Fabrication of Co-Hf Doped Ba-Sr Hexagonal Ferrite for Economical and Lightweight Microwave Absorber Application: Functionalization of Structural, Morphological, Hysteresis, Electromagnetic Metrics and Tunable Bandwidth

Microwave absorbers are used to mitigate the unwanted wireless/electromagnetic signals that interfere with the functioning of electrical/electronic devices. In this paper, Co²⁺-Hf⁴⁺ doped M-type Ba-Sr hexagonal ferrite samples have been synthesized using a solid-state ceramic method. The phase structure and surface morphology were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The magnetic hysteresis was investigated with a vibrating sample magnetometer. A two-port vector network analyzer characterized the microwave absorption in terms of frequency, dopants, and geometrical thickness of the samples. XRD confirmed that the produced powders are of M-type hexaferrite structure without any additional phases. SEM revealed inhibition of grain growth with dopants and small grain size as well as grain cluster formation. This morphological behavior caused a reduction in coercivity and enhanced microwave absorption. As a consequence, the dopants caused a better soft magnetic nature with a room temperature (r.t.) coercivity (H_c) varying between 788 Oe (x=0.0) and 489 Oe (x=1.0). The measured dielectric constant/dielectric loss and permeability/magnetic loss parameters were increased with dopants. The dopants caused an increase in the absorption from -20.67 dB at 6.5 mm in undoped composition (x=0.0) to -40.21 dB at 1.6 mm in doped composition (x=1.0). Additionally, -10 dB absorption bandwidth of 3.13 GHz was exhibited in x=1.0 from 8.47-11.60 GHz frequency band, while x=0.4 revealed -10 dB absorption bandwidth of 1.92 GHz from 10.48-12.40 GHz. Microwave absorption has been substantiated by impedance matching/λ/4 mechanism and absorption peaks have been tuned in the desired frequency spectrum by careful selection of thickness/dopants. Significant variations were seen associated with dopants-based tuning of absorption peaks in the frequency spectrum, an increase in microwave absorption, and a decrease in the thickness of the samples. The tunable performance metrics and economical synthesis imply the potential application of prepared compositions for microwave absorbers and passive device applications for wireless communication.