A processing route for bulk, high coercivity, rare-earth free, nanocomposite magnets based on metastable iron oxide

Abstract

High coercivity magnets are crucial in technology today, but dependence on rare earth elements and/or precious metals limits the application of the state of the art magnets. Here we introduce a method for producing large sized nanocomposite magnets comprised primarily of a metastable material. They have a very high coercivity (∼12 kOe) and are based on three of the most earth abundant elements—Fe, Si and O. The key is leveraging length scale and kinetic control to stabilize a large volume content of the metastable ε-Fe₂O₃ nanophase in a thin silica matrix to obtain a high density of magnetic phase. X-ray diffraction, electron microscopy and magnetic measurements confirm densely packed nanorods consisting primarily of ε-Fe₂O₃. These nanocomposites have some of the highest coercivities reported in dense millimeter-sized magnets that do not contain rare earths or precious metals.