The influence of material properties on the assembly of ferrite nanoparticles into 3D structures

• Magnetically-directed assembly of ferrite nanoparticles into 3D structures.
• Strength of an applied field and particles size influence assemblies' morphology.
• Under an applied field of 0.5 T 20 nm-sized CoFe2O4 particles assemble into columns.
• The assembled columns displayed promising magnetic properties.
• The CoFe2O4 columns can be used for the preparation of 1–3 magneto-electric composites.

The fabrication of patterned arrays consisting of magnetic nanoparticles is gaining more and more research activities due to the possibility of enhancing a material's properties for the use in various fields (i.e. magneto-electrics). Here we present the influence of the particle size, the magnetic interactions between the particles and the strength of the applied magnetic field on the magnetically-directed assembly of ferrite nanoparticles. The assemblies were prepared from cobalt ferrite and maghemite suspensions using the drop-deposition technique without or with applied magnetic fields of different strengths. The cobalt ferrite particles with diameter of 6, 8, 10, 12, 20 nm, and maghemite nanoparticles with a diameter of 14 nm were used. The particles' size influences their magnetic properties, the magnetic interactions between the particles and, consequently, the assemblies' morphology. At an applied magnetic field of 0.5 T the morphology of the assembled structures was gradually changing as the particles' size was increasing from flat films for the 6 nm-sized cobalt ferrite nanoparticles to ordered 3-dimensional structures with columnar shape for the 20 nm-sized cobalt ferrite and 14 nm-sized maghemite nanoparticles. The assembled 3-dimensional structures displayed promising magnetic properties and can be used as a basis for the preparation of magneto-electric composites.