Adding Ag to FePt Nanoparticles Lowers the Temperature for L10 Ordering
Chemically synthesized FePt nanoparticles have attached considerable attention as future, high density magnetic recording media. A fundamental problem has been that the chemical synthesis gives the disordered A1 phase and the particles must be heated to temperatures above 550ºC to obtain the chemically ordered L10 phase with high magnetocrystalline anisotropy. However the heating also caused undesirable grains growth and this has led to a search for means of obtaining the L10 phase without grain growth. The Nanoparticles Team in our MRSEC was the first to develop the chemistry to prepare ternary alloys, FePtM, where M is Ag, Au, Co, Cu, Mn, Ni or Pd. The particles had a chemically disordered fcc structure with Fe, Pt and M randomly placed on the fcc lattice. Adding Ag or Ag lowers the temperature for chemical ordering; however for self-assembled films the addition also lowered the temperature for the onset of grain growth. A recently developed salt annealing procedure (J. P. Liu at U. T. Arlington) allowed us to heat the particles without sintering. We found that adding Ag intrinsically lowers the temperature required fro chemical ordering, while adding Mn had no effect. Further studies will seek to understand the mechanism of the temperature lowering and the fate of the Ag additive.