The synthesis of rhodium substituted ε-iron oxide exhibiting super high frequency natural resonance

In this study, we demonstrate a synthesis of rhodium substituted e-iron oxide, e-RhxFe2−xO3 (0 ≤ x ≤ 0.19), nanoparticles in silica. The synthesis features a sol–gel method to coat the metal hydroxide sol containing Fe3+ and Rh3+ ions with a silica sol via hydrolysis of alkoxysilane to form a composite gel. The obtained samples are barrel-shaped nanoparticles with average long- and short-axial lengths of approximately 30 nm and 20 nm, respectively. The crystallographic structure study using X-ray diffraction shows that e-RhxFe2−xO3 has an orthorhombic crystal structure in the Pna21 space group. Among the four non-equivalent substitution sites (A–D sites), Rh3+ ions mainly substitute into the C sites. The formation mechanism of e-RhxFe2−xO3 nanoparticles is considered to be that the large surface area of the nanoparticles increases the contribution from the surface energy to Gibbs free energy, resulting in a different phase, e-phase, becoming the most stable phase compared to that of bulk or single crystal. The measured electromagnetic wave absorption characteristics due to natural resonance (zero-field ferromagnetic resonance) using terahertz time domain spectroscopy reveal that the natural resonance frequency shifts from 182 GHz (e-Fe2O3) to 222 GHz (e-Rh0.19Fe1.81O3) upon rhodium substitution. This is the highest natural resonance frequency of a magnetic material, and is attributed to the large magnetic anisotropy due to rhodium substitution. The estimated coercive field for e-Rh0.19Fe1.81O3 is as large as 28 kOe.