R&D: Step Closer to Single-Atom Storage

From EPFL (Ecole Polytechnique Fédérale de Lausanne)

Physicists at EPFL (Ecole Polytechnique Fédérale de Lausanne) used Scanning Tunneling Microscopy to successfully test the stability of a magnet made up of a single atom.

Left: Scanning Tunneling Microscope image of Holmium single-atom magnets.
Right: Cobalt helper atoms on magnesium oxide.

Despite the rise of solid-state drives, magnetic storage devices such as conventional hard drives and magnetic tapes are still very common. But as our data-storage needs are increasing at a rate of almost 15 million gigabytes per day, scientists are turning to alternative storage devices.

One of these are single-atom magnets: storage devices consisting of individual atoms stuck (‘adsorbed’) on a surface, each atom able to store a single bit of data that can be written and read using quantum mechanics. And because atoms are tiny enough to be packed together densely, single-atom storage devices promise enormous data capacities.

But although they are no longer science fiction, single-atom magnets are still in basic research, with many fundamental obstacles to be overcome before they can be implemented into commercial devices. EPFL has been at the forefront of the field, overcoming the issue of magnetic remanence, and showing that single-atom magnets can be used to read and write data.

In a new study published in Physical Review Letters, physicists at EPFL’s Institute of Physics have used Scanning Tunneling Microscopy to demonstrate the stability of a magnet consisting of a single atom of holmium, an element they have been working with for years.

“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 Fabian Natterer, EPFL who is the paper’s first author. “This would be the last piece of the puzzle to atomic data recording.”

The scientists exposed the atom to extreme conditions that normally de-magnetize single-atom magnets, such as temperature and high magnetic fields, all of which would pose risks to future storage devices.

Using a Scanning Tunneling Microscope, which can ‘see’ atoms on surfaces, the scientists found that the holmium atoms could retain their magnetization in a magnetic field exceeding eight Tesla, which is around the strength of magnets used in the Large Hadron Collider. The authors describe this as ‘record-breaking coercivity’, a term that describes the ability of a magnet to withstand an external magnetic field without becoming demagnetized.

Next, they turned up the heat: The researchers exposed a series of Holmium single-atom magnets to temperatures of up to 45 Kelvin, (-233.15°C), which, for single atoms, is like being in a sauna. The Holmium single-atom magnets remained stable up to a temperature of 35K. Only at around 45K, the magnets began to spontaneously align themselves to the applied magnetic field. This showed that they can withstand relatively high temperature perturbations and might point to the way forward for running single-atom magnets at more commercially viable temperatures.

“Research in the miniaturization of magnetic bits has focused heavily on magnetic bistability,” says Natterer. “We have demonstrated that the smallest bits can indeed be extremely stable, but 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.”

Other contributors :

• Institute for Basic Science (Korea)

• Ewha Womans University (Korea)

Funding :

Swiss National Science Foundation (FNS)

Article: Thermal and Magnetic-Field Stability of Holmium Single-Atom Magnets

Physical Review Letters has published an article written by Fabian Donat Natterer, Fabio Donati, François Patthey, and Harald Brune.

 (a) STM image of coadsorbed Ho and Co atoms on MgO/Ag(100). (b) Line profile of a Ho atom measured with a SP-tip at 150mV showing apparent height contrast between the two magnetic states, as opposed to the profile across the paramagnetic Co atom. © Pump-probe measurement of the Co atom demonstrating spin polarization of the tip along the surface normal. (d) Telegraph signal in apparent height due to magnetic switching of a Ho atom. The histogram indicates a preference for one magnetization direction, identifying the magnetic ground state being UP in the external field pointing UP [(a)–(d) Bz=4  T, TSTM=4.3  K, V=150  mV, I=50  pA; © Vpump=150  mV, Vprobe=40  mV].

Abstract: “We use spin-polarized scanning tunneling microscopy to demonstrate that Ho atoms on magnesium oxide exhibit a coercive field of more than 8T and magnetic bistability for many minutes, both at 35K. The first spontaneous magnetization reversal events are recorded at 45K, for which the metastable state relaxes in an external field of 8T. The transverse magnetic anisotropy energy is estimated from magnetic field and bias voltage dependent switching rates at 4.3K. Our measurements constrain the possible ground state of Ho single-atom magnets to either Jz=7 or 8, both compatible with magnetic bistability at fields larger than 10mT.”