What's Head

Head of hard disk is the key component to read data. Its main role is to convert magnetic information on hard disk to electrical signal in order to transmit the information outward. Generally, the number of head on hard disk is 1 or 2. But when there is trouble in the development of storage per disk enhancement technology, the hard drive manufacturers will increase disk capacity by increasing the number of disk. This requires more than two heads to operate.

1.An Introduction to Number of Head


Number of head

The role of head is that it can convert magneto-electricity. The cost of head holds around 40% of the total cost of hard disk. If people want to have breakthrough in storage per disk, they must improve head technology. Typically, there is one head on one disc, but because of the latest technology, two heads can read one disc at the same time. In LBA mode, the maximum number of heads can be set up to 255.

2.An Introduction to Head

Hard disk head is the key component for disk to read data. Its working principle is that it can read and write data on disk by using special materials whose resistance value changes along with the change of magnetic field. The storage density of hard disk is largely determined by the quality of head. Now, GMR (Giant Magneto Resistive) is commonly used. Compared with the previous traditional head and MR (Magneto Resistive), GMR uses materials with better magneto resistive effect and the multi-layer film structure to make it more sensitive. The relative change of magnetic field of GMR can cause changes of large resistance value. This can achieve higher storage densities.

Head, which is one of the most sophisticated parts of hard disk, is a reading and writing tool on hard disk. Head is made by made of coils winding on the magnetic core. When hard disk works, head reads data by sensing the changes of magnetic field on the rotating disc. It writes in data by changing magnetic field on disc. In order to avoid wear of head and disc, when head is at work, it suspends over the disc with high rotating speed rather than directly contacts with disc. Only after power is turned off, head will automatically return to a fixed position on the disc (it is called the landing zone which doesn't store data and is the starting position of the disc).

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Due to the work nature of the disc, there is a high requirement in its magnetic induction sensitivity and precision. Earlier head uses ferromagnetic material. This results in its bad magnetic induction sensitivity. So, storage per disk of the earlier hard disk is small. The larger the storage per disk is, the lager the track density will be. Data can't be accurately read out, if head sensitivity degree is not enough. This results in a limited capacity. With the development of technology, magnetic induction sensitivity and precision of head have been improved a lot.

Reading and writing functions of earlier head are together with each other. This has a high requirement in manufacturing process and technology of head. But for personal computer, in the process of exchanging data with disk, data reading speed is much faster than data writing speed. The characteristics of reading and writing operations are quite different from each other. This leads to the appearance of separate heads with reading and writing function respectively. The two heads work separately without interference to each other.

3.Thin Film Inductive Head (TFI Head)

During 1990 to 1995, hard disk used TFI reading and writing technology. Actually, TFI head is a winding magnetic core. The induced voltage will produce when disc passes under the winding magnetic core. TFI reading/writing head reaches its capacity limitation, because magnetic sensitivity degree is improved while writing ability is weakened.

4.Anisotropic Magneto Resistive Head (AMR head)

During the mid-1990s, Seagate Company introduced hard disk using AMR head. AMR head uses TFI head to finish operations, but it uses magnetic material with thin strip as reading component. In the presence of magnetic field, resistance of thin strip will change along with the change of magnetic field. This will produce strong signal. Hard disk decodes the change of strip resistance caused by the change of magnetic field polarity. This improves reading sensitivity of hard disk. AMR head further improves surface density while reduces the number of components. Because of the amount of resistance change of AMR head has a certain limitation, AMR technology can support at most 3.3 GB/square inch of recoding density. This makes the sensitivity degree of AMR head also has a limit. So, the development of GMR head comes into being.

5.Giant Magneto Resistive (GMR)

GMR head inherited the reading and writing technologies of TFI head and AMR head. But its reading head has higher sensitivity degree to magnetic transformation on hard disk. GMR head is composed of four layers of conductive material and magnetic film: a sensible layer, a non-conductive mediation layer, a magnetic plug layer and an exchangeable layer. The sensitivity degree of GMR is three times of that of AMR head, so, it can improve density and performance of disc.

The number of head in hard disk is determined by the number of discs. There are data saved on both sides of disc, so, if a disc works properly, it must have two heads. For example, a hard disk with the total capacity of 80 GB uses a disc whose storage per disk is 80 GB. There are data on both sides of this disc. So, the disc has two heads. However, the disk with total capacity of 120 GB uses two discs. There are only three heads on this disk, because one side of one disc among them doesn't have head.