Magnetic Characterization of Ferrite Nanoparticles

Magnetic nanoparticles (NPs) of different compositions (FeO/Fe$_{3}$O$_{4}$, g-Fe$_{2}$O$_{3}$, FePt, and CoFe$_{2}$O$_{4})$ have been synthesized using high temperature organometallic routes described elsewhere. NPs (16.6 nm in diameter) of a mixed FeO/Fe$_{3}$O$_{4}$ (wuestite/magnetite) composition were prepared by thermal decomposition or iron oleate in the presence of oleic acid as a surfactant in dodocane at 370C in argon atmosphere. After the thermal treatment of the reaction solution at 200 C under air for 2 hours these NPs are transformed into maghemite (g-Fe$_{2}$O$_{3})$, the magnetization of which is significantly enhanced. NPs of CoFe$_{2}$O$_{4}$ (8 nm) have been prepared by simultaneous decomposition of Co(II) and Fe(III) acetylacetonates in the presence of oleic acid and oleylamine. The X-ray diffraction profile of these NPs is characteristic of cobalt ferrite. Alternatively, alloyed 1.8 nm FePt NPs prepared by simultaneous decomposition of Fe and Pt acetylacetonates in the reductive environment demonstrate a completely disordered structure, which is reflected in their magnetic properties. SQUID magnetometry was used to measure the magnetization of NPs at high and low temperatures. Zero-field cooling and field-cooling measurements were taken to demonstrate superparamagnetic behavior and an associated blocking temperature.