The nature of super exchange interactions in transition metal-doped ε-Fe₂O₃ nanoparticles

ε-Fe₂O₃ is the least common and understood iron oxide polymorph, isomorphic to GaFeO₃ and κ-Al₂O₃. ε-Fe₂O₃ has attracted significant interest recently for its unique properties, including a huge coercivity at room temperature, millimeter wave ferromagnetic resonance and magnetoelectric coupling. This research examines transition metal-doped ε-Fe₂O₃ (M=Cr, Mn, Co, Ni, Cu, Zn) nanoparticles to identify the electronic configurations of the different ion sites and how they impact the overall properties. X-ray diffraction reveal the samples have the expected orthorhombic structure while EDS and ICP-OES measurements confirm successful doping. Temperature dependent susceptibility and hysteresis measurements show the characteristic spin reorientation transition (SR) at 150 K, where the system transforms from an incommensurate state to a high-coercivity canted antiferromagnet. While this SR is preserved in all samples, the type and concentration of dopant ion does alter the magnetism. ⁵⁷Fe Mössbauer spectroscopy and extended x-ray absorption fine structure analysis quantify the exchange interactions, revealing changes to the super transferred hyperfine interactions and charge ordering.