Effects of Annealing on the Magnetic and Structural Properties of FePt Nanoparticles by Chemical Synthesis*

In this paper, we have studied the effects of annealing atmospheres on the particle size, the degree of ordering, and the magnetic properties of FePt nanoparticles made by chemical synthesis. Measurements were made on particle assemblies with controlled dimension and thickness. Annealing was done under forming gas and vacuum atmospheres. Particles did not grow much after annealing at 550 $^{o}$C for 30 min under forming gas, resulting in an average particle size D $\approx $ 5 nm and a coercivity of 4 kOe. After annealing under vacuum using the same annealing temperature and time, the morphology of particles is similar to that of particles annealed under forming gas but with a coercivity less than 1 kOe. We believe that this large difference in coercivity is due to the fact that the forming gas not only assists the transformation from the chemically disordered fcc phase to the chemically ordered L1$_{0}$ phase but also makes the atomic composition of FePt nanoparticles more uniform. M\"{o}ssbauer studies showed a higher percentage of L1$_{0 }$phase in the sample annealed under forming gas than that under vacuum, which further confirms the hypothesis stated above.