Structural secrets of storage media

Physicists at the Forschungszentrum Jülich have explained the structure and processes in materials that form the memory of rewritable optical storage media, such as DVD-RAM and Blu-ray discs. Only the underlying principle was clear. The material can be switched selectively with a laser writing head from an ordered, crystalline form into a disordered state and back. These processes are accompanied by changes in reflectivity, which can be measured by laser. “The rearrangement of the atoms lasts only a few nanoseconds, i.e. is extremely fast. We asked ourselves what sort of structure makes this possible,” explains Dr. Robert Jones.

Jones and his team carried out simulations on the supercomputer JUGENE to find out what occurs when molten DVD material cools into an amorphous mass, as happens when information is written. They adjusted their model sequentially using measurements of the crystal structures which Japanese colleagues had made on the synchrotron SPring-8. The supercomputer JUGENE calculated the position and motion of 460 atoms during the 0.3 nanoseconds of the cooling phase. The computation took a total of four months. The results provide insights that no microscope can supply. Ring-like structures comprising four atoms are decisive for the rapid switching, and they occur in both the amorphous and the crystalline material. The presence of cavities is also important, so that the building blocks can rearrange quickly without breaking too many atomic bonds. “With this insight, we can approach the material design of new storage media much more systematically,” says Jones.