Nanoceramics of metastable ε-Fe2O3: an effect of sintering on the magnetic properties and sub-terahertz electron resonance

In this study, we demonstrate the sintering of metastable ε-Fe2O3 nanoparticles into nanoceramics containing 98 wt% of the epsilon iron oxide phase and with a specific density of 60%. At room temperature, the ceramics retain a giant coercivity of 20 kOe and a sub-terahertz absorption at 190 GHz inherent in the initial nanoparticles. The sintering leads to an increase in the frequencies of the natural ferromagnetic resonance at 200–300 K and larger coercivities at temperatures below 150 K. We propose a simple but working explanation of the low-temperature dynamics of the macroscopic magnetic parameters of the ε-Fe2O3 materials via the transition of the smallest nanoparticles into a superparamagnetic state. The results are confirmed by the temperature dependence of the magnetocrystalline anisotropy constant and micromagnetic modeling. In addition, based on the Landau–Lifshitz formalism, we discuss features of the spin dynamics in ε-Fe2O3 and the possibility of using nanoceramics as sub-terahertz spin-pumping media. Our observations will expand the applicability of ε-Fe2O3 materials and promote their integration into telecommunication devices of the next generation.