Scientists from the Max Planck Institute in Germany claim to have discovered the basis for the next generation of memory devices.
In collaboration with scientists from the Forschungszentrum Jülich, they have, for the first time, been able to observe directly how dipoles, which store information in ferroelectric materials, continuously rotate and therefore may be organised in circular structures. The findings were obtained using a type of high resolution transmission electron microscopy with especially sharp contrast, developed by the Jülich scientists. The team claims arranging the dipoles in a circular structure could allow for significantly denser data storage than previously possible, while still ensuring fast writing and reading processes. In addition, it believes it may be possible to achieve greater data density in them than previously assumed, meaning they could soon be the material of choice for working memories with a density of several terabits per square inch. The results were obtained through the use of a ferroelectric material produced at the Max Planck Institute of Microstructure Physics in Halle. The material, lead zirconate titanate (PZT), contains lead, zirconium, titanium and oxygen. Researchers Chun-Lin Jia and Knut Urban studied the sample of PZT using a particularly sensitive atomic resolution transmission electron microscope. In contrast to conventional transmission electron microscopes, this permitted the localisation of the oxygen atoms in the PZT, where they were otherwise almost impossible to detect due to their weak scattering yield. By determining the exact positions in the PZT sample of the oxygen atoms on the one hand, and the zirconium and titanium atoms on the other hand, the scientists identified the dipole orientation in all of the 250 unit cells. The sample consisted of a cross section of a PZT layer, which was approximately twenty unit cells thick. The ferroelectric material was deposited on a monocrystalline strontium titanate substrate which was additionally equipped with a thin marker layer of ruthenium oxide in order to better distinguish the interface between the ferroelectric film and the substrate. Even the boundaries between two domains with inverse polarisation could be detected accurately in the transmission electron microscopic image of the sample cross section. Where the domain boundary met the ruthenium oxide marker layer, the scientists from Jülich observed an unexpected additional domain measuring only a few square nanometres, in which the orientation of the dipole system continuously rotated at 180 degrees. The scientists called this a flux closure domain. "Such domains are known from magnetic materials and had been predicted in theory for ferroelectric materials", said Urban. "However, we are the first to have observed them directly". The team is now looking to examine the exact conditions under which the structures with circular polarisation form. "We already have ideas for new research along those lines", concluded Urban. "However, it will be a few years before we see data storage that can store several billion data points per square inch and that can write and read them as quickly as the memory devices currently on the market."