Formation of Columnar Structures by the Magnetically Directed Assembly of Cobalt Ferrite Nanoparticles

We have studied the assembly of cobalt ferrite nanoparticles in a magnetic field. Nanoparticles of different sizes, i.e., 5–40 nm, were synthesized with coprecipitation or under hydrothermal conditions. Their saturation magnetization values ranged between 31 and 68 Am
$^{2}$
/kg, respectively, resulting in a strong magnetic attraction and agglomeration between the nanoparticles. In aqueous ferrofluids this was prevented by the adsorption of citric acid on the nanoparticles' surfaces. The estimated interaction energies show that the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory fails to explain the stability of cobalt ferrite ferrofluids and that the solvation interaction cannot be neglected. Cobalt ferrite ferrofluids were deposited on alumina substrates and dried in a magnetic field of 0.03 or 0.5 T. The homogeneity of the deposits decreased with the increasing saturation magnetization of the nanoparticles and/or with the increasing magnetic field, until columnar structures were formed from the nanoparticles with
${\rm Ms}\geq 55$
Am
$^{2}$
/kg under a magnetic field of
${\rm B} = 0.5$
T