Annealing effect on microstructure and magnetic properties of flake-shaped agglomerates of Ni-20wt%Fe nanopowder

The present study has been performed to develop a flake-shaped Ni-20wt%Fe magnetic nanopowder agglomerate for the application of electromagnetic absorption media in a high frequency range. The spherical agglomerates with 10 μm in diameter consisting of 30 nm nanoparticles were deformed into thin plate-like powders of 1 μm in thickness by milling and then annealed at various temperatures. The structural analysis revealed that with elevating annealing temperature the crystallite size was increased by sintering effect. In magnetic properties, the saturation magnetization was not changed regardless of milling and annealing conditions. On the other hand, the milling treatment induced nanopowder agglomerates to have shape anisotropy. The coercivity was increased due to the generation of internal stress but it vanished by annealing. It is concluded that the flake-shaped nanopowder agglomerates can be easily magnetized and demagnetized to incident magnetic wave by post annealing treatment.