Part 2: Disk Media Technology
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.
