Terabit-Per-Square-Inch Hard Drive Areal Densities

Terabit-per-square-inch hard drive areal densities, the goal for present magnetic recording research, will probably not be achieved with giant magnetoresistance (GMR) or with tunneling magnetoresistance (TMR) technology.

Much of applied physics is transduction--turning one thing into another thing--and in the case of GMR this means sensing the orientation (0 or 1) of tiny magnetic domains with heads consisting of a sandwich of alternating thin layers of magnetic and nonmagnetic atoms. The domain's magnetism is transduced into a change in electrical resistance.

The multi-billion-dollar GMR industry is successful [with commercial (areal) densities of about 15 Gb/in²] but, according to Stuart Solin of NEC Research Institute, may not exceed an areal density of about 150 Gb/in² because of limitations imposed by magnetic noise.

Solin's answer is to use the magnetoresistive effect in a nonmagnetic semiconductor-metal composite read head, which is immune from magnetic noise, has a faster response time than with GMR, and would entail a fabrication process of perhaps 20 steps, as opposed to 200-300 steps for GMR (or TMR, in which spins can "tunnel" from one orientation to another). He believes that GMR or TMR will continue to furnish efficient read heads until such time as higher areal densities are desirable, at which time alternative approaches, such as the nonmagnetic sensor, might be viewed favorably.

Solin (609-951-2610, solin@research.nj.nec.com) and his colleagues are reporting the first mesoscopic device to employ the "extraordinary" magnetoresistive (EMR) effect in a nonmagnetic sample-a Si doped InSb quantum-well structure (see figure). The areal density for this sensor exceeds 100 Gb/in², while the power signal-to-noise ratio is compatible with practical requirements in working read heads. (Solin et al., Applied Physics Letters, 27 May 2002.)