Friday, October 14, 2011

Perpendicular Hard Disk Drives

What is a Hard Disk Drive?

A Hard Disk Drive (HDD) is a device used by modern computers to permanently store information. The Hard Disk Drive is arguable the most essential part of a computer system in that all the information that is permanently stored is contained within its enclosure, including your computer’s Operating System (OS). Thanks to Hard Disk Drives, long gone are the days when you would have had to keep all your programs and documents stored on removable media such as Floppy Disks or CD-ROMs.

Originally invented in the mid 1950’s and made commercially available in 1956 by International Business Machines (IBM). Called RAMAC (Random Access Method of Accounting and Control), the first Hard Disk Drives contained as much as 50 platters which were 24 inches in diameter and were computers in their own right albeit with a single purpose – to store data. The entire unit which housed the hard drive was the approximate size of two large refrigerators placed side by side. In the 50 or so years since their invention, Hard Disk Drives have steadily and aggressively far out paced Moore’s law. Which stipulates that memory in computers will increase by 100% approximately every 18 months. Hard Disk Drives on the other hand have increased capacity in the same period by approximately 130%, an increase of 100% every nine months in many cases. Such capacity increases are being threatened, however.

In the years since the first Hard Disk Drive very little has changed apart from logical steps in technology such as the increased speed or improved interfaces, the basic technology has changed very little. There have been no technological leaps, as it were, for Hard Disk Drives beyond their increased miniaturisation. Apart from miniaturisation and recording media improvements the Hard Disk Drive as a device is almost identical technologically speaking, to the very first, the RAMAC.Hard Disk Drives use a similar technology as is employed in audio and video cassettes. Such audio and video cassettes use a magnetic ribbon wound around a two wheels to store data. To access a particular portion of the data contained on the magnetic ribbon, the device must wind the tape such that the beginning of the section containing the data is underneath the device that reads the data (the magnetic read/write head). This process is called sequential data retrieval because in the process of accessing the particular data, the device must sequentially read each piece of data until the data it’s looking for is found. This process is very time consuming and contributes to wear.

Hard Disks on the other hand use a circular disk-shaped platter upon which the magnetically sensitive compound is laid. Such platters are similar in concept to a Compact Disk (CD) in that the data they hold can be accessed randomly, that the recordable media is in a circular (disk) shape, and that the data is sectioned off into tracks and sectors. Data on a Hard Disk Drive can be accessed randomly because the recordable medium of Hard Disk Drives uses these separated tracks and sectors. By separating the data in such a way, it can be positioned at random intervals of the disk, depending upon the space requirements.

Anywhere from one to seven recordable platters are contained within a modern Hard Disk Drive’s metallic enclosure. Hard Disk Drive platters are perfectly circular disks made from either an aluminium alloy or a more recently a glass ceramic substrate which is a ceramic disk suspended in a glass outer shell. Onto the surfaces of a disk’s platter is laid a thin layer of a magnetically sensitive coating called the recording medium, in modern drives the mixture is a complex amalgam of different materials such as cobalt chromium platinum boron (CoCrPtB) and other such rare metals.

How does a Hard Disk Drive store data?

All information located on a computer is expressed as a series of ones and zeros (1/0), as binary digits (bits). Taking advantage of the nature of magnetic particles, that they can be polarised to magnetic north or south and that their magnetic poles can be alternated or switched when a sufficient magnetic field of the correct polarity is applied, Hard Disk Drives can store the very same sequence of bits onto a disk by polarising the required magnetic particles on the recording medium such that they represent the data being stored. Hard Disk Drives are sectioned off such that they contain both intersecting tracks and sectors. The purpose of which is to provide a logical data structure, to provide a way to distinguish between areas of data. Within each track there are a number of sectors. It is within these sectors of the Hard Disk which data is stored.

The platter of a Hard Disk Drive is coated with a magnetically sensitive coating comprised primarily of magnetically charged particles or filings which as a whole may be called the recording medium. These particulates can be magnetically aligned such that they represent binary digits, by inducing an electromagnetic field upon them via a devices read/write head. The recording media contains many billions of microscopic particles which when viewed extremely close resemble miniature metal filings. When a Hard Disk Drive records data onto the medium it takes many hundreds (usually anywhere from 500 to 100) of these magnetically sensitive particles to store a single binary digit. The increased reduction of the amount of particles required to record data is highly limited by the precision of the read/write head (the miniature device that reads and records data onto the recording medium) because the magnetic field which is used by the drive’s read/write head to read and/or record (write) data is such that it already tentatively borders nearby data.

Should it be shrunk much further in an attempt to increase precision, the likelihood of data corruption would increase vastly. Research by various parties has been on-going to find a workable solution to recording data onto much fewer or even single particles for some time now. A hard drive may record data onto the Hard Disk Drive by applying a sufficient magnetic field to the section of the recording medium (which is suspended upon the Hard Disks platter) such that the data (a series of ones and/or zeros which correspond to the information being stored) is recorded onto the medium by aligning the specified particles to the desired magnetic pole (north or south). In doing so, any previous data which was present is therefore destroyed.

Perpendicular verses Longitudinal

Ever since the late 1980’s and early 1990’s magnetic media drive manufacturers have been researching the feasibility of switching from longitudinal to perpendicular recording techniques. The advantage is clearly one of capacity: when longitudinal magnetic particles are packed together, they take up much more space than if they were to stand upright, if they stood perpendicular to the platter. More than merely a matter of initial capacity gain, perpendicular recording technology avoids a problem which has been well known in the field for many years: the super-paramagnetic effect (SPE), which affects magnetically charged particles of such small size as that used in Hard Disk Drives. “The super-paramagnetic effect is a phenomenon observed in very fine particles, where the energy required to change the direction of the magnetic moment of a particle is comparable to the ambient thermal energy” (source: theories have cropped up over the years as to what density magnetic particles (described by a disks areal density) may achieve before becoming subject to SPE. At present it is suggested that anything from 100Gbit/inch2 to 150Gbit/inch2 is the physical limitation for longitudinal Hard Disk Drives, although perpendicular media solutions have been made as high as 230Gbit/inch2.

In the layering of the magnetic particulates atop a magnetic suspension layer and orienting the particles perpendicular to the platter, the recording medium can pack many more magnetically sensitive particles together in the same space than previously possible whilst keeping SPE at bay. Perpendicular recording technology does not however preclude SPE from limiting capacity in the future, more than anything perpendicular recording technology can been described as a way to give manufacturers breathing room to develop more permanent technological solutions such as holographic lithography or a multilayered recording medium. Traditional recording media manufacture consists of the spreading of recording material over a disk platter via a centrifugal force induced by spinning the platter whilst the recording material is placed atop its surface. The centrifugal force would spread the recording material across the surface, evenly spreading it in all directions. Perpendicular recording media manufacture on the other hand requires a much different technique.

The exact manufacturing process of perpendicular recording media is unsurprisingly a closely guarded secret, especially considering its recent arrival on the marketplace. From patents filed at the United States Patent and Trademark Office (USPTO), it can be taken that the predominant technique involves the laminating of magnetic and non-magnetically charged metals such as chromium, cobalt, platinum and alloys of similar; sandwiching unique layers to affect the desired result – a recording medium such that the magnetic particles are aligned perpendicular to the platter. In US patent number 6387483, filed by the NEC Corporation of Tokyo; it describes the technique as follows:The perpendicular magnetic recording medium of the embodiment is formed by laminating a Cr film, a soft magnetic under layer film, and a perpendicular magnetizing film on a substrate in this order. (Source: USPTO no. 6387483)

In longitudinal media manufacture too, laminating multiple supportive metals is achieved; in perpendicular media however, the difference is the magnetizing film as described above. Whereas traditional lamination ordinarily serves only to prevent wear and noise (both electro-mechanical and audible noise), in perpendicular media manufacture it would appear that at least some of the lamination process is used to magnetize the magnetic media particles to a perpendicular orientation. Precisely how the reorientation of magnetic media particulate is accomplished is not easy to determine, most probably because the technology is so new that such details are sketchy at best and obscure or guarded at worst. This fact is not at all surprising concerning a new technology such as perpendicular magnetic media development.

The future of storage technology

Perpendicular magnetic media technology as discussed earlier is merely a temporary solution, to find more permanent solutions we must look to much more advanced technologies. One such technology is patterned magnetic media. The process of patterned magnetic media aims to make singular magnetic particulates the object of recording bits, you will remember that current technologies requires approximately 500 to 1000 magnetic particles to store a single bit. The object of patterned media is to cut this dramatically down to a single particle per bit. Advantages of such a technology are such as reduced statistical noise associated with granular media and more increased areal density (as much as 64Gbit/inch2).

Patterned magnetic media aims to prevent the SPE barrier, or at least further decrement its effect through the use of so-called mesas and valleys. The technique uses the creation of barriers between magnetic particles, thereby avoiding the SPE complication which affects closely packed particles. Holographic Storage (a.k.a. Holographic Lithography) too is a technology that aims to increase storage capacity which is also under heavy research, and claims to be a much more permanent solution. Unlike Patterned Magnetic Media, Holographic Storage is a revolutionary step away from magnetic media and previous optoelectronic technologies.

Hard Disk Drives will always be subject to inertia and centrifugal force induced by the moving parts of the drives mechanical components (platter, read/write head), Holographic Storage has no such issues; the holographic process uses lasers in replacement of the read/write head of a Hard Disk Drive and the media itself requires no momentum (unlike the platters in Hard Disk Drives).

Such holographic storage is far from realisation, in fact it is postulated by some that it may be as much as ten years before the technology can be made into a workable solution. In direct symmetry to early memory research, research on Holographic Storage technologies seems to have banded into two camps: one of super fast data retrieval and extraordinarily high capacity storage; no doubt there will be extremely profitable markets for both.

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