Electromagnetic properties of Ni and La doped strontium hexaferrites in the microwave region

The complex permeability and permittivity spectra of novel Ni and La substituted SrCo2Fe16O27 W-type hexaferrites are studied in the 2-18 GHz microwave band. While permittivity exhibits practically no dispersion in the specific testing frequency range, both doping elements yield an increase of dielectric polarization. The permeability of composite specimens is governed by the inherent tendency of Ni and La to induce the anisotropy transformation from planar to axial configuration. Actually, the decrease of the in-plane and out-of-plane anisotropy fields H-phi and H-theta, along with the high saturation magnetization M-s, for low substitution rates, result in the significant rising of mu(max)'' and its shift to substantially lower frequencies. Thus, the alteration of the Ni or La proportion enables the tuning of the magnetic loss peak over the range 5.8-16.7 GHz with maximum values as high as 0.85. Moreover, with the aim to investigate the impact of anisotropy on the dynamic magnetic properties, some essential formulations are reviewed. Through this analysis, the minimization of H-phi of the intermediate compounds in the vicinity of the anisotropy transition, which indicates the existence of an isotropic easy magnetization surface, is highlighted as a basic design requirement for the fabrication of soft hexagonal ferrites with increased mu' and mu '' values. (C) 2011 Elsevier B.V. All rights reserved.