Self-assembly and magnetism in core-shell microspheres

We report on the fabrication and characterization of magnetic composite colloids with a defined shape, composition and multilayer shell thickness. They consist of a core of a polystyrene microsphere (640 nm diameter) coated with consecutive shells of Fe3O4 nanoparticles (12 nm diameter), polyelectrolytes and Au nanoparticles (15 nm). The homogeneity of the coating was confirmed by transmission electron microscopy. Composite core-shell microspheres were self-assembled into periodically ordered chains up to 2 mm in length by magnetophoretic deposition. The self-organization was visualized by optical and atomic force microscopy. Magnetic properties were determined by angular dependent ferromagnetic resonance (FMR) and superconducting quantum interference device magnetometry between 5 and 300 K. The FMR reveals long-range magnetic order at 300 K due to dipolar coupling and an easy axis in plane along the chains. We find a magnetic moment that is reduced in comparison with the magnetite bulk value. (C) 2003 American Vacuum Society.