Ultra-High-Frequency Behavior of BaFe $_{12}$ O $_{19}$ Hexaferrite for LTCC Substrates

M-type barium hexaferrite (BaFe₁₂O₁₉) is investigated with an aim of using with low temperature co-fired ceramic (LTCC) substrate material. The as-calcined powders from combustion synthesis at neutral and acidic precursor solutions are used, which exhibit excellent properties of low coercive field values (up to 1350 Oe) and high magnetization (up to 60 emu/g). With an appropriate amount of Bi₂O₃ as sintering aid, high density ferrites have been realized at LTCC processing temperatures. The sintered ferrites possess bulk resistivity in excess of 10⁹ Ω·cm, indicating their suitability for high-frequency applications. Complex magnetic permeability and magnetic loss factors are studied up to 1 GHz frequency. Complex permeability between 1.2 to 2.2 and magnetic loss tangent between 0.05-0.5 have been realized for the sintered ferrites. The influence of Bi₂O₃ content on resistivity and high-frequency magnetic properties is discussed. It is observed that the sample prepared by employing neutral pH during combustion preparation and calcined at 1200°C for 12 h shows low coercivity of 1350 Oe. After sintering at 900°C with 2 wt% Bi₂O₃, the sample shows better results with permeability of 1.5-1.7, magnetic loss tangent below 0.3 up to 700 MHz frequency and a high resonance frequency of 140 and 715 MHz in VHF and UHF band, respectively. Since the synthesis conditions provide better control over basic magnetic properties, the use of combustion synthesized BaFe₁₂O₁₉ material augers well for magnetic materials applications in LTCC technology.