Nanoparticles*

Halide ion-mediated synthesis of L1₀ FePt has previously been used to form magnetically hard L1₀-FePt phase without the need of any post-annealing or 3^rd element addition [1,2]. In this work, we chemically synthesized L1₂ FePt₃ by the co-reduction of iron and platinum-based precursors in the presence of elemental iodine. Thermomagnetic data showed that the magnetic properties depend strongly on the degree of atomic ordering. The as-made structure showed the L1₂ phase but with a low degree of ordering. Magnetic measurements showed that the ordering temperature of the as-made sample is at 380 K. The chemical ordering increased to 0.71 when the samples were annealed at 700^oC for 30 minutes. The annealed samples showed a different ordering temperature of 251 K. Our data agreed with those reported by Margeat et al [3]. Room temperature hysteresis loops showed that the coercivity of our samples increased with annealing temperatures, up to 0.23 kOe for samples annealed at 700^oC. Thermomagnetic measurements from 50 K to 380 K also provide evidence of a phase transformation in the L1₂ FePt₃ nanoparticles, which we believe to be the transition from the AFM state to a FM state.