ε-Fe2O3: An Advanced Nanomaterial Exhibiting Giant Coercive Field, Millimeter-Wave Ferromagnetic Resonance, and Magnetoelectric Coupling

Nanosized iron oxides still attract significant attention within the scientific community, because of their application-promising properties. Among them, ε-Fe2O3 constitutes a remarkable phase, taking pride in a giant coercive field at room temperature, significant ferromagnetic resonance, and coupled magnetoelectric features that are not observed in any other simple metal oxide phase. In this work, we review basic structural and magnetic characteristics of this extraordinary nanomaterial with an emphasis on questionable and unresolved issues raised during its intense research in the past years. We show how a combination of various experimental techniques brings essential and valuable information, with regard to understanding the physicochemical properties of the ε-polymorph of Fe2O3, which remained unexplored for a long period of time. In addition, we recapitulate a series of synthetic routes that lead to the formation of ε-Fe2O3, highlighting their advantages and drawbacks. We also demonstrate how magnetic properties of ε-Fe2O3 can be tuned through the exploitation of various morphologies of ε-Fe2O3 nanosystems, the alignment of ε-Fe2O3 nanoobjects in a supporting matrix, and various degrees of cation substitution. Based on the current knowledge of the scientific community working in the field of ε-Fe2O3, we finally arrive at two main future challenges: (i) the search for optimal synthetic conditions to prepare single-phase ε-Fe2O3 with a high yield, desired size, morphology, and stability; and (ii) the search for a correct description of the magnetic behavior of ε-Fe2O3 at temperatures below the characteristic magnetic ordering temperature.