Perpendicular recording: the next generation of magnetic recording. (Tape/Disk/Optical Storage).
For the past 40 years, longitudinal recording has been used to record information on a disc drive. In longitudinal recording, the magnetization in the bits on a disc is flipped between lying parallel and anti-parallel to the direction in which the head is moving relative to the disc. Each bit itself is made up of approximately 100 magnetic grains.
Increasing areal densities to allow greater capacities is no small task. Today it is becoming more challenging to increase areal densities in longitudinal recording--previous growth rates have typically been well over 100 percent annually, while the average is approximately 60 percent currently.
To increase areal densities in longitudinal recording, as well as increase overall storage capacity, the data bits on a disc must be made smaller and put closer together. However, there are limits to how small the bits can be made. If the bit becomes too small, the magnetic energy holding the bit in place may also become so small that thermal energy can cause it to demagnetize. This phenomenon is known as superparamagnetism. To avoid superparamagnetic effects, disc media manufacturers have been increasing the coercivity (the "field" required to write a bit) of the media. However, the fields that can be applied are limited by the magnetic materials from which the write head is made.
According to Dr. Mark Kryder, senior vice president at Seagate Research, longitudinal recording still has time left before reaching the superparamagnetic limit. "We expect today's longitudinal recording methods to take us beyond 100 gigabits per square inch in density. A great challenge, however, is maintaining a strong signal-to-noise ratio for the bits recorded on the media. When the bit size is reduced, the signal-to-noise ratio is decreased, making the bits more difficult to detect, as well as more difficult to keep stable."
Perpendicular recording is widely seen as the next method of recording that will be adopted to help push areal densities further. Dr. Kryder estimates that the switch to perpendicular recording will occur sometime between 100 and 200 gigabits per square inch areal density. In perpendicular recording, the magnetization of the disc-- instead of lying in the disc's plane as it does in longitudinal recording--stands on-end perpendicular to the plane of the disc. The bits are then represented as regions of upward or downward directed magnetization (see Figure 1).
The ideal perpendicular recording media will have an M-H loop with unity squareness (S) to avoid excess noise in the DC saturated state, a negative nucleation field in excess of the fields produced under the return pole of the recording head, a high anisotropy to provide thermal stability and a small average grain size and small intra-granular exchange to reduce transition jitter. These requirements to varying degrees have been met by two material systems used in recent demonstrations of perpendicular recording, CoCrPt alloys and Co/Pd (Co/Pt) multi-layers (see Figure 2).
The recording head for perpendicular recording consists of a single pole inductive write head with a suitable flux return path designed for high efficiency, low stray field sensitivity and sharp field gradient capable of writing on perpendicular media with coercivities in excess of 5000 Oe. In perpendicular recording, the media is deposited on a soft magnetic underlayer that functions as part of the write field return path and effectively produces an image of the recording head that doubles the recording field, thus enabling one to record bits at a higher density than longitudinal recording.
Using industry-standard test procedures, Seagate has achieved a recording density in perpendicular recording of 100 Gbits per square inch, at 700 kbpi by 143 ktpi, with a data rate of 300Mbits per second.
The demonstration was carried out with a hardware channel under realistic and stringent drive conditions that incorporated a full set of adjacent data tracks on multiple heads and media. Moreover, the heads used were fully integrated read/write heads specifically designed both for perpendicular recording with a magnetic write width of l50nm. The readback portion of the head is a conventional permalloy shielded, bottom spin valve sensor with a GMR ratio of 18 percent and a shield-to-shield spacing of 85nm. The media was a double-layered perpendicular structure designed for robust thermal stability.
"These demonstrations are evidence of the increasing maturity of the technology, which we intend to utilize in Seagate products in the future," said John Weyandt, Seagate senior vice president, Product and Process Development. "The shift to perpendicular recording highlights a technology roadmap for magnetic recording to continue to address the needs of an increasingly data-intensive, digital world."
Time of Perpendicular Productization
The shift to perpendicular recording will correlate with the need of higher densities as viewed in the marketplace. For the past several years, areal density demonstrations have grown in excess of 100 percent per year. Some slowdown of areal density achievements using longitudinal recording has occurred, with a current approximate average of 60 percent annual growth rate (see Figure 3).
The shift to perpendicular recording technology offers the potential for significant areal density increases. "At this time, we estimate that perpendicular recording methods may take us all the way to one terabit per square inch," Dr. Kryder continued. "When that level is reached, a single 3.5-inch disc will store over one terabyte of information." However, from the time of an areal density demonstration to productization can range from 18 months to 2 years or more. Current disc drives in production are at levels of nearly 50 gigabits per square inch. Following the time curve from scientific demonstration to productization, Seagate's 107gigabit per square inch demonstration using longitudinal recording that occurred in November 2001 may translate into available disc drive products in the 2004-2005 timeframe. Seagate's demonstration of perpendicular recording technology illustrates the viability of this technology, and is an important milestone in staging it for productization. In this regard, perpendicular re cording holds promise as the next step in recording technology, but also shows that longitudinal recording hasn't quite run out of steam.
[FIGURE 3 OMITTED]
Mike Covault is vice president of Seagate's Advanced Technology Integration Team (Scotts Valley, Calif.)
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|Publication:||Computer Technology Review|
|Date:||May 1, 2003|
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