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

In this study, we demonstrate a synthesis of rhodium substituted epsilon -iron oxide, epsilon -Rh sub(x)Fe sub(2-x)O sub(3) (0 less than or equal to x less than or equal to 0.19), nanoparticles in silica. The synthesis features a sol-gel method to coat the metal hydroxide sol containing Fe super(3+) and Rh super(3+) 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 epsilon -Rh sub(x)Fe sub(2-x)O sub(3) has an orthorhombic crystal structure in the Pna2 sub(1) space group. Among the four non-equivalent substitution sites (A-D sites), Rh super(3+) ions mainly substitute into the C sites. The formation mechanism of epsilon -Rh sub(x)Fe sub(2-x)O sub(3) 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, epsilon -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 ( epsilon -Fe sub(2)O sub(3)) to 222 GHz ( epsilon -Rh sub(0.19)Fe sub(1.81)O sub(3)) 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 epsilon -Rh sub(0.19)Fe sub(1.81) O sub(3) is as large as 28 kOe.