Unveiling a New High-Temperature Ordered Magnetic Phase in ε-Fe2O3

Abstract

Iron oxides are among the most abundant materials on Earth, and yet there are some of their basic properties which are still not well-established. Here, we present temperature-dependent magnetic, X-ray, and neutron diffraction measurements refuting the current belief that the magnetic ordering temperature of e-Fe2O3 is ∼500 K, i.e., well below that of other iron oxides such as hematite, magnetite, or maghemite. Upon heating from room temperature, the e-Fe2O3 nanoparticles’ saturation magnetization undergoes a monotonic decrease while the coercivity and remanence sharply drop, virtually vanishing around ∼500 K. However, above that temperature the hysteresis loops present a nonlinear response with finite coercivity, making evident signs of ferrimagnetic order up to temperatures as high as 850 K (TN1). The neutron diffraction study confirms the presence of ferrimagnetic order well above 500 K with Pna'21' magnetic symmetry, but only involving two of the four Fe3+ sublattices which are ordered below TN2 ≈ 480...