One bit per atom: This would be the favourite principle for building the magnetic data storage devices of the future. A team including researchers from Karlsruhe Institute of Technology (KIT), the Max Planck Institute of Microstructure Physics in Halle and the University of Leipzig has now come much closer to achieving this. As they report in the current edition of the prestigious scientific journal Nature, they fixed an individual holmium atom to a metal surface so that the spin of one holmium electron remains stable for more than 10 minutes. The spin can be descriptively understood as a rotation direction of an electron, giving it a magnetic moment that can align itself in a particular direction in an external magnetic field. At present, a network of several hundred million atoms is necessary for a magnetic bit to remain stable enough for hard disk data to remain safe for years.

Individual atoms can store data: The image taken by a scanning tunneling microscope shows holmium atoms on a platinum surface. In this quantum system, the spins and thus the magnetic moments of individual holmium atoms remain stable for more than 10 minutes. This creates the basis for storing one data bit in an individual spin (KIT).

“One individual atom fixed to a substrate is usually so sensitive that it keeps its magnetic orientation for mere fractions of a microsecond (200 nanoseconds),” explains Wulf Wulfhekel from Karlsruhe Institute of Technology. Working with colleagues from the Max Planck Institute of Microstructure Physics, the University of Leipzig and the University of Halle he has now succeeded in extending this time by a factor of around one billion to several minutes. “This not only opens the door to denser computer storage devices, but could also lay the foundation for constructing quantum computers,” says Wulfhekel. Quantum computers are based on the quantum physical properties of atomic systems and could, at least in theory, solve numerous computing tasks many times faster than conventional computers.

In their latest experiment, the researchers placed one individual atom of the rare earth metal holmium onto a platinum substrate. At temperatures close to absolute zero, i.e. around minus 272 degrees Celsius, they used the fine tip of a scanning tunnelling microscope to measure how the spin of the atom and thus its magnetic moment aligns. They observed that it was almost ten minutes before the magnetic moment changed its direction. “So once the system has established its magnetic spin, it keeps it for a billion times longer than comparable atomic systems,” says Wulfhekel. The researchers used KIT’s innovative scanning tunneling microscope in their experiment. Its special cooling system for the temperature range close to absolute zero reduces vibrations to a particularly low level and allows long measuring times.

“In order to extend the spin flip times we have removed the interfering effect of the atomic environment,” explains Arthur Ernst, who researches and teaches at the Max Planck Institute of Microstructure Physics and the universities of Leipzig and Halle. The theoretical sumulations carried out by Ernst and his colleagues have made it possible to interpret the experiments of their Karlsruhe colleagues.

Normally, the electrons of the substrate and the atom interact frequently with each other on the quantum mechanical level and destabilise the spin of the atom in microseconds or faster. Holmium and platinum form a quantum system whose symmetry properties switch off the interfering interactions at very low temperatures. “Basically, holmium and platinum are mutually invisible, as far as the spin scattering is concerned,” says Ernst. With the aid of external magnetic fields, however, it is possible to align the spin of the holmium and thus to write information. This is precisely what the team of researchers now wants to attempt. If they are successful, this would lay the foundations for the development of compact data storage devices or quantum computers.