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 degreesC in H-2 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 similar to50 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. (C) 2003 American Institute of Physics.