Magnetic Storage Taken to the Atomic Scale
International team of scientists learns to read and write data on islands of atoms
PHOTO: Stefan Krause/University of Hamburg
14 September 2007—When it comes to data storage, there’s no such thing as too much. But hard drive makers are finding that as they try to pack more magnetic bits onto their discs, it becomes impossible to target just one with the magnetic field needed to write the data, because the field needed to change one bit would also rewrite its neighbors.
Scientists from the United States and Germany say they’ve come up with a new way of reading and writing data that does away with those sloppy magnetic fields. In today’s issue of Science , the researchers report that a particular type of electric current from a scanning tunneling microscope will switch the magnetic polarization of 100-atom iron ”nanoislands,” which take up only one six-hundredth the area of magnetic bits on a typical hard disc. The new method is still experimental: the data bits it creates are temporary, and the nanoislands must be cooled below the temperature of liquid nitrogen.
When left alone, each nanoisland normally fluctuates between two equally preferable magnetic states, like a compass that spends equal time pointing north and south. But the scientists were able to break the equilibrium by adding or subtracting electrons, all of which had the same value of a quantum property related to magnetism called spin. Passing a stream of electrons from the microscope tip to a nanoisland will make the nanoisland’s magnetic state point more often in the direction of the electrons’ spin. Reversing the current, drawing polarized electrons out of the nanoisland, makes the iron’s magnetic state tend to point opposite to the electrons’ spin.
By breaking the nanoislands’ magnetic equilibrium, you force them to act as tiny binary bits—physical structures you can set to one of two states, says Stefan Krause, a doctoral student at the University of Hamburg, Germany, and the study’s lead author.
”These are novel mechanisms of manipulating magnetic bits when they get very small,” says Jonathan Sun, an expert in magnetic storage at IBM who wasn’t involved in the study. He adds that although other groups previously showed that such manipulation is possible, Krause and his collaborators applied the technique to by far the smallest structure to date.
Bit size isn’t the only advantage of the new technique. Today’s hard drives need separate circuits for reading and writing data, but Krause’s group used the same tip of the microscope to do both. A quantum mechanics phenomenon, called tunneling, allows electrons to leap the nanometers from the microscope’s movable tip to the nanoislands. The tip writes data when it moves in close to the sample and enough polarized electrons flow to change the nanoisland’s polarity. From farther away, fewer electrons can make it across, and the resulting current is enough to read the polarity but not enough to change it. Reading is just a matter of measuring the current from the microscope tip. If the tip is over a nanoisland whose magnetic polarization matches the spin polarization of the electrons in the tip, more current will flow than if the nanoisland has the opposite polarization.
”It’s conceptually much simpler if you just use the same device for reading as you do for writing,” notes Andreas Heinrich, who researches nanomagnetics at IBM’s Almaden Research Center, in San Jose, Calif., and was not involved in the study. Heinrich says that although the discovery of spin-polarized currents has generated a flurry of research, previous experiments required the reading and writing structures to be integrated into the bit itself. Being able to control such small structures with an external device, he says, is ”a pretty big step.”
But don’t get your hopes up for a new type of hard drive just yet. Luis Berbil-Bautista, a postdoctoral researcher at the University of California, Berkeley, and co-author of the report, points out that the iron nanoislands are not true bits: equilibrium breaks only while electrons are flowing, and even then the polarity fluctuates back and forth.
”In our case, they are switching between the two states, and we are just changing the equilibrium to one side,” Berbil-Bautista says. For a storage device, ”you would want your bits to be stable.”
Applying their successful switching technique to stable bits is the next step for Hamburg University’s Krause. The stability of the iron nanoislands increases as you lower the temperature, so Krause plans on studying the structures at 25 Kelvin instead of the 55 Kelvin he used this time.
”We now showed that we can switch thermally activated islands, but it’s very important to show that we can also switch stable islands,” he says, because that’s how you really store data. He emphasizes that practical applications remain far in the future.
”The engineering may be very daunting, and whether it’s cost-effective or not remains to be seen,” says IBM’s Sun. ”But this is certainly a very exciting experiment.”
To Probe Further
See video MAGNETIC MOMENTS: Passing a stream of electrons from the tip of a scanning tunneling microscope (yellow) to 100-atom islands of iron (red and green discs) will make the island’s magnetic state (large arrow) point more often in the direction of the electrons’ spin (small green arrows). Reversing the current, drawing electrons out of the nanoisland, makes the iron’s magnetic state tend to point in the opposite direction.