HARD DISK PLATTER

A hard disk drive platter (or disk) is the circular disk on which magnetic data is stored in a hard disk drive. The rigid nature of the platters in a hard drive is what gives them their name (as opposed to the flexible materials which are used to make floppy disk). Hard drives typically have several platters which are mounted on the same spindle. A platter can store information on both sides, requiring two heads per platter.

Contents

                1 Design

                2 Manufacture

                3 See also

                4 References

                5 External links

Design

The magnetic surface of each platter is divided into small sub-micrometer-sized magnetic regions, each of which is used to represent a single binary unit of information. A typical magnetic region on a hard-disk platter (as of 2006) is about 200–250 nanometers wide (in the radial direction of the platter) and extends about 25–30 nanometers in the down-track direction (the circumferential direction on the platter), corresponding to about 100 billion bits per square inch of disk area (15.5 Gbit/cm2).[1] The material of the main magnetic medium layer is usually a cobalt-based alloy. In today's hard drives each of these magnetic regions is composed of a few hundred magnetic grains, which are the base material that gets magnetized. As a whole, each magnetic region will have a magnetization.

One reason magnetic grains are used as opposed to a continuous magnetic medium is that they reduce the space needed for a magnetic region. In continuous magnetic materials, formations called Neel spikes tend to appear. These are spikes of opposite magnetization, and form for the same reason that bar magnets will tend to align themselves in opposite directions. These cause problems because the spikes cancel each other's magnetic field out, so that at region boundaries, the transition from one magnetization to the other will happen over the length of the Neel spikes. This is called the transition width.

Grains help solve this problem because each grain is in theory a single magnetic domain (though not always in practice). This means that the magnetic domains cannot grow or shrink to form spikes, and therefore the transition width will be on the order of the diameter of the grains. Thus, much of the development in hard drives has been in reduction of grain size.

Manufacture

Platters are typically made using an aluminium or glass and ceramic substrate. In disk manufacturing, a thin coating is deposited on both sides of the substrate, mostly by a vacuum deposition process called magnetron sputtering. The coating has a complex layered structure consisting of various metallic (mostly non-magnetic) alloys as underlayers, optimized for the control of the crystallographic orientation and the grain size of the actual magnetic media layer on top of them, i.e. the film storing the bits of information. On top of it a protective carbon-based overcoat is deposited in the same sputtering process. In post-processing a nanometer thin polymeric lubricant layer gets deposited on top of the sputtered structure by dipping the disk into a solvent solution, after which the disk is buffed by various processes to eliminate small defects and verified by a special sensor on a flying head for absence of any remaining impurities or other defects (where the size of the bit given above roughly sets the scale for what constitutes a significant defect size). In the hard-disk drive the hard-drive heads fly and move radially over the surface of the spinning platters to read or write the data. Extreme smoothness, durability, and perfection of finish are required properties of a hard-disk platter.

In 2005–06, a major shift in technology of hard-disk drives and of magnetic disks/media began. Originally, in-plane magnetized materials were used to store the bits but perpendicular magnetization is now taking over. (see perpendicular recording).

The reason for this transition is the need to continue the trend of increasing storage densities, with perpendicularly oriented media offering a more stable solution for a decreasing bit size. Orienting the magnetization perpendicular to the disk surface has major implications for the disk's deposited structure and the choice of magnetic materials, as well as for some of the other components of the hard-disk drive (such as the head and the electronic channel).

MATERIL

Media really means material. The inside of a hard drive contains one or more platters or plates that are used to hold the magnetic information. These platters consist of a substrate layer and several thin layers of material designed to hold the data and protect it.

Each platter has its own set of read/write heads. Sometimes, there is only one read/write head for a given platter but usually there are 2. Each head sits on a metallic arm that functions as a spring and extension. The spring acts to keep the heads as close to the platters as possible. Head technology may be addressed in a future post.

The platters start with a "substrate" layer such as aluminum or glass. The substrate layer is actually the support for the other materials. The substrate is polished to a flat surface to keep it as smooth as possible. The less defects here the more data you can pack in. Additional smoothing may be accomplished by adding a "thin film" of substrate. Once the surface is prepared, additional "thin film" layers are applied using either electroplating methods (cheaper but almost phased out) or "sputtering" methods. These layers vary in thickness from an amazing 1 nanometer (1 billionth of a meter) to 30 nanometers.

The first layer is the magnetic layer. It was originally iron oxide paint. Rumor has it that the first platters used the same paint used on the Golden Gate Bridge. Modern drives use a cobalt iron mix to create a harder layer with better magnetic characteristics. So, it has better durability and is able to hold much more data. There are actually 3 "sub-layers" to create the magnetic layer. There are 2 layers of magnetic material with a layer of the element Ruthenium in between, creating a super magnetic sandwich. The 2 magnetic layers act to reinforce each other's magnetism.

The second layer is a hardening layer. It is usually a sputtered carbon layer. While we mainly think of carbon as soft, such as graphite in lead pencils or coal, diamonds are also made of carbon. When sputtered on in a thin film, most of the carbon is deposited as an amorphous solid (like coal) but a portion of the carbon crystallizes and acquires the hardness of diamond. This layer can be from 3 nanometers to several nanometers in thickness.

The last layer is the lubricating layer. Lubricants used are chemicals with names like z-dol and Z-tetraol. They form a regular, smooth and very thin layer on the surface of the carbon. These layers can be little as 1 nanometer thick. Unlike standard oil based lubricants these synthetics have unique properties. Besides thinness, they are more durable and don't evaporate like oils. However, they are sensitive to heat. Heat can cause the lubricant to break down or evaporate. Loss of this layer is a major factor in media damage.

Looking ahead to the future of disks the latest news is that there may still be as much as 5 times the density available for data. A new technology called, exchange coupled composite, which is made from alternating layers of fast changing and slow changing magnetic materials. The 2 layers act to dampen each other eliminating errors at such small distances. Another interesting approach being investigated is "bit patterned media" which uses etching of the media to create discreet pockets to hold the data.