Storing data in single-atom magnets

As memory devices are becoming increasingly smaller, it was hypothesized whether the elementary storage unit could one day be as small as a single atom. A step towards that direction was made in 2016 when EPFL scientists demonstrated magnetic remanence for a collection of holmium atoms. However, this left the question as to how it would be possible to read from and write information to these single-atom magnets in ways that resemble conventional hard disc drives. Publishing in Nature, scientists from IBM and EPFL have conclusively shown the reading from and writing of data to single atom magnets.

Single-atom magnets

What is the smallest possible stable magnet? The answer is a single atom. Scientists have known for a while that certain atoms are magnetic when they are placed on a surface through a method called adsorption. However, despite tremendous research efforts, the magnetization of single atoms was never stable enough due to spontaneous fluctuations.

The key aspect here is something called “magnetic remanence”, which essentially refers to the atom’s ability to remain magnetized, similar to a bar magnet that retains its north and south poles over time. If we ever wanted to build atom-scale storage devices, remanence will be the crucial ingredient; we don’t have much use for encoding data into bits that will spontaneously lose their information.

Can we read from and write data to a single atom?

The study was lead by Fabian Natterer, in the course of an SNSF Ambizione grant (hosted by Harald Brune’s lab at EPFL). Guided by previous insights from Brune’s lab in 2016, Natterer and his colleagues at IBM aimed to describe the properties of single atom magnets and to demonstrate the control of the magnetic properties in a quest to answer the question: if a single atom can indeed be magnetized, how can we encode it in a meaningful way?

The scientists looked at single holmium atoms that they adsorbed on magnesium oxide surfaces. Holmium is a rare-earth metal that is used in some of the strongest magnets available today.

To read from and write to single-atom magnets the scientists used two high-precision methods based on a scanning tunneling microscope, which is essentially a sharp, atom-thick metallic tip that is scanned along a surface.

Writing was accomplished with the scanning tunneling microscope using pulses of electrical current to allow electrons to tunnel through the tip and to reverse the magnetization of the holmium atoms. To read the state of the holmium bits, the scientists relied on a phenomenon called “tunnel magnetoresistance”, which enabled them to see the direction of the Ho atom’s magnetization.

They also double-checked their results using a novel technique based on single-atom electron spin resonance, which has been published separately in a paper in Nature Nanotechnology.

Using these methods, the scientists showed how the magnetic remanence of single atoms can be used to store information in them. The data remained stored in these single atom magnets over many hours and showed that single-atom memory is possible.

“Common hard-disk technology achieves a data density of about one terabit per square inch; using single-atom magnets, we could reach a thousand times that density,” says Fabian Natterer. But when will this happen? “It’s hard to say. But the numbers already show that we are actually really close to the fundamental limit of classical storage technology, only three more orders of magnitude.”

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This study was carried out at the IBM Almaden Research Center (San Jose, California). The lead author has now joined EPFL’s Institute of Physics in the course of an Ambizione grant from the Swiss National Science Foundation. The study also received contributions from the University of Chinese Academy of Sciences, the University of Göttingen (Germany), the University of Zürich, the Center for Quantum Nanoscience (Seoul), and Ewha Womans University (Seoul).

The study was funded by the Office of Naval Research, the Swiss National Science Foundation (SNSF), the National Natural Science Foundation of China, the Natural Sciences and Engineering Research Council of Canada, and the German academic exchange service.

Reference

Fabian D. Natterer, Kai Yang, William Paul, Philip Willke, Taeyoung Choi, Thomas Greber, Andreas J. Heinrich, Christopher P. Lutz. Reading and Writing Single-Atom Magnets.Nature 08 March 2017. DOI: 10.1038/nature21371