Silica coated nanoparticles: synthesis,magnetic properties and spin structure

In the recent years, magnetic nanoparticles have been extensively studied for their superparamagnetic properties providing useful labels in biology or for fundamental aspects including the size dependence of magnetic atomic moment and the effect of surface anisotropy. In most cases, the particles were smaller than 10 nm and interestingly, the sizes ranging between 10 and 100 nm have been poorly investigated until now. This is mainly due to the fact that usual chemical routes produce 5-10 nm oxide or metallic particles or eventually 20 nm at most. On the over side, atomization techniques yields particles in the micrometer range. Metallic particles are particularly interesting for better magnetic propeties compared to oxides, but they have two big drawbacks: they are not biocompatible and they are conducting electricity. Consequently, it's necessary to produce core-shell particles, for witch the shell is biocompatible and insulating and with a perfect control of thickness and uniformity of that shell. In this work, we are studying metallic particles synthetized by an original evaporation-condensation technique that produces particles of sevral tens of nanometers. We prepared hard magnetic cobalt particles and soft FeNi ones coated by a silica shell using a mofied sol-gel metod. Morphological and magnetic properties are presented, showing the efficiency of ultrasonic sol-gel process for that purpose.