Magnetic field orientation and spatial effects on the retention of paramagnetic nanoparticles with magnetite

The magnetic force exerted on a paramagnetic nanoparticle by a 3-D array of magnetite particles was investigated to extend previous work that only involved one magnetite particle. The separation between the magnetite: particles in the array, the orientation of the magnetic field, and the distance between the nanoparticle and the surface of the magnetite array were studied. At magnetite particle separations of less than two magnetite diameters, a reduction in the net force on the nanoparticle compared to that associated with a single magnetite particle was realized due to the overlapping behavior of the magnetic fields of the magnetite array. The net force was still strong to overcome thermal (Brownian) motion and attract and retain the nanoparticle. The magnetite array also gives rise to retention zones at any orientation of the magnetite field; this was not true of a single magnetite particle, which exhibited repulsive zones depending on the orientation of the field. When the separation between the magnetite particles was greater than two magnetite diameters, the effect of the array was lost and the nanoparticle interacted essentially with only one of the magnetite particles. The close proximity of the magnetite particles and the associated smoothing effect translated into long-range interactions that leveled off with distance between the nanoparticle and the array. Although too weak to retain the nanoparticle, they were suspected to be strong enough to retain much larger particles approaching the size of the array. These effects could be explained only by the fact that when the magnetite particles were closer to each other they started behaving as a single large particle. Overall, these results suggest that both nanoparticles and particles of the size the array can be attracted to and retained by a magnetic matrix comprising clusters of small and similarly sized magnetite particles.