Magnetic cores of passive components are required to have low hysteresis loss, which is dependent on the coercive force. Since it is well known that the coercive force becomes zero at the superparamagnetic regime below a certain critical size, we attempted to synthesize Ni nanoparticles in a size-controlled fashion and investigated the effect of particle size on the magnetic properties. Ni nanoparticles were synthesized by the reduction of Ni acetylacetonate in oleylamine at 220 °C with trioctylphosphine (TOP) as the capping agent. An increase in the TOP/Ni ratio resulted in the size decrease. We succeeded in synthesizing superparamagnetic Ni nanoparticles with almost zero coercive force at particle size below 20 nm by the TOP/Ni ratio of 0.8. However, the saturation magnetization values became smaller with decrease in the size. The saturation magnetizations of the Ni nanoparticles without capping layers were calculated based on the assumption that the interior atoms of the nanoparticles were magnetic, whereas the surface-oxidized atoms were non-magnetic. The measured and calculated saturation magnetization values decreased in approximately the same fashion as the TOP/Ni ratio increased, indicating that the decrease could be mainly attributed to increases in the amounts of capping layer and oxidized surface atoms.
Keywords: Ni; chemical reduction method; coercive force; nanoparticle; phosphine; saturation magnetization; superparamagnetism.