Chemical ordering of FePt nanoparticle self-assemblies by rapid thermal annealing

Self-assembled 4nm FePt nanoparticle arrays were treated with rapid thermal annealing (RTA) process. The phase transformation from the chemically disordered face-centered-cubic structure of the as-synthesized FePt nanoparticles to the chemically ordered face-centered-tetragonal structure was realized by RTA, with both the annealing temperature and time being greatly reduced, as compared to conventional annealing. The onset of chemical ordering occurred at the annealing condition of around 400°C for only 5s. Temperature-dependent coercivity measurements revealed strong thermal effects of the nanoparticle assemblies. Curie temperatures (T_c) of the annealed assemblies were derived from the temperature-dependent hysteresis measurements. T_c of the annealed assemblies increases with increasing annealing temperature, but is about 200-300 K lower than that of bulk FePt (750 K). The drastic reduction of T_c may be attributed to size effect and partial chemical ordering of FePt nanoparticles.