Understanding the role of Fe-O hybridization in the characteristic transitions of iron oxide nanoparticles

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Nickel, Rachel
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Iron oxides have long attracted interest for both their fascinating scientific properties and potential applications due to their strong electron correlations and competing degrees of freedom. The evolution of these interactions underpins the electronic and magnetic transitions that occur, resulting in states that exhibit everything from simple ferrimagnetism to complex multiferroicity. However, the relationship between the underlying interactions and the overall material properties is not well-understood. In this work, the characteristic transitions of Fe3O4 and ε-Fe2O3 are studied to identify the role of Fe-O hybridization. While the trimeron model has been accepted as the mechanism behind the Verwey transition in bulk Fe3O4, the disappearance of this metal-insulator transition in nanoparticles has not been addressed. By studying three sizes of Fe3O4 nanorods, a clear relationship between strain and the Verwey transition temperature (TV ) appears. Isotropic compressive strain (which typically occurs in spherical nanoparticles) reduces TV , while uniaxial tensile strain increases TV . Detailed study of the largest Fe3O4 nanorods confirms the formation of orbitally-ordered trimerons in the low temperature insulating phase, albeit a modified trimeron state from that of bulk Fe3O4. Such altered hybridization causes TV to shift in strained nanoparticles. In ε-Fe2O3, the DOh1 octahedral chains create a complicated electronic structure which yields complex hybridization. As a result, the physical mechanism behind the characteristic transition of ε-Fe2O3 is unresolved. Full characterization of three sizes of ε-Fe2O3 reveal that this transition is purely electronic with no structural changes observed. Perturbed transition metal-doped ε-Fe2O3 provide further insights. In particular, temperature dependent hyperfine parameters (from Mössbauer spectroscopy) show high temperature charge ordering in the Cr-doped nanoparticles. Combined with the undoped ε-Fe2O3 behaviour, the transition is caused by the onset of supertransferred hyperfine interactions between Td sites.
nanoparticle, magnetism, hybridization