In the drive to cram ever more information into handy data-storage devices, researchers have reduced the size of a bit of data to the ultimate limit—a single atom (Nature 2017, DOI: 10.1038/nature21371). The study may lead to ways of increasing the data-storage density of devices such as computer hard drives, in which information is encoded in magnetic materials.
To store data on a computer hard drive, a device known as a read-write head rapidly magnetizes nanometer sized regions of the hard disk. That process sets the magnetic polarities of these domains or bits in one of two states, corresponding to the zeros and ones of digital data. The head reads the data by sensing the magnetic state of the domains.
For decades, hard-disk manufacturers have been increasing their devices’ data-storage densities by gradually shrinking the size of the magnetic domains in which data are stored. One challenge has been ensuring that the microscopic domains are magnetically stable. If the polarity of a domain spontaneously flips, data will be lost. That problem is related to the domains’ composition and size, as well as their thermodynamic properties.
The domains in today’s commercial devices, which have lateral dimensions in the nanometer range and are a few atomic layers thick, typically consist of hundreds of thousands of atoms. Meanwhile, various researchers have continued to shrink that bit size to just a handful of atoms in lab demonstrations.
By using a custom-made scanning tunneling microscope (STM) at ultralow temperatures and under ultrahigh vacuum, the team isolated a few holmium atoms on a magnesium oxide film and applied brief electric pulses to set the atoms’ magnetic states, or spins. Then the team detected the orientation of the spins via an STM technique known as tunnel magnetoresistance, which showed that the spins can be switched at will and remain stable for several hours. To confirm for themselves that they had achieved spin switching, the researchers placed one iron atom near the holmium atoms and used it as a magnetic sensor in a single-atom electron spin resonance measurement.
Describing the experimental techniques at the heart of the study as “ingenious,” Roberta Sessoli, a specialist in magnetic materials at the University of Florence, asserts that the IBM team has “unambiguously achieved the ultimate limit of writing and reading information.”
The complexity of the method means that the work is still far from real-world applications, she notes, yet it shows that it is possible to store and retrieve magnetic information with a single atom.