Low loss NiZn spinel ferrite–W-type hexaferrite composites from BaM addition for antenna applications

A series of NiZn ferrite composites with various BaFe12O19 hexaferrite (BaM) additions (x) were successfully synthesized by the solid-state reaction method for their potential application as magneto-dielectric antenna substrate materials. XRD and energy-dispersive spectrum analyses reveal that a W-type hexagonal phase is formed when the mixture of NiZn ferrite and BaM are sintered at 1200 °C, and diphase composite ferrite that consists of NiZn spinel ferrite and BaW hexaferrite is finally obtained. The composites display much smaller grain size and a refined microstructure. With the increasing x, the initial permeability decreases from ∼84.5 to ∼15.5, while the cut-off frequency is dramatically improved from 75 to 570 MHz. BaM addition significantly inhibits the frequency dispersion of permittivity spectrum and stable permittivity ε′ around 15 is observed for the doped samples from 1 MHz to 1 GHz, which is proved to be closely related to the refined grains according to Koops' theory. Besides, compared with the undoped NiZn ferrite, both the magnetic loss and dielectric loss are found to be reduced in the doped samples. Almost equal values of μ′ and ε′ is obtained in the sample with x = 30 wt%. Related magnetic and dielectric mechanisms are also discussed. (paper)