Verwey Transition in Ultrasmall-Sized Octahedral Fe₃O₄ Nanoparticles

We observed Verwey transition in very small (6–14 nm) amine-coated octahedral magnetite (Fe 3 O 4 ) 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 spherical particles. The electrical transport characteristics indicate that these nanoparticle assemblies or thin films could be useful for future generation switching and memory devices.