Orientational dependence of apparent magnetic susceptibilities of superparamagnetic nanoparticles in planar structured arrays: Effect on magnetic moments of nanoparticle-coated core–shell magnetic beads

A theoretical and experimental study of the interactions of magnetic nanoparticles within planar structured arrays indicates that the magnetization curves for these arrays depend on their orientations relative to externally applied magnetic fields. When the layers and magnetic field are co-directional, the apparent magnetic susceptibilities are enhanced, but a decrease is seen when the field is normal to the nanoparticle array. The magnetic response of a core–shell type bead coated with one or more ordered layers of magnetic nanoparticles was evaluated by treating the particles at any position on the surface as being in a planar array tangent to the surface of the bead at that position; the effective magnetic susceptibilities of the beads were estimated by integration over the bead surface of the orientation-dependent responses of these layers to the applied magnetic fields. The effective susceptibility of magnetite nanoparticle monolayer-covered beads can increase by up to about 30%, but the effects are reduced as the number of layers increases.