We demonstrate fabrication of novel magnetically controllable photonic crystals formed through the self-assembly of highly charged, monodisperse superparamagnetic colloidal spheres. These superparamagnetic monodisperse charged polystyrene particles containing nanoscale iron oxide nanoparticles were synthesized through emulsion polymerization. They self-assemble into crystalline colloidal arrays (CCAs) in deionized water and Bragg diffract visible light. The diffraction from these superparamagnetic CCAs can be controlled by imposition of magnetic fields, which readily alter the CCA lattice constant. We also observe magnetically induced self-assembly of these superparamagnetic particles into CCAs in media such as NaCl aqueous solutions and organic polar solvents, which normally do not permit spontaneous CCA self-assembly. We also find that magnetic fields can strain the face-centered cubic lattice of superparamagnetic CCAs polymerized within hydrogels.The lattice symmetry of this photonic crystal becomes tetragonal. The observed magnetically induced CCA self-assembly enables the development of novel photonic crystal materials and devices.

Photonic crystals consisting of monodisperse superparamagnetic colloidal particles have been synthesized. The particles (see Figure) are obtained by emulsion polymerization of styrene in the presence of freshly precipitated surface‐modified iron oxide nanoparticles. A magnetic field self‐assembles the particles and controls the diffraction wavelength and crystal parameters of the array.

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