Effect of annealing on magnetic properties of Ni ₈₀Fe ₂₀ permalloy nanoparticles prepared by polyol method

Ni ₈₀Fe ₂₀ permalloy nanoparticles with narrow size distribution and homogeneous composition have been prepared by the polyol processing at 180 °C for 2 h and their particle sizes can be tunable in the size range of 20∼440 nm by proper addition of K ₂PtCl ₄ agent. X-ray diffraction results show that the NiFe nanoparticles are of face centered cubic structure. The addition of K ₂PtCl ₄ does not affect the composition of NiFe NPs but decreases the particle size remarkably. Both saturation magnetization and coercivity of the as-prepared NiFe nanoparticles decrease with decreasing particle size. Annealed at 280 °C, however, the saturation magnetization of various sized NiFe nanoparticles increases drastically and approaches to the bulk for the ∼440 nm NiFe particles, and a maximum coercivity (∼270 Oe) happens at a critical size of ∼50 nm. The magnetic property dependency of these NiFe nanoparticles on annealing has been discussed by considering the surface chemistry.