Hexaferrite–FeCo nanocomposite particles and their electrical and magnetic properties at high frequencies

Nanocomposites are realized by chemical reduction whereby the conducting magnetic particles of Fe–Co alloy are generated within the insulating ferrimagnetic BaCo₂Fe₁₆O₂₇ or Ba₂Co₂Fe₁₂O₂₂ hexaferrite matrix. Transmission electron microscopy revealed that metal nanoparticles precipitate coherently as thin flakes along the a–b planes of the derivative magnetoplumbite lattice of the hexaferrites above the characteristic reduction temperature, T_R≫375 °C in H₂ atmosphere. The coercivity increases with T_R in the early stages of the solid-state precipitation and then decreases with the formation of larger fractions of Fe–Co alloy; a converse trend is noticed for magnetization. The complex permittivity increases with reduction to ∼50 in the broad frequency range of 4–18 GHz. The complex permeability is also enhanced with the content of Fe–Co nanoparticles. It is proposed that the spin reorientation at the Fe–Co/hexaferrite interface gives rise to broadband response, rendering these composite particles useful as electromagnetic microwave absorbers. © 2003 American Institute of Physics.