Perpendicular hard drive explanation

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Show #58:

  • Date: 2006-05-15
  • Subject: Perpendicular hard drive explanation
  • Duration: 13:22
  • Format: MP3
  • Size: 5,525 kb

Topics Discussed:

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  • Patterned media
  • Holographic recording technology

Additional Information:

Perpendicular hard drive explanation.

For nearly 50 years, the disk drive industry has focused exclusively on a method called longitudinal magnetic recording, in which the magnetization of each data bit is aligned horizontally in relation to the drives spinning platter. Perpendicular hard drive recording will be able to deliver up to 10 times the storage density of longitudinal recording on the same piece of media.

Perpendicular recording was first demonstrated in the late 19th century by Danish scientist Valdemar Poulsen, the first person to demonstrate that sound could be recorded magnetically. Computer storage density is a measure of the amount of bits of information that can be stored on a given area of surface. Back in the 1980s there was some interest in using the system on floppy disks but it was never reliable. Now that current hard drive technology is reaching a fundamental limit there is a renewed interest in perpendicular drive technology.

Current technology with longitudinal recording has as expected limit of 100 to 200 gigabit per square inch due to the Superparamagnetic effect. Perpendicular recording is predicted to allow information densities of up to around 1000 Gbit/sq. inch. The Superparamagnetic effect is a phenomenon observed in very fine particles. Basically it is where the energy required to change the direction of the magnetic moment of a particle is comparable to the ambient thermal energy. At this level the rate at which the particles will randomly reverse direction becomes significant. This is important in hard disk technology because the superparamagnetic effect limits the minimum size of particles that can be used, and consequently the data densities possible. The result is flipped bits - bits whose magnetic north and south poles suddenly and spontaneously reverse and can corrupt data, rendering it and the storage device unreliable and thus unusable.

Perpendicular recording can be technically equal to the current generation of longitudinal devices but there are a number of technical challenges remain. For example, engineers at Hitachi GST are engaged in research to invent new kinds of read/write heads; to experiment with new materials that have enhanced magnetic properties and improved surface finishes; to maintain signal-to-noise ratios as the magnetic bits and signals become smaller; and to detect and interpret the magnetic signals using ever more advanced algorithms.

When designing magnetic hard drive storage media you want to retain the magnetization of the medium despite thermal fluctuations. If the thermal energy is too high, there may be enough energy to reverse the magnetization in a region of the medium, destroying the data stored there. The energy needed to actually reverse the magnetization of a hard disk region is proportional to the size of the magnetic region. As it turns out a larger magnetic region is more stable and there is a minimum size for a magnetic region at a given temperature. Perpendicular recording keeps the same region sizes as in standard magnetic media, but organizes the magnetic regions in a more space-efficient way.

Perpendicular hard disk recording can achieve a higher storage density because it aligns the poles of the magnetic elements, which represent bits, perpendicularly to the surface of the disk platter. By doing this type of bit alignment it takes less of the hard disk platter than what would have been required had they been placed longitudinally. They can be placed closer together on the platter, thus increasing storage density by a factor of 10.

These new hard drives involve recording data in vertical, three-dimensional columns rather than in two dimensions of the current hard drive technologies. This is like having people in a crowded city center move from single family homes to high rises. Although the shift to perpendicular recording methods greatly increases the amount of data that can be stored in a small space, it has forced the drive industry to put extra work into developing disk media, new heads and new electronics.

In January 2006, Seagate Technology began shipping its first laptop sized, 2.5 inch hard drive using perpendicular recording technology and able to hold up to 160GB of storage on a 5,400 RPM hard disk. Just recently in April, Seagate announced a series of 3.5 inch hard drives utilizing perpendicular recording with a maximum capacity of 750 GB. Additionally, Hitachi promises a 20 GB Microdrive and 1 TB 3.5 inch drive next year using this technology.

Equally important, perpendicular recording is not the final method for all storage requirements. Rather, it is a stepping stone that will give the disk drive industry breathing room to explore and invent new methods of extending magnetic recording. One method called patterned media, for example, may one day reduce the size of a bit to a single grain as compared to the 100 or so grains that comprise a bit today. The approach uses lithography to etch a pattern onto the platter. Once engineered, it is a technology that should be easily and economically replicated, adding no significant cost to the drive and potentially improving areal densities by another factor of 10. Significant research is being undertaken in Hitachi GST laboratories on this approach.

There is also Holographic disk storage. In its basic form, a hologram is the photographic record of the spatial interference pattern created by the mixing of two coherent laser beams. One of the beams usually carries spatial information and is labeled the "object" beam. The other is distinguished by its particular direction of travel and is labeled the "reference" beam. Illuminating the recorded hologram with the reference beam will yield or reconstruct the object beam and vice versa. As the holographic material becomes thicker, the reconstruction becomes very sensitive to the particular angle of incidence of the reference beam, which allows multiple objects to be recorded in the same volume and accessed independently by using an appropriate set of associated reference beams. Such holograms would be recorded sequentially, each object beam illuminating the holographic material simultaneously with its unique reference beam. In addition to such technology demonstrations, Holoplex (Pasadena, CA) has recently developed and delivered a commercial holographic memory product that stores up to 1000 images, each consisting of 640 x 480 pixels, and is capable of reading out its entire contents in one second. Holographic recording technology could possibly enable existing discs the same size as today\'s DVDs to store as much as one terabyte of data (200 times the capacity of a single layer DVD), with a transfer speed of one gigabyte per second (40 times the speed of DVD).

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