Monodisperse magnetic nanoparticles: Effects of surfactants on the reaction between iron acetylacetonate and platinum acetylacetonate

Abstract

Magnetic properties of monodisperse nanoparticles for ultrahigh density recording and biomedical applications are sensitive to their shape and size distributions. These attributes are, in turns, dictated by several parameters during the synthesis and heat treatments. In this work, monodisperse FePt-based magnetic nanoparticles around 5 nm in diameter were synthesized by the reaction between iron acetylacetonate (Fe(acac)$_{3})$ and platinum acetylacetonate (Pt(acac)$_{2}$) in benzyl ether. X-ray absorption near-edge structure (XANES) spectra agreed with transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) that as-synthesized nanoparticles were composed of Pt-rich nuclei and iron oxides. Whereas their composition and size was not sensitive to the variation in the amount of surfactants (oleic acid and oleylamine), the nanoparticles exhibited a larger variation in shape with the increase in each surfactant from 1.5 to 4.5 mmol. After annealing in argon atmosphere at 650$^{\circ}$C for 1 hour, the nanoparticles tended to agglomerate. Higher amounts of surfactants surrounded the nanoparticles apparently allowed more sintering because the decomposed carbon from the excess surfactants facilitated the reduction of iron oxides. More Fe in the large annealed particles then resulted in ferromagnetic properties. By contrast, the ferromagnetic behavior and the highest coercivity were obtained without such agglomeration in the case of annealed particles synthesized by using the minimum surfactants.