Formation of FePt nanoparticles in annealed FePt/C multilayers

Abstract

The formation of FePt nanoparticles by annealing FePt/C multilayer in the range of 600–800 °C was studied. During annealing at 700 °C, the coercivity of FePt (5 Å)/C (5 Å) sample increases from 0.2 to 6 kOe after 2 min of annealing, reaching 14.5 kOe after 1 h, while the coercivity of FePt (5 Å)/C (20 Å) sample shows a much slower change with annealing, exhibiting a value of 3.5 kOe after 1 h of annealing. Transmission electron microscopy results did not show a layer structure in as-deposited 5 Å/5 Å samples, and a clear superlattice reflection was observed in 5 Å/5 Å samples subjected to 2 min of annealing, indicative of the L10 fct phase. The degree of atomic ordering of the fct structure increases with further annealing. After 1 h of annealing, the 5 Å/5 Å samples show aggregates of particles with a well-ordered structure and a wide particle size distribution. In the as-deposited 5 Å/20 Å sample, a layer structure was observed with a uniform particle size distribution. After 5 min annealing at 700 °C, the layers are completely broken with the appearance of weak superlattice reflections. Both particle size and degree of atomic ordering increase slowly with further annealing in the 5 Å/20 Å sample. After 1 h annealing, isolated particles were observed with an average particle size of 6 nm. Depositing thicker carbon layers appear to restrict the growth of particles and delay the onset of atomic ordering with annealing.