Emergent Magnetodielectric Materials for 5G Antenna Miniaturization—18H Hexaferrites

Antenna miniaturization technology based on traditional materials with high dielectric constant has the disadvantages of low efficiency, narrow bandwidth, and difficulty in impedance matching. Compared to high-permittivity dielectrics, magnetodielectric materials, which exhibit both relative permeability and permittivity greater than unity, can achieve competitive miniaturization performance and bandwidth enhancement due to their less capacitive nature and low electromagnetic energy concentration. The fifth generation (5G) communications call for high-performance magnetodielectric materials with high values of permeability and permittivity, low magnetic and dielectric losses, and high cutoff frequency over the sub-6 GHz frequency bands. Due to limited resonant frequencies and large natural resonance linewidth, conventional microwave ferrites and their composites, which are usually used over the frequency range from several hundred MHz to several GHz, do not meet these requirements.<br/><br/>Here, a unique type of hexagonal ferrites—Mg-Zn 18H hexaferrite (Ba₅Mg₂_-<i>_x</i>Zn<i>_x</i>Ti₃Fe₁₂O₃₁), whose crystal structure can be interpreted as the structure of Y-type hexaferrites with intercalation of a three-layered hexagonal barium titanate block into the middle of the T-block, has been synthesized by a solid-state reaction route, and its microwave properties have been comprehensively investigated for the first time. Depending on the Zn content, the polycrystalline Mg-Zn 18H hexaferrite exhibits a saturation magnetization of 1–1.23 kG and a coercivity of 15–68 Oe. The small coercivity indicates the ease of domain wall motion through the lattice upon demagnetization and the great potential for low-loss applications. The permeability and permittivity spectra of the Mg-Zn 18H hexaferrite are extracted from the S-parameters measured with a vector network analyzer over 0.1–10 GHz. Narrow and strong magnetic resonant peaks up to 5 GHz are observed. The magnetic resonance shifts to lower frequencies and slightly broadens with increasing the Zn content. Fitted with the superposition of the domain wall resonance and gyromagnetic spin resonance, the Mg-Zn 18H hexaferrite shows an extremely low damping coefficient of 0.1–0.2, indicating concentrated magnetic loss within the narrow frequency range and implying a narrow ferrimagnetic resonance (FMR) linewidth. The FMR linewidth is confirmed to be 486-660 Oe by a resonant perturbation technique using a TE₁₀ rectangular waveguide resonator at X band. Owing to this remarkably low damping coefficient compared to that of other microwave ferrites, which typically varies from 0.2 to 0.9, Mg-Zn 18H hexaferrite exhibits an excellent magnetic loss tangent as low as 0.06, as well as a small dielectric loss tangent less than 0.006. By modifying the atomic ratio of Zn:Mg in the formula, this low-loss property covers the entire S band from 2 to 4 GHz, representing the best magnetodielectric performance among the reported microwave ferrites for the S band applications. Moreover, the temperature dependence of the damping coefficient is 0.0004 K^-¹ over the range of 300–410 K, indicating an excellent thermal stability for the temperature variation of most communication applications.<br/><br/>For antenna miniaturization, depending on the Zn content, the miniaturization factor of the Mg-Zn 18H hexaferrite varies from 5.3 to 7.6. As an example, we demonstrate this capability for antenna size reduction by applying the Mg 18H hexaferrite to the substrate material of a 3.6-GHz patch antenna, which shows a miniaturization factor of ~5 and significant bandwidth improvement of ~50–110% over conventional dielectric substrates. These results imply the great potential of the 18H hexaferrites to be Co-free and low-cost alternatives to most microwave hexaferrites and reveal their great technological and commercial value for 5G communications.