Structural and magnetic properties of gold and silica doubly coated γ-Fe₂O₃ nanoparticles

Extensive structural and magnetic characterization measurements were carried out on gold and silica doubly coated γ-Fe₂O₃ nanoparticles, which were recently demonstrated to have an efficient photothermal effect and high transverse relaxivities for MRI applications. Powder X-ray diffraction and X-ray absorption spectroscopy show the phase of the uncoated and coated nanoparticles to be that of the γ-Fe ₂O₃ structure. The sizes, structure, and chemical compositions of the nanoparticles were determined by transmission electron microscopy. The magnetization results indicate that coating of the iron oxide nanoparticles by gold/silica decreases the blocking temperature from 160 to 80 K. Such a decrease can be well-explained by spin disorder, causing reduction of the effective volume of the γ-Fe₂O₃ core. Moreover, it was found that in the temperature (T) range between 100 K and room temperature, the gold/silica coating can cause a slight magnetic change in the γ-Fe₂O₃ cores from superparamagnetic to almost superparamagnetic. Finally, it was found that the coercivity for both the uncoated and the coated nanoparticles decreases almost linearly with T ^1/2 with the former decreasing faster than the latter, and this coercivity result confirms that the blocking temperature is decreased by gold/silica coating. These results are valuable for evaluating the future applications of this class of multifunctional, hybrid magnetic nanoparticles in biomedicine.