Magnetic ferrites such as Fe3O4 and Fe2O3 are extensively used in a range of applications because they are inexpensive and chemically stable. Here we show that rhodium-substituted ε-Fe2O3, ε-RhxFe2−xO3 nanomagnets prepared by a nanoscale chemical synthesis using mesoporous silica as a template, exhibit a huge coercive field (Hc) of 27 kOe at room temperature. Furthermore, a crystallographically oriented sample recorded an Hc value of 31 kOe, which is the largest value among metal-oxide-based magnets and is comparable to those of rare-earth magnets. In addition, ε-RhxFe2−xO3 shows high frequency millimetre wave absorption up to 209 GHz. ε-Rh0.14Fe1.86O3 exhibits a rotation of the polarization plane of the propagated millimetre wave at 220 GHz, which is one of the promising carrier frequencies (the window of air) for millimetre wave wireless communications.

ε-Fe 2 O 3 is known to exhibit a large coercive field of 20 kOe at room temperature. In this work, we examine the electronic structure and magnetic properties of ε-Fe 2 O 3 using first-principles calculation and discrete variational (DV)-Xα molecular orbital calculation. The first-principles calculation shows that ε-Fe 2 O 3 is a charge-transfer type insulator with a valence band of O2p and a conduction band of Fe3d. The optical transition is an indirect transition from Γ to S point. The density of states (DOS) of the four nonequivalent Fe sites (Fe A , Fe B , Fe C , and Fe D ) indicates that ε-Fe 2 O 3 has ferrimagnetic ordering due to α spins on Fe B and Fe C and β spins on Fe A and Fe D . The charge density map of the occupied Fe3d band displays a strong hybridization between Fe3d and O2p. Molecular orbital calculation for each Fe site also supports the existence of a strong Fe3d−O2p hybridization. Such a strong hybridization induces nonzero orbital angular momentum L on Fe3d through the partial charge transfer from O2p to Fe3d. The appearance of L causes a large magnetic anisotropy through spin−orbit interaction, which induces the large coercive field.

A large thermal hysteresis loop was observed in the phase transition on rod-shaped ɛ-InxFe2−xO3 (x ∼ 0.04) nanomagnets. The width of the thermal hysteresis loop, ΔT, increased with increasing rod length (l), i.e., ΔT = 6 K (l = 25 nm), 14 K (40 nm), 25 K (80 nm), and 47 K (170 nm). The observed ΔT value of 47 K is one of the largest values among insulating ferromagnetic materials. The thermal hysteresis loops were analyzed by the Slichter and Drickamer model, and the results showed that the transition enthalpy and entropy do not change. However, the elastic interaction parameter between the transition sites increases with an increasing l value. Maybe the correlation length of a propagating phonon due to elastic interaction competes with the rod length of the samples, causing the rod-length dependence of the thermal hysteresis loop.

A series of Al 3+ -substituted ε-Fe 2 O 3 nanomagnets, ε-Al x Fe 2−x O 3 (x = 0, 0.21, 0.40), with large coercive field values was studied by 57 Fe Mössbauer spectroscopy. The hyperfine field and absorption coefficient in the Mössbauer spectra changed with x. These behaviors can be explained by the selective replacement of Fe 3+ ions with Al 3+ ions. This is the first report on the Mössbauer spectra of metalsubstituted ε-Fe 2 O 3 .

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