"We have answered the question, how small can we make a magnetic structure and still be able to use it?" Andreas Heinrich, an IBM researcher who worked on the project, says. The answer that the scientists give in the journal Science today: You can make a bit of data from just 12 atoms.
A bit, the most basic unit of digital computing, stores information as either a 1 or a 0. In a typical hard drive today, a single bit is made up of nearly 1 million atoms, and companies are in a constant race to make that number smaller, compressing information into smaller and smaller areas each year.
"Our mission is to start with the ultimate end of Moore?s law and do something interesting," Heinrich says. Moore?s law is the prediction made by Intel?s Gordon Moore in the 1960s that the number of transistors on a computer chip would double every two years. Transistors aren?t bits, but the rate of shrinkage in storage size is similar to that of computer chips. So the researchers went with the prediction to its logical conclusion?atoms.
Heinrich and colleagues started off with a single iron atom and tried to see if they could reliably store information on it. One atom alone was too unreliable, but when they got up to 12 atoms in a bit, they were able to store information for hours at a time in assemblages of 8 bits (a typical byte). By reversing the configuration of the electron spins within a 12-atom bit, the team could differentiate between a 1 and a 0.
The key to this experiment is that the iron particles weren?t magnetized in the same way your refrigerator magnet is. Those kinds of magnets, and the ones used in modern hard drives, are ferromagnets, materials made of atoms whose electrons are all spinning in the same way. In Heinrich?s tiny bits, though, the iron atoms are lined up in rows and linked by antiferromagnetism, which just means that each atom is spinning in the opposite direction from its neighbors.
Antiferromagnets have distinct benefits when it comes to digital storage. Because the atoms have opposite spins, the magnetic fields at the atomic level cancel out, leaving the material itself without a distinct outside magnetic field. In an ordinary storage device made of ferromagnets, the device can only get so small before the magnetic field around each bit starts interfering with its neighbors. But with antiferromagnets, there are no pesky overarching magnetic fields to worry about at that small scale. That means that computer engineers could pack more bits into smaller spaces, and that antiferromagnetic storage would be less susceptible (though not totally impervious) to influence from outside magnetic fields. (Powerful magnets can destroy the memory on a normal hard drive.)
Ramamoorthy Ramesh, an engineering professor at UC Berkeley who has worked with antiferromangets, said he was impressed with the new study. "If you look at storage devices everyone wants to do more with a very small amount of area," Ramesh says.
As huge as this tiny development is in the world of computing, don?t expect atomic hard drives to appear in the store at any point soon. The bytes that the scientists assembled were only stable for a few hours at temperatures barely above absolute zero: about 0.5 Kelvin, or -458 degrees Fahrenheit.
"At low temperatures the atoms hold still and you can move them where you want them," says Sebastian Loth, lead author of the paper.
The magnetic bonds between the atoms, which were spaced out on a copper nitride base to make them easier to detect, got weaker at warmer temperatures. But Loth and Heinrich think that this problem would be overcome with simply a larger volume of atoms. If they scaled up from 12 to about 150 atoms in a bit, they estimate, then the bit could be stable at room temperature. (Assembling a bit out of 150 atoms would take more time than the bit they built from a dozen atoms, a process the researchers estimated at two days with their scanning tunneling microscope.)
Heinrich acknowledges that this small-scale production is far from the mass market. "If you really want to build something like this you need to figure out how to manufacture something atomically, cheaply," he says. But while atomic-scale hard drives are a thing for the future, Heinrich predicts that larger antiferromagnets could be used in storage devices much sooner, even as early as they next 5-10 years.
Maybe someday, dropping a magnet near your computer won?t incite panic.
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