Abstract
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.
Highlights
Magnetic nanoparticles are being studied by a number of researchers owing to their potential applications in a variety of areas including magnetic devices, magnetic fluids, magnetic recording media, medical diagnosis, and hyperthermia [1,2,3,4]
For a reaction time of 15 min, 2.7 g of particles are obtained, whereas 3.5 g of particles are obtained for reaction durations of 30 min and 60 min. These results suggest that while Ni nanoparticles can be obtained at 15 min, more than 30 min is required to reduce Ni(acac)2 completely because 60 mmol of Ni corresponds to 3.5 g
The saturation magnetization decreased with reduction in the Ni nanoparticle size
Summary
Magnetic nanoparticles are being studied by a number of researchers owing to their potential applications in a variety of areas including magnetic devices, magnetic fluids, magnetic recording media, medical diagnosis, and hyperthermia [1,2,3,4]. Since hysteresis loss is dependent on the coercive force, a decrease in the size of the magnetic nanoparticles effectively decreases the hysteresis loss [7]. Since high operating frequencies enhance hysteresis loss, the use of superparamagnetic nanoparticles in passive components is expected to lead to low loss. Chemical reduction synthesis is advantageous because the particle size can be effectively controlled in this method by changing the capping and reducing agents and with inexpensive materials and apparatus [8,9]. A number of reports on the chemical reduction synthesis of noble metal nanoparticles have already been published because of the easy reduction of these ions and their high stability in the liquid phase. The dependence of the magnetic properties of these metals on the nanoparticle size needs to be examined
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