Mn-substitution effects on the magnetic and zero-field ferromagnetic resonance properties of ε-Fe2O3 nanoparticles†

Metal substitution is an important way to tune the magnetic properties of ferrites. In the present study, to investigate the effects of Mn substitution on the magnetic properties and millimeter wave absorption properties on ε-Fe₂O₃ for the first time, Mn-substituted epsilon iron oxides, ε-Mn_xFe_2−xO_3−x/2 (x = 0 (Mn0), 0.10 (Mn1), and 0.20 (Mn2)) were synthesized by sintering iron oxide hydroxide with manganese hydroxide in a silica matrix. Transmission electron microscopy shows particle sizes of 18.7 ± 5.8 nm (Mn0), 19.0 ± 6.2 nm (Mn1), and 19.8 ± 6.7 nm (Mn2). Energy dispersive X-ray spectroscopy confirms a uniform manganese distribution across all particles, while the powder X-ray diffraction patterns demonstrate that ε-Mn_xFe_2−xO_3−x/2 has an orthorhombic crystal structure with a space group of Pna2₁ (e.g., the lattice constants in Mn2 are a = 5.1031(4) Å, b = 8.7759(8) Å, and c = 9.4661(7) Å). As the Mn substitution ratio increases, the Curie temperature decreases from 487 K (Mn0) to 469 K (Mn2). As for the magnetic properties at 300 K, the coercive field increases from 17.2 kOe (Mn0) to 18.2 kOe (Mn2), while the saturation magnetisation decreases from 17.1 emu g⁻¹ (Mn0) to 13.9 emu g⁻¹ (Mn2), with increasing substitution ratio. Terahertz time-domain spectroscopy demonstrates that the samples exhibit electromagnetic wave absorption in the millimetre-wave region, due to zero-field ferromagnetic resonance. As the Mn substitution ratio increases, the resonance frequency increases from 174 GHz (Mn0) to 182 GHz (Mn1) and 187 GHz (Mn2). Due to the substitution of Fe³⁺ with Mn²⁺, the saturation magnetisation decreases and the coercive field and the resonance frequency increase.