Vacancy-engineered Gd3+-substituted yttrium iron garnet with narrow ferrimagnetic linewidth and high Curie temperature

Hexaferrites with high permeability and low magnetic loss have been attracting great attention from both academic and industrial communities as promising high-frequency materials for modern communication applications. The magnetic properties of hexaferrites are closely related to the crystallographic texture and can be improved by orienting the crystallites. The traditional magnetic field orientation technique, however, has the disadvantages of complicated process and low efficiency. Here, a tunable crystallographic texture of polycrystalline hexaferrite composites achieved by a non-magnetic-field orientation technique is reported. Tabular Co2Z hexaferrite crystallites are synthesized from BaM and Co2Y hexaferrite mixtures and oriented in a form of lamination during the grain growth stage of the sintering process. These layered grains with paralleled easy-magnetization planes play an important role in enhancing the magnetic permeability and suppressing the eddy current from penetrating highly insulating grain boundaries at high frequencies. The textured hexaferrite composite with optimized composition exhibits a high permeability of 16.6, low magnetic specific loss of 0.006, and enhanced Snoek's product of 17.4 GHz at 200 MHz. These results demonstrate an easy and effective method to achieve textured hexaferrite composites without magnetic fields and reveal the potential of hexaferrite composites with sintering-induced texture for high-frequency low-loss magnetic applications. 

Hexaferrite composites with sintering-induced texture for Snoek's product enhancement and magnetic loss suppression

High Remanence Ratio of Aluminum Substituted Hexagonal Barium Ferrite Films for Self-Biased Microwave Devices

High remanence ratio of aluminum substituted hexagonal barium ferrite films for self-biased microwave devices

In this study, Ge4+-substituted cordierite, Mg2Al4(Si1–xGex)5O18 ceramics, were successfully prepared by the traditional solid-state method to broaden its application potential. Notably, the excellent dielectric properties with εr = 4.90, Q·f = 128,200 GHz, and τf = −21.01 ppm °C–1 were achieved. The increase in εr value is mainly due to the heightened content of Ge4+ with high polarizability. The Q·f value improved by 2.21 times compared to the cordierite matrix, which can be primarily attributed to enhanced lattice energy, bond covalency, and hexagonal ring symmetry. The alteration in τf value arises from the variation of bond energy, bond strength, and distortion in the [MgO6] octahedra. These conclusions provide valuable insights for the design of silicate ceramics with higher Q·f values. In addition, the dielectric properties in the microwave and terahertz bands were compared. The higher Q·f and lower εr values in the terahertz band mainly result from the withdrawal of partial polarization mechanisms and differences in measurement methods. Mg2Al4(Si0.92Ge0.08)5O18 ceramics, demonstrating an ultra-low εr value of 4.54 and an ultra-high Q·f value of 286,533 GHz in the terahertz band, emerge as formidable contenders for future terahertz communications materials. Finally, a microstrip patch antenna was fabricated, achieving a bandwidth of 150 MHz at 4.78 GHz, which confirms the application in the n79 band for wireless communication. 

Effect of Ge4+-substituted on the structure characteristics and microwave/terahertz dielectric properties of ultra-low εr, high Q·f cordierite ceramics

Barium hexagonal ferrites exhibiting unique self-biasing characteristics have great potential for application in planar microwave devices, such as self-biased circulators. Cu doping is an effective method to tailor their anisotropy field and ferrimagnetic resonance (FMR) linewidth to meet the requirements for low-frequency low-loss microwave devices. However, the regulation mechanism of Cu doping is still obscure, and its regulation effect is not optimized. Here, the magnetic and microwave properties of two groups of barium hexaferrites with site-controlled Cu doping are reported. The diffusion dynamics of Cu2+ ions are comprehensively investigated, revealing significant differences in the concentration distribution and polycrystalline morphology, when comparing CuO as a reactant and sintering additive. The accumulation of Cu2+ ions at grain boundaries contributes to the increase in coercivity, whereas the dispersion of Cu2+ ions in crystallites leads to the decrease in the anisotropy field. Moreover, by introducing Cu2+ ions into the interstitial positions of the lattice, barium hexaferrites with a narrow FMR linewidth of 303 Oe and a high remanence ratio of 0.82 are achieved. These results represent the lowest FMR linewidth reported in polycrystalline hexagonal ferrites and prove the great technological value and commercial potential of Cu-doped barium hexaferrites for next-generation planar microwave devices. 

Barium hexaferrites with narrow ferrimagnetic resonance linewidth tailored by site‐controlled Cu doping

Excellent gyromagnetic properties of textured, bulk Ba-hexaferrite samples are required for low-loss, self-biased applications for microwave and millimeter-wave (MMW) devices. However, conventionally processed bulk Ba-hexaferrite ceramics typically demonstrate low remanent magnetization values, 4πMr, of 2.0~3.0 kG, and relatively large ferromagnetic resonance (FMR) linewidths, ΔHFMR, of 0.8~2 kOe. These properties lead to the development of high-performance, practical devices. Herein, crystallographically textured Ba-hexaferrite samples, of the composition Ba0.8La0.2Fe11.8Cu0.2O19, having excellent functional properties, are proposed. These materials exhibit strong anisotropy fields, Ha, of ~14.6 kOe, high remanent magnetization, 4πMr, of 3.96 kGs, and a low ΔHFMR of 401 Oe at zero-bias field at the Q-band. Concomitantly, the broadband millimeter-wave transmittance was utilized to determine the complex permeability, μ*, and permittivity, ε*, of textured hexaferrites. Based on Schlöemann’s theory of complex permeability, μ*, the remanent magnetization, 4πMr, anisotropy field, Ha, and effective linewidth, ΔHeff, were estimated; these values agree well with measured values.

/pdf/crystallographically-textured-and-magnetic-lacu-substituted-2hvy55bf.pdf

Crystallographically Textured and Magnetic LaCu-Substituted Ba-Hexaferrite with Excellent Gyromagnetic Properties

Temperature stability and low dielectric constant of Li+ substituted Mg2Al4Si5O18 ceramics

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Barium hexaferrites with narrow ferrimagnetic resonance linewidth tailored by site‐controlled Cu doping

Crystallographically Textured and Magnetic LaCu-Substituted Ba-Hexaferrite with Excellent Gyromagnetic Properties

Temperature stability and low dielectric constant of Li+ substituted Mg2Al4Si5O18 ceramics