  The influence of an external magnetic field upon the capture of 130 nm
magnetic nanoparticles (MNPs) by ferromagnetic nanostructures was investigated.
The magnetophoretic forces acting upon a nanoparticle were simulated in
external magnetic fields parallel and perpendicular to ferromagnetic
nanostructures consisting of arrays of antidots and dots. Changing the
direction of the external field was found to dramatically alter the
magnetophoretic forces acting on the particle and the trajectories of the MNPs.
A field parallel to the nanostructures' surfaces generated magnetophoretic
forces that directed the nanoparticle into the nanostructures. A perpendicular
field produced forces directing particles onto the structures' surfaces.
Nanostructures were etched into the surfaces of Permalloy films using ion beam
lithography. MNPs were then deposited onto the films' surfaces under a parallel
or perpendicular magnetic field. The distributions of particles in the
nanostructures were analysed to obtain the capture efficiencies of each
structure which indicate the proportion of particles trapped inside. Without an
external field, the highest efficiency was displayed by arrays of circular
antidots with circular dot arrays displaying the lowest. Antidot arrays
displayed higher capture efficiencies than dot arrays. Addition of a field
parallel to the surface significantly increased the capture efficiencies and
addition of a field perpendicular to the surface decreased the efficiencies.
Under the perpendicular field, the particles were instead caught on the outer
edges of the nanostructures. These results suggest that application of a
parallel external magnetic field promotes the capture of MNPs within
ferromagnetic nanostructures and a perpendicular field increases the capture of
MNPs onto the outer surface and edges of nanostructures.
