MIT Technology Review

Thursday, January 12, 2012

Magnetic Memory Miniaturized to Just 12 Atoms

A memory-storage element made at IBM Research points to future computing systems built atom by atom.

The smallest magnetic-memory bit ever made—an aggregation of just 12 iron atoms created by researchers at IBM—shows the ultimate limits of future data-storage systems.

The magnetic memory elements don't work in the same way that today's hard drives work, and, in theory, they can be much smaller without becoming unstable. Data-storage arrays made from these atomic bits would be about 100 times denser than anything that can be built today. But the 12 atoms making up each bit must be painstakingly assembled using an expensive and complex microscope, and the bits can hold data for only a few hours and at low temperatures approaching absolute zero, so the miniscule memory elements won't be found in consumer devices anytime soon.

As the semiconductor industry bumps up against the limits of scaling by making memory and computation devices ever smaller, the IBM Almaden research group, led by Andreas Heinrich, is working from the other end, building computing elements atom-by-atom in the lab.

The necessary technology for large-scale manufacturing at the single-atom scale doesn't exist yet. Today, says Heinrich, the question is, "What is it you would want to build on the scale of atoms for data storage and computation, in the distant future?"

As engineers miniaturize conventional devices, they're finding that quantum physics, which never had to be accounted for in the past, makes devices less stable. As conventional magnetic memory bits are miniaturized, for example, each bit's magnetic field begins to affect its neighbors', weakening each bit's ability to hold on to a 1 or a 0.

The IBM researchers found that it was possible to sidestep this problem by using groups of atoms that display a different kind of magnetism. The key, says Heinrich, is the magnetic spin of each individual atom.

In conventional magnets, whether they're found holding up a note on the refrigerator or in a data-storage array, the magnetic spins of the atoms are aligned. It's this alignment that leads to instability when magnetic-memory elements are miniaturized. The IBM researchers made their tiny memory elements by lining up iron atoms whose spins were counter-aligned.

The researchers both constructed and wrote data to the tiny memory elements using a scanning tunneling microscope, a device developed at IBM Zürich in 1981. This microscope has a very thin conducting probe that can be used to image a surface and push individual atoms around.

Heinrich says his team found it could make antiferromagnetic memory using fewer than 12 atoms, but these were less stable. With 12 atoms, the memory elements obey classical physics, and the read-and-write pulses applied through the microscope probe are similar to those used in today's hard drives. This research is described today in the journal Science.

Any realistic nonvolatile data storage technology has to be able to hold onto the data for 10 years at temperatures well over room temperature, says Victor Zhirnov, a research scientist at the Semiconductor Research Corporation, who was not involved with the work. The IBM bits can hold onto a 1 or a 0 for just a few hours, and only at very low temperatures, but Heinrich says it should be possible to increase their stability for operation at more realistic temperatures by using 150 atoms per bit rather than 12—still a miniscule number compared to existing forms of memory.

However, making a realistic technology was not the aim of the current work, says Heinrich. His aim is to explore whether other kinds of computing elements can be made from a few atoms, perhaps by embracing quantum. "We have to have the foresight not to worry about the next step, but to jump to something potentially revolutionary," he says.