Effects of annealing on the magnetic and structural properties of FePt nanoparticles prepared by chemical synthesis

In this work, we have studied the effects of annealing on the magnetic and structural properties and atomic ordering of FePt nanoparticles made by chemical synthesis. Measurements have been made on particle assemblies with controlled dimension and thickness. Particles with an average particle size D≈5nm did not grow much after annealing at 550 °C for 30 min in forming gas (95% of N2 and 5% of H2) while they developed a coercivity of 3.95 kOe. When annealing the sample particles in vacuum, at the same annealing temperature and time, the particles became much bigger than those annealed in forming gas and had a much smaller coercivity of about 0.92 kOe. Our studies show that the reason for the large coercivity difference between the particles annealed in forming gas and vacuum is due to the fact that forming gas can promote the transformation from the chemically disordered face-centered cubic (FCC) phase to the chemically ordered L10 phase which is responsible for the high coercivity. Mössbauer studies showed a higher percentage of L10 phase in the sample annealed in forming gas than in vacuum. Also the average atomic composition distribution of particles annealed in forming gas was found to be more uniform than that in vacuum-annealed samples.