Structural and magnetic properties of size-controlled Fe–Ni nanoparticles synthesized by diffusing Fe atoms into preformed Ni nanoparticles

Abstract

Fe–Ni nanoparticles are attractive candidates for magnetic cores of transformers and inductors that are operated at high frequencies. When applying Fe–Ni nanoparticles in magnetic cores, particle size must be controlled to enhance magnetic properties. A two-step synthetic method can be useful for fabricating size-controlled Fe–Ni nanoparticles. In the first step, Ni nanoparticles are synthesized by reducing Ni (II) acetylacetonate in oleylamine (OA) with trioctylphosphine (TOP). Thereafter, Fe (III) acetylacetonate is reduced and Fe atoms diffuse into the preformed Ni nanoparticles in OA. The size of the Fe–Ni nanoparticles depends on that of the Ni nanoparticles. However, no studies have conducted extensive investigations of their synthesis and magnetic properties with various particle sizes. Here, we attempted particle-size-controlled synthesis of Fe–Ni nanoparticles and investigated their magnetic properties. The particle size of the Fe–Ni nanoparticles was varied between 13.1. and 20.5 nm by varying the size of Ni nanoparticles. The saturation magnetization (M_s) gradually reduced with the decrease in particle size because of the increased amount of oxidized surface atoms and phosphides. Coercive forces (H_c) also decreased as the particle size decreased to 15 nm because of the increased effect of superparamagnetism, while the H_c greatly increased with the particle size reduction below 14 nm because of the ferromagnetism exhibited by the Fe layer owing to undiffused Fe atoms. Thus, using this synthetic method, Fe–Ni nanoparticles with a size of approximately 15 nm can be produced, which have a minimum H_c and are the most appropriate for magnetic cores.