Phase transformation of FePt nanoparticles

To transfer face-centered cubic (fee) FePt nanoparticles to face-centered tetragonal (fct) structure with high magnetic anisotropy, heat treatments are necessary. The heat treatments, however, often lead to agglomeration and sintering of the nanoparticles. To prevent the particles from sintering, a new method, the salt-matrix annealing, has been adopted in our experiments recently. The fee nanoparticles produced by chemical synthesis were mixed with NaCl or other salt powders. The mixture was then annealed at temperatures up to 750 degrees C to complete the fcc to fct phase transition. After the annealing, the salt was washed out by water and monodisperse fct FePt nanoparticles free of non-magnetic impurities were obtained. Magnetic measurements show very high coercivity (up to 30 kOe) of the monodispersed fct nanoparticles. Size dependence of the phase transition and alignment of the ferromagnetic nanoparticles have also been investigated.