Heating Up Magnetic Memory

Demand for data storage keeps going up, even while consumers expect the cost per bit to keep going down. However, the magnetic-recording materials used in today’s hard disks are reaching their storage limits and will probably max out within five years. To compete with newer technologies such as flash, the companies that make them need something new.

Now researchers at Seagate have demonstrated the feasibility of a new technology that could extend the capacity of magnetic data recording for many years more. Called heat-assisted magnetic recording, it involves blasting the magnetic regions of a disk with heat to make it possible to use more stable recording media. It should make it possible to record data at densities 50 times greater than will be possible when today’s technologies reach their limits.

“Within a few years, the magnetic-recording industry is going to have to find a new way forward,” because the materials currently used are nearing their physical limits, says Randall Victora, a professor of electrical and computer engineering at the University of Minnesota.

The hard drive inside most computers is made up of one or more spinning disks coated with a magnetically sensitive film consisting of tiny, jagged grains. Data is recorded when a small head moves over the disk, flipping the magnetization of one of these grains so that it either points up or down, for a 1 or a 0.

“As we make the storage density greater, we have to make the grains smaller,” says Ed Schlesinger, head of the department of electrical and computer engineering at Carnegie Mellon University, in Pittsburgh. “But you reach a point where the grains get so small, they become unstable,” and their magnetic state can be altered by small temperature fluctuations.

The problem cannot be overcome simply by switching to more stable recording media because today’s recording heads can’t write to them. So Seagate has been developing magnetic-recording heads that integrate a heating element. Blasting more magnetically stable grains with a short pulse of heat makes it much easier to flip them. When the media cools down again, the data is “frozen.”

Heat-assisted magnetic recording still presents a tremendous scientific and engineering challenge, though. The heat is provided by a rapid laser blast that must be focused down to a spot the size of an individual grain–less than 100 nanometers in diameter. This is impossible to do using conventional optics. Instead, it requires a new generation of optics that work in what’s known as the near field. The Seagate technology uses optical antennas, which can focus light energy onto areas smaller than any lens-based instrument can.

Researchers at Seagate have now demonstrated that heat-assisted magnetic recording can be done reliably. They used a magnetic-writing head outfitted with near-field optics to write data to a hard disk coated with stable recording media. Today in the journal Nature Photonics, the researchers describe their system and report recording data at densities of 250 gigabits per square inch.

This density only matches that of the hard disks found in today’s laptops. But that’s not the point, say researchers. “This is a tour de force in the science and engineering of this technology,” says Schlesinger.

The Seagate prototype is made almost entirely out of components that are found in today’s hard drives, says Ed Gage, executive director of research on recording systems at the company. The prototype uses a different recording medium than do today’s hard disks, but it can be laid down using the same processes already employed in the industry. Likewise, the writing head is the same as those already being made by the company, except for the addition of the optics.

The company now plans to bring the recording density up. “The experimental system needs additional engineering work,” says William Challener, another researcher on the Seagate project. The size of light achieved in the prototype was about 70 nanometers; other researchers have demonstrated 20 nanometers in the lab, and the company hopes to match this. There also remains some work to be done on integrating an electronic control system for the laser into a hard drive.

Meanwhile, others are working on a second technology for boosting magnetic storage. This approach, called bit patterning, involves increasing the density and stability of magnetic bits by creating patterned arrays of very regularly shaped, nanoscale magnetic grains.

“These approaches each have very different strengths and weaknesses,” says Barry Schechtman, executive director emeritus of the Information Storage Industry Consortium. “But there’s a strong consensus that five to ten years out, only one won’t be enough. We’ll need a combination of bit patterning and heat-assisted magnetic recording.”