Terahertz Phonon Resonance and Broadband Dielectric Response of Lanthanum Hexaferrite: Unlocking Its Potential for 6G Communications

Developing materials exhibiting a high refractive index, low dielectric loss, and a resonance frequency beyond the operational terahertz (THz) range is essential for advancing next-generation communication technologies. In this study, phase-pure lanthanum-substituted M-type hexaferrite, LaFe12O19 (LaM) has been synthesized and systematically investigated to achieve desirable optical and dielectric properties in the THz regime. The substitution of La3+ induces a partial Fe3+/Fe2+ valence transition at the octahedral 2a sites within the P63/mmc crystallographic structure. A high static dielectric constant (ε′ ≈ 4.38 × 104) and low dielectric loss (tan δ ≈ 1.1) at 1 kHz, driven by space charge polarization at grain boundaries. Relaxation dynamics follow non-Debye behavior, modeled via Cole–Cole and Kohlrausch–Williams–Watts functions. Interfacial polarization dominates at higher frequencies due to conductivity inhomogeneities, consistent with the Maxwell–Wagner mechanism. The temperature- and frequency-dependent impedance analysis reveals the semiconducting behavior and Warburg-type diffusion. Terahertz time-domain spectroscopy (THz-TDS) reveals a phonon-activated resonance process in LaM. At 1.4 THz, the material exhibits a high refractive index (n ≈ 3.71), elevated dielectric constant (ε′ ≈ 14), and low dielectric loss (tan δ ≈ 0.08). The Lorentz model confirms damped harmonic oscillations and a resonance above the operational THz band, highlighting LaM’s potential for next-generation THz antennas and communication systems.