Holmium Element Is The Key To Single-Atom Storage

Single-atom magnets show stability needed for data storage


Holmium magnets
Magnetic singles: individual holmium atoms light-up this STM image of a magnesium oxide surface. (Courtesy: Fabian Natterer/EPFL)

Individual holmium atoms adsorbed on magnesium oxide films can form highly stable magnets, according to a study done by Fabian Natterer at Switzerland’s Federal Institute of Technology in Lausanne (EPFL) and colleagues. The team showed that the single-atom magnets can endure relatively high temperatures and strong external magnetic fields. The work could lead to the development of extremely high-density data storage devices.

Increasingly, data centres are coming under strain as we produce more and more information. One potential solution could lie in single-atom magnets, on which bits of data could be stored as long-lived magnetic quantum states. Previous studies have shown that these states can be easily manipulated, allowing data to be easily written and read out from the atoms. Furthermore, densely packing many atoms onto a surface would allow for vast amounts of data to be stored.

“Single-atom magnets offer an interesting perspective because quantum mechanics may offer shortcuts across their stability barriers that we could exploit in the future,” says Natterer. “This would be the last piece of the puzzle to atomic data recording.”

Significant challenges

Single-atom magnets are still in the early stages of development, and the technology faces significant challenges relating to the thermal stability of the atoms’ magnetic quantum states. The coercivity of the magnets –  their ability to resist demagnetization in external magnetic fields – is also low, which is not appropriate for data storage.

In their study, Natterer’s team used a scanning tunneling microscope to observe individual holmium atoms adsorbed to a film of magnesium oxide. This system that had previously been identified as collection of highly stable, single-atom magnets.

To test the atoms’ ability to withstand demagnetization, the team first subjected them to external magnetic fields up to 8 T – which is about 100,000 times the strength of Earth’s magnetic field. Remarkably, the atoms retained their magnetization for many minutes – the highest coercivity yet observed in individual atoms.

Hot and cold

Next, the atoms were exposed to temperatures of over 45 K. Their magnetic states remained stable up to 35::K and began to align with an external field at above 45 K. Although this is about 260 degrees below room temperature, it is very hot for single-atom magnets and reveals an ability to resist thermal perturbations.


Tiny switch toggles the position of a single atom

While the holmium atoms adsorbed on magnesium oxide are remarkably stable for a system of single magnets, Natterer and colleagues acknowledge that further studies are needed before the system can be implemented in commercial data storage. “We have demonstrated that the smallest bits can indeed be extremely stable,” Natterer continues.  “Next, we need to learn how to write information to those bits more effectively to overcome the magnetic ‘trilemma’ of magnetic recording: stability, writability, and signal-to-noise ratio.”

The team also included scientists at Korea’s Institute for Basic Science and Ewha Womans University. The research is described in Physical Review Letters.

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