Soft Magnetic Properties of Agglomerates of Fe Nanoparticles Fabricated by Cold-Spray Technique

The development of soft magnetic devices requires high-performance soft magnetic materials, in addition to a high degree of design freedom. The aim of this study is to obtain excellent soft magnetic properties and a high degree of design freedom in molding using high-performance soft magnetic powders by additive manufacturing. Fe nanoparticles (FeNPs) have attracted significant attention as a soft magnetic material due to their high saturation magnetization and low coercivity. Achieving a high packing density without decreasing soft magnetic properties is a challenge when nanoparticles are used as the starting material. Bulk-like structures comprised of pure FeNPs with a high packing density are successfully fabricated using the cold-spray deposition technique. The estimated packing density of the particles is 96%, which is 24% larger than that of conventional compression molded samples at room temperature. Although the size of the FeNPs slightly increases after the post-annealing process, the grain growth is limited after cold spraying. The coercivity of the cold-sprayed (CS) sample was lower than that of the compression molded samples because of the higher packing density of the nanoparticles. From the observation of the magnetic domain, the exchange coupling length is estimated to be approximately 18.8 nm, and a comparison with the particle size indicated that coercivity may be slightly decreased due to averaged magnetic anisotropy energy. A further reduction in coercivity can be achieved by optimizing the molding and post-annealing conditions. The findings of this study are a first step of the manufacture high-performance soft magnetic devices.