Chemists from 8 different chemical institutions have joined forces to bring you this new recipe for magnetic nanorings (Jia et al. 2008 DOI). Dimensions: height 80 -120 nanometer , outer diameter 150 nm and inner diameter 70 nm and each one made out of a single-crystal piece of iron oxide. As often in this brand of nanotechnology you can do the chemistry in the kitchen sink. Chemical analysis on the other hand takes a battery of rather expensive equipment.
Ferric chloride together with bits of sodium dihydrogen phosphate and sodium sulphate are dissolved in water and placed in an autoclave at 220 °C. A hydrothermal process forms hematite (alpha-Fe2O3) with correct nanoring morphology. The interplay of ions determine the morphology. Whereas the phosphate ions facilitate anisotropic crystal growth and promote the formation of tubes, the sulphate ions facilitate growth around the faces of the dodecasonal prism building blocks. The acidity increases as the reaction progresses and dissolution of Fe2O3 starts at the core eventually creating the nanoring hole. Residence time in the autoclave is important, eventually the hole in the ring will disappear again.
In two more steps hematite is converted to magnetite by high temperature reduction and then to ferrimagnetic maghemite in an oxidation process. Electron holography shows that the magnetic field lines form circles (the lines do not venture outside of the rings) and that is exactly what you need for magnetic data storage where the clockwise and counterclockwise magnetic fields represent the 1 and 0 states.