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Hard-disk-drive (HDD) manufacturers are hard at work on ways to pack multiple terabytes (Tbytes) of storage onto a single device. At October’s Perpendicular Magnetic Recording Conference in Tokyo, two key players in the HDD market detailed their latest work in expanding storage capacity. Hitachi Global Storage Technologies (GST) revamped its readheads to make possible a 4-Tbyte drive, while Western Digital shaped current read-head technologies to deliver 3-Tbyte drives. The companies are taking different approaches to increasing areal density, which now maxes out in devices with 1 Tbyte of storage (see the figure).

Back to the drawing board
The key to Hitachi’s goal lies in overhauling its read-heads, which it hails as the world’s smallest. In a hard-disk drive, a metal disk spins while an arm with an electromagnetic head detects information about the magnetism of the particles on the disk. Drive heads, however, need to be shrunk as areal density increases. Electrical resistance increases as heads become smaller, leading to increased noise output and potential signal degradation.

Hitachi resolved the problem by replacing today’s dominant tunneling magnetoresistive (TMR) technology with giant magnetoresistive (GMR) technology. Where current drive heads can read information on tracks that are 70 nm apart, Hitachi’s drive heads—based on current perpendicular-to-the-plane GMR (CPPGMR) technology—can read information on tracks that are 50 nm and eventually 30 nm apart.

CPP-GMR technology has a lower electrical resistance level than TMR, due to its reliance on metallic rather than tunneling conductance. TMR technology relies on electrical tunneling to produce logic 1’s and 0’s, where electrons tunnel through an insulating layer that sits between two magnetic layers. In GMR, a conductor like copper replaces the insulating layer.

GMR technology is making somewhat of a comeback in HDDs. In 1997, IBM employed the first GMR heads in its Deskstar 16GXO, but the industry moved on to TMR drives over the years. Now, some researchers say TMR heads may not work reliably at areal densities above 500 Gbits/in.² So companies instead may revisit GMR, a property that two European physicists won a Nobel Prize last month for discovering in 1988.

TMR still on top
TMR technology, however, still dominates the HDD market. Hitachi delivered the first 1-Tbyte HDD earlier this year at densities of 148 Gbits/in.² based on TMR. Also, the technology is found in today’s prevalent 200-Gbit/in.² densities.

Western Digital is continuing to use TMR in the 520-Gbit/in.² densities it detailed at the Perpendicular Magnetic Recording Conference. To do so, it had to scale down current TMR read-head sizes to correspond with track width, which shrinks with each progressive generation of areal density.

“[Using TMR read-head technology] is important because it illustrates the extendibility of TMR head technology generations into the future,” said Hossein Moghadam, chief technology officer for Western Digital.

“The hard-drive industry has repeatedly proven that existing technology often can be refined and extended for further than originally anticipated,” he added. “Our demonstration of 520 Gigabits per square inch using our own perpendicular magnetic recording/tunneling magnetoresistive head technology is a perfect example.”

The company’s anticipated 3-Tbyte devices will be contained in a 3.5-in. hard drive that stores 640 Gbytes per platter. While this is achievable with the TMR read-head technology, Moghadam said, Western Digital will likely move to GMR for future solutions. A select number of its devices currently use GMR technology.

Western Digital’s 3-Tbyte drives are expected to be on the market in 2010, while Hitachi’s 4-Tbyte desktop drives and 1-Tbyte laptop drives will be out in 2011.