Surface-induced reversal of a phase transformation for the synthesis of epsilon-Fe2O3 nanoparticles with high coercivity

A metastable 8-polymorph of iron(III) oxide (epsilon-Fe2O3) is a very attractive material from the technological, engineering, and scientific points of view. In comparison with other iron oxides, it is characterized by unusual magnetic properties and a giant coercivity of similar to 20 kOe, which is the largest value among metal oxides. The routine method of epsilon-Fe2O3 formation is based on the thermal annealing of maghemite (gamma-Fe2O3) nanoparticles confined in a silica matrix where the epsilon-Fe2O3 appears as an intermediate phase between the maghemite and an alpha-polymorph (alpha-Fe2O3) hematite (gamma ->epsilon ->alpha pathway). In this study, it is demonstrated that the epsilon ->alpha transformation can be reversed when hematite nanoparticles with an anisotropic hollow morphology are annealed above 600 degrees C. The observed reversal of the phase stability is explained in terms of an increased nanoparticle surface area and surface energy related to the hollow structure. This study demonstrates the applicability of surface-induced phase transformation to stabilize and control epsilon-Fe2O3 nanostructures with anisotropic shape and high coercivity similar to 1600 kA/m that is one of the key properties of functional magnetic materials for information processing and storage. The understanding of epsilon-Fe2O3 formation mechanism can provide a new viewpoint and guidance for designing metastable polymorphs and applicative properties. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.