Structural and magnetic properties of Ni/NiOxide- and Co/CoOxide core/shell nanoparticles and their possible use for ferrofluids

Ni- and Co-nanoparticles of average sizes of about 4 to 10 nm were produced by “physical” means with the cluster source LUCAS based upon laser evaporation into seeding gas and adiabatic expansion into an UHV experimentation chamber. They were deposited and subsequently oxidized in situ at about 400 °C. The structures of the nanos were characterized by HRTEM and chemically analyzed by EFTEM. Clear core/shell structures were established with crystalline metallic core and crystalline oxide shell of final thickness of about 2 nm. In one sample it might be that the shell is amorphous. An analysis of magnetic properties by SQUID was performed at temperatures between 5 and 300 K and for three different states of oxidation. Surprisingly strong influences of magnetic defects in the nominally (but probably incompletely saturated) antiferromagnetic oxide layers were observed which result in additional exchange anisotropy, in drastic exchange bias shifts of the hystereses and, in one sample, in a second hysteresis loop contribution. In all samples we found suppression of the superparamagnetic state. The special sample with two loop contributions was numerically evaluated (by co-author H. Krenn). In the final chapter a method is proposed how to transform these “physically” produced magnetic nanoparticles into a ferrofluid of well known composition