Perpendicular recording: the next generation of magnetic recording - Tape/Disk/Optical Storage

Computer Technology Review,  May, 2003  by Mike Covault

• E-mail
• Print
• Link

As the demand for storage has increased dramatically over time, technologists nave worked toward increasing the amount of information that can be stored onto disc drives. By increasing the areal density--or the amount of information that can be placed within a given area on a disc drive--technologists in fact have been able to deliver densities in excess of 100 percent annually over the course of the last several years. A key end-result or benefit of this dramatic areal density curve is that disc drive manufacturers have also been able to drive down the cost of the disc drives themselves because they can offer higher capacity disc drives using fewer platters, heads, and mechanical parts.

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.

Technical Details

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.