Hitachi, HGST Achieve 610Gb/inch2 HDD Recording Density w/ Existing Technology

Aug 4, 2008 19:35 Tadashi Nezu, Nikkei Electronics

Hitachi Ltd and Hitachi Global Storage Technologies Inc (HGST) achieved a surface recording density of about 610Gbit/inch² by making several improvements to the existing perpendicular magnetic recording technology for HDDs.

This surface recording density is about 2.5 times higher than that of the existing mass-produced HDDs, Hitachi said.

"There are some problems remaining, but we consider it possible to commercialize the technology in the not-so-distant future," the company said.

Hitachi and HGST plan to focus on the research and development of the new technology aiming at mass production. They did not specify the commercial production date but said, "The surface recording density of our existing products is about 250Gbit/inch². And it took us two to three years to develop this technology from the research and development stage. We aims to develop the 610Gbit/inch² technology in the same length of time."

The companies' achievement was evaluated by "747 curve," a common method to measure surface recording densities. In that method, a track density is calculated from the relationship between (1) the interval between adjacent tracks and (2) off-track capability (OTC).

This time, the linear recording density is 1,606kBPI, and the track density is 381kTPI. The magnetic recording width of the recording head is 80nm, and that of the reproducing head is 40nm. The magnetic coercive force of the vertical recording medium is 4.0kOe.

Three improvements realized 610Gbit/inch²

There are three major reasons why Hitachi and HGST could achieve the surface recording density of about 610Gbit/inch², the improvements of the magnetic head and recording medium, and the adoption of LDPC codes as error correcting codes.

First, the companies made improvements to the existing recording and reproducing heads. If a track width is narrowed to enhance a surface recording density, it is possible that the magnetism from the recording head rewrites or deletes the information recorded on the adjacent tracks. Therefore, the companies' existing product has the "WAS Structure," which covers the area surrounding the main pole of the magnetic head.

This time, Hitachi and HGST improved this structure so that it can deal with narrower track widths. A TMR element is used for the recording head.

For the recording medium, the companies used a vertical recording medium called "graded medium." In general, when the magnetic cluster of a medium is miniaturized to enhance its surface recording density, the thermal stability of the medium deteriorates causing rewriting and deletion of the data.

On the other hand, when a surface recording density is increased, larger magnetism is required to generate magnetization reversal. Therefore, on the recording layer of the graded medium, thermal stability-related anisotropy field is altered in accordance with the film-thickness direction to enhance both thermal stability and recordability.

The anisotropy field is smaller in the upper layers near the magnetic head and larger in the lower layers. When recording, magnetization reversal occurs in the upper layers first. Then, it travels towards the lower layers where it is difficult to generate magnetization reversal.

Low density parity-check (LDPC) codes require less amount of data compared with Reed-Solomon codes, which Hitachi and HGST used as error correcting codes before. By this method, the recording area of an HDD can be increased by about 4%, the companies said.

Therefore, they consider that if they use the new recording medium and magnetic head, and LDPC codes to develop an HDD, the surface recording density of about 635Gbit/inch² can be realized.

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