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

Abstract

In this study, we demonstrate a synthesis of rhodium substituted ε-iron oxide, ε-Rh_xFe_2−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 ε-Rh_xFe_2−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 ε-Rh_xFe_2−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, ε-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 (ε-Fe₂O₃) to 222 GHz (ε-Rh_0.19Fe_1.81O₃) 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 ε-Rh_0.19Fe_1.81O₃ is as large as 28 kOe.