A micellar route to ordered arrays of magnetic nanoparticles: From size-selected pure cobalt dots to cobalt-cobalt oxide core-shell systems

Starting with Co-salt-loaded inverse micelles, which form if the diblock copolymer polystyrene-block-poly(2-vinylpyridine) is dissolved in a selective solvent like toluene and CoCl2) is added to the solution, monomicellar arrays of such micelles exhibiting a significant hexagonal order can be prepared on top of various substrates with tailored intermicellar distances and structure heights. In order to remove the polymer matrix and to finally obtain arrays of pure Co nanoparticles, the micelles are first exposed to an oxygen plasma, followed by a treatment in a hydrogen plasma. Applying in-situ X-ray photoelectron spectroscopy, it is demonstrated that: 1) The oxygen plasma completely removes the polymer, though conserving the original order of the micellar array. Furthermore, the resulting nanoparticles are entirely oxidized with a chemical shift of the Co 2P(3/2) line pointing to the formation Of Co3O4. 2) By the subsequent hydrogen plasma treatment the nanoparticles are fully reduced to metallic Co. 3) By exposing the pure Co nanoparticles for 100 s to various oxygen partial pressures p(O2), a stepwise oxidation is observed with a still metallic Co core surrounded by an oxide shell. The data allow the extraction of the thickness of the oxide shell as a function of the total exposure to oxygen (p(O2) x time), thus giving the opportunity to control the ferromagnetic-antiferromagnetic composition of an exchange-biased magnetic system.