Optical Properties of Epsilon Iron Oxide Nanoparticles in the Millimeter- and Terahertz-Wave Regions

Abstract Epsilon iron oxide (ε-Fe2O3) is attracting global attention as a magnetic material with a large magnetic anisotropy. In this article, the optical properties of ε-Fe2O3 nanoparticles and the metal-substituted series of ε-MxFe2−xO3 (M = Ga, In, and Al) are studied over a wide frequency range from the millimeter-wave to terahertz-wave region, 30 GHz–30 THz, using terahertz time-domain, far-infrared, and Raman spectroscopies. To understand the spectroscopic data, first-principles calculations of the electronic structure and phonon modes are performed. First, an ε-Fe2O3 bar magnet is introduced and its atomic movements are calculated by phonon mode calculations. Second, the phonon modes of Ga-substituted ε-Fe2O3 are calculated. Far-IR, mid-IR, and Raman spectroscopies confirm that the calculated and observed spectra show good agreement. Third, the influences of In-substitution on the crystal structure, magnetic properties, and millimeter-wave absorption are described. In high-frequency millimeter-wave absorption due to magnon, the resonance frequency decreased with In-substitution. Finally, the millimeter-wave absorption property of ε-AlxFe2−xO3 is described. An absorption peak due to the natural resonance occurs at 100 GHz. The rotation data of the transmitted millimeter wave are determined by millimeter-wave–polarization-plane measurements.