Synthesis of low-coercivity ordered-phase Fe-64%Ni nanoparticles via diffusion-based method

Abstract

FeNi nanoparticles are promising candidates for the magnetic cores of transformers and inductors operating at high frequencies. When applied to magnetic cores, reducing the coercive force (Hc) is crucial to suppress heat generation. Face-centered cubic FeNi is a low Hc material with two phases: a disordered phase and an ordered phase. The disordered phase exhibits the lowest Hc in Fe-78.5%Ni, while the ordered phase has the lowest Hc in Fe-64%Ni. These phases can be controlled through specific heat-treatment conditions. However, an in-depth investigation into the relationship between heat treatment, crystal structure, and Hc of FeNi nanoparticles has not been conducted until now. In this study, we elucidated the changes in the crystal structure, and Hc of Fe-64%Ni and Fe-78.5%Ni nanoparticles synthesized through a diffusion-based liquid-phase method employing heat treatment to reduce Hc. Initially, the Fe-78.5%Ni nanoparticles demonstrated low Hc but also low saturation magnetization (Ms). Upon performing heat treatment to enhance Ms, an increase in Hc occurred due to the transformation from the disordered phase changes to the ordered phase. Consequently, achieving both low Hc and high Ms proved challenging. Conversely, Fe-64%Ni nanoparticles exhibited high Hc and Ms in their synthesized state. However, through heat treatment, Hc could be lowered owing to the disordered-ordered transition. Therefore, by controlling the composition and crystal phase, FeNi nanoparticles with low Hc and high Ms can be obtained. Notably, Fe-64%Ni with the ordered phase heat-treated at 250 °C exhibited the lowest Hc.