Cobalt-based superparamagnetic nanorings

Remarkable optical and magnetic properties were observed in a Co-thiol-polymer composite. The Co and the thiol ligand used to passivate the Co surface, instead of generating isolated nanoparticles, grow into nanorings with a radius around 2 nm. These structures consist of small clusters of metallic Co connected by alkylthiol chains. These small nanorings self-assemble into larger nanorings of ∼10 nm, which in turn self-assemble into larger nanorings in the range of 100-1000 nm in size. We formed a composite by embedding the nanorings in a polymeric matrix. The resulting material presented interesting optical properties such as absorption and emission in the visible zone of the electromagnetic spectrum, and a strong nonlinear optical behavior. The absorption and emission are mainly due to the presence of an alkylthiol-cobalt complex; and the nonlinear optical behavior is due to the presence of the Co clusters inside the polymer matrix. These composites exhibit a superparamagnetic effect down to 250 K, and below this temperature presented features very similar to those observed in spin-glass-like systems. In the nanorings a remarkable phenomenon is that, in addition to the superparamagnetic behavior, we found that during measurements with an applied external field they act as giant aromatic macromolecules and a current circulates through them. The evidence of a current circulating through the nanorings in the presence of an external field tells us about the possibility to use these structures in the design of nanosystems. Our work opens new possibilities of using magnetic transition metals to fabricate nanostructured magnetooptical materials.