Structural and Magnetic Characterizations of Gold and Silica Doubly Coated $\gamma $-Fe$_{2}$O$_{3}$ Nanoparticles

An extensive structural and magnetic characterization has been carried out on the gold (Au) and silica (SiO$_{2})$ doubly coated $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles. The phase of the uncoated and Au/SiO$_{2}$ coated nanoparticles were studied by powder x-ray diffraction (XRD) and x-tray absorption spectroscopy (XAS), which show that the iron oxide particles before and after coating have the phase of $\gamma $-Fe$_{2}$O$_{3. }$The transmission electron microscopy (TEM) results indicate that the average sizes of the $\gamma $-Fe$_{2}$O$_{3.}$cores, the Au/SiO$_{2}$ coated Fe$_{2}$O$_{3 }$ nanoparticles, and the Au nanoparticles in the Au nanoshells are about 12 nm, 92 nm, and 7 nm, respectively. We have the following observations from the magnetization measurements: (i) both the uncoated and coated nanoparticles are superparamagnetic at room temperature (T), and the Au nanoparticles are diamagnetic at all temperatures, (ii) the blocking temperature is decreased by the Au/SiO$_{2}$ double coating, and (iii) the coercivity for both the uncoated and coated nanoparticles decreases almost linearly with T$^{ \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$ with the latter decreasing faster than the former.