Effect of annealing on magnetic properties of Ni80Fe20 permalloy nanoparticles with various sizes prepared by polyol method

Ni₈₀Fe₂₀ permalloy nanoparticles (NPs) 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 K₂PtCl₄ agent. X-ray diffraction results show that the NiFe NPs are of face centered cubic (FCC) structure. The addition of K₂PtCl₄ does not affect the composition of NiFe NPs but decreases the particle size remarkably. Both saturation magnetization and coercivity of the as-prepared NiFe NPs decrease with decreasing particle size. Annealed at 280°C, however, the saturation magnetization of various size NiFe nanoparticles increases drastically and approaches to the bulk for the ~440 nm NiFe particles, however, a maximum coercivity (~270 Oe) happens at a critical size of ~50 nm. The magnetic property dependency of these NiFe NPs on annealing has been discussed by considering the surface chemistry.