Development of Microwave and Millimeter Wave Absorber with Flexible Hexaferrite-Rubber Sheets

With the rapid development of information and communication technology (ICT) and the increasing integration and multifunctionality of automotive electronics, the frequencies of electromagnetic (EM) waves generated by electronic devices are becoming more diverse, extending beyond microwaves to millimeter wave bands. This trend highlights the importance of EM wave absorption technology to mitigate potential malfunctions and signal degradation. EM wave absorption typically relies on the dielectric loss and magnetic loss mechanisms of materials. One effective method for absorbing frequencies over GHz ranges is the utilization of ferromagnetic resonance (FMR) of magnetic materials. FMR is directly proportional to the magnetic anisotropy of magnetic materials. Most metallic and ceramic magnetic materials do not have sufficient magnetic anisotropy, so their FMR band cannot exceed the GHz range. M-type hexaferrite materials, with the basic chemical formula (Ba, Sr)Fe₁₂O₁₉, exhibit high crystal magnetic anisotropy and an FMR frequency around 50 GHz. La-Ca-Co substituted M-type hexaferrite, known for high-performance ferrite magnets, have even higher crystal magnetic anisotropy, leading to higher FMR frequencies above 50 GHz and are expected to absorb even higher frequency bands.<br/>In this study, we synthesized powders of SrFe₁₂O₁₉ (SrM), the basic composition of M-type hexaferrite, and Sr_0.2La_0.4Ca_0.4Fe_9.75-xAl_xCo_0.25O₁₉ (SLCAM) with x = 0, 0.2, and 0.4, considered high-performance permanent magnet compositions, using the solid-state reaction method. We measured the M-H curves of the sintered samples using a vibrating sample magnetometer (VSM) and evaluated their magnetic properties. The powdered samples were combined with nitrile butadiene rubber (NBR) to create flexible absorber sheets. The crystal phases and microstructures of the sheets were analyzed using X-ray diffraction (XRD) and a scanning electron microscope (SEM). The high-frequency complex permittivity and complex permeability of each sample were measured in the 33<b>–</b>75 GHz range using a vector network analyzer and a free space measurement system, and the reflection loss (RL), indicating EM wave absorption capability, was calculated based on transmission line theory.<br/>All samples were synthesized in a single-phase M-type hexaferrite structure. Compared to the x = 0 sample, the x = 0.2 and 0.4 samples had larger grains and increased c-axis orientation in the sheet. The FMR frequency (<i>f_r</i>), identified by the peak of the imaginary part of the permeability, was 48.5 GHz for the SrM sample and 63.3 GHz, 66.2 GHz, and 68.5 GHz for the SLCAM samples with x = 0, 0.2, and 0.4, respectively. The SrM-rubber sheet showed a minimum RL (RL_min) of -33.5 dB at 49.7 GHz with a thickness of 0.37 mm. The x = 0, 0.2, and 0.4 sheets, with a thickness of ~0.3 mm, showed RL_min values of -35.0, -38.8, and -43.0 dB at frequencies of 63.2, 66.3, and 68.5 GHz, respectively. The substitution of Al increases the <i>f_r</i> and the frequency of RL_min, corresponding with the <i>f_r</i> increase. The maximum EM wave absorption frequency band (Δ<i>f</i>), satisfying RL &lt; -10 dB, was 45.0–53.4 GHz for SrM, and 52.9–75 GHz, 50–75 GHz, and 59–74.2 GHz for the x = 0, 0.2, and 0.4 samples, respectively. The x = 0.2 sheets showed very promising results, demonstrating extremely wideband absorption characteristics, capable of absorbing 99% of EM waves over the entire V-band (50–75 GHz) range. The Al-substituted La-Ca-Co M-type hexaferrite-rubber sheets, with a highly flexible thickness of 0.3 mm that can be bent or folded without damage, exhibited excellent wideband EM wave absorption characteristics in the microwave and millimeter wave bands, indicating their potential as highly promising materials for EM wave absorbers in automotive and various electronic device systems.