Magnetic field-induced growth and self-assembly of cobalt nanocrystallites

The growth and assembly behavior of cobalt magnetic nanocrystallites under an external magnetic field were studied. Co polycrystalline wires with an average length of 2 mm and diameter of 13 µm were formed by the self-assembly of Co nanocrystallites (15 nm on average) under the induction of a 0.25 T external magnetic field. The wires were nearly parallel because their axes were all parallel to the magnetic line of force. The Ms and Hc values of the sample, 111 emu g−1 and 389 Oe, are higher than those of the sample prepared without an external magnetic field applied (91 emu g−1 and 375 Oe), which might be associated with the special nanostructure in which Co nanocrystallites were arranged in polycrystalline wires acting as permanent magnetic dipoles. The process could be used to fabricate large arrays of uniform wires of some magnetic materials and improve the magnetic properties of nanoscale magnetic materials.