Low loss and tailored high‐frequency performances of BaO‐doped NiZnCo magneto‐dielectric ferrites

• Published in: Journal of the American Ceramic Society 2020-02, Vol.103 (2), p.1248-1257
• Main Authors: Zheng, Zongliang, Wu, Xu, Feng, Quanyuan, Harris, Vincent G.
• Format: Article
• Language: English
• Subjects: Barium oxides; Density; Dielectrics; Doping; Effective medium theory; Ferrites; Frequency ranges; Grain size; high‐frequency; low loss; Magnetic circuits; Magnetic permeability; magneto‐dielectric; NiZnCo spinel ferrite; permeability and permittivity; Ultrahigh frequencies; Very high frequencies
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.16831

Spinel ferrite ceramics of nominal composition Ni0.5Zn0.3Co0.2Fe2O4 (NiZnCo) with various BaO doping were successfully synthesized for applications as novel high‐frequency magneto‐dielectric materials. The influences of BaO doping on the crystal phase, density, microstructure, and magnetic and dielectric performances in the frequency range of 0.1 to 5 GHz were systematically studied. It is revealed that the doped Ba2+ ions aggregate to the grain‐boundary regions and lead to the formation of BaFe2O4 phase that significantly restrains the growth of NiZnCo ferrite grains. Correspondingly, permeability and permittivity are effectively tailored through the varied grain size and density, which is demonstrated by the magnetic circuit model and the modified effective medium theory proposed herein. As BaO content x = 1.2‐1.8 mol%, the NiZnCo ferrites reveal the excellent performance with almost equal values of μ′ and ε′, yielding a characteristic impedance to be nearly identical as that of free space over a wide frequency range of the VHF and UHF bands. Furthermore, the magnetic loss is effectively reduced at high frequencies, where the typical tan δμ at 0.5 GHz is decreased to ~0.043 with a reduction of up to 37% and the loss factor (tan δμ/μ′) is as low as ~0.006.