Sr ferrites were synthesized by a sol‑gel route with varying Fe/Ba(Sr) ratios, followed by a two‑step heat treatment that produced single‑domain powders with high magnetization and anisotropy, and Al‑substituted variants were characterized by ferromagnetic resonance to probe hexaferrite formation. The sol‑gel derived powders exhibited single‑domain behavior with σS≈649 A cm²/g and Hc≈402 kA/m for Ba ferrite and σS≈695 A cm²/g and Hc≈416 kA/m for Sr ferrite, and hexaferrite formation was observed to initiate at annealing temperatures below 700 °C.

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 σS=649 A cm2/g and coercive field strength HcM=402 kA/m in the case of Ba ferrite and σS=695 A cm2/g and HcM=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.