Direct Release of Sombrero-Shaped Magnetite Nanoparticles via Nanoimprint Lithography

Magnetic nanoparticles exhibit high potential for biological and biomedical applications [ 1–4 ], ranging from magnetic particle imaging (MPI) [ 5 ], to direct drug delivery and hyperthermia [ 6 ] as well as stemcell tracking and gene therapy [ 7 ]. For successful in vivo applications, magnetic nanoparticles must be bio-compatible, imposing conditions other than magnetic properties on materials selection. Most of the conventionally used ferromagnetic materials demonstrating high saturation magnetization (Co, Ni, Fe, and their alloys) are cytotoxic due to their oxidation state, or solubility. However, Fe 3 O 4 magnetite, demonstrates sufficiently adequate magnetic properties with bio-compatibility [ 1 ]. Magnetite nanoparticles, in spherical form, are superparamagnetic for sub-100 nm sizes [ 8–11 ] for a measurement time of 100 s. Larger particles, >100 nm in diameter, tend to be ferrimagnetic and agglomerate at room temperature due to dipolar coupling. However, for multilayered elements consisting of alternating layers of Fe 3 O 4 and Ti, the magneto-static interactions between the layers of the heterostructure will lead to antiparallel orientation of the magnetization in the layers and in the ideal case, effectively cancel their magnetization giving zero remanence. Thus, these synthetic antiferromagnets (SAF) can be fabricated in much larger sizes providing the principal motivations for fabricating and releasing heterostructured elements [ 12 ].