Verwey Transition in Ultrasmall-Sized Octahedral Fe3O4 Nanoparticles

Abstract

We observed Verwey transition in very small (6–14 nm) amine-coated octahedral magnetite (Fe3O4) nanoparticles that is not present in spherical similarly sized (4–13 nm) nanoparticles. Electron microscopy shows that octahedral nanoparticles have {111} facets with better cationic coordination symmetry as their surface. Spherical shape illustrates conventional superparamagnetic behavior; on the contrary, a characteristic Verwey transition near 120 K is prominent in field-cooled/zero-field-cooled curves of octahedral nanoparticles. Higher saturation magnetization in octahedral nanoparticles indicates lesser surface spin disorder and well-established anisotropy. Better surface coordination offers a reduced number of oxygen vacancies at the surface and, therefore, better stoichiometry results in a Verwey transition in octahedral nanoparticles. Electrical resistivity measurements show a sharp change in resistance for octahedral particles below the Verwey transition temperature which is completely hindered in sph...