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Function of the Read/Write Heads
In concept, hard disk heads are relatively simple. They are energy converters: they transform electrical signals to magnetic signals, and magnetic signals back to electrical ones again. The heads on your VCR or home stereo tape deck perform a similar function, although using very different technology. The read/write heads are in essence tiny electromagnets that perform this conversion from electrical information to magnetic and back again. Each bit of data to be stored is recorded onto the hard disk using a special encoding method that translates zeros and ones into patterns of magnetic flux reversals.
Older, conventional (ferrite, metal-in-gap and thin film) hard disk heads work by making use of the two main principles of electromagnetic force. The first is that applying an electrical current through a coil produces a magnetic field; this is used when writing to the disk. The direction of the magnetic field produced depends on the direction that the current is flowing through the coil. The second is the opposite, that applying a magnetic field to a coil will cause an electrical current to flow; this is used when reading back the previously written information. (You can see a photograph showing this design on the page on ferrite heads.) Again here, the direction that the current flows depends on the direction of the magnetic field applied to the coil. Newer (MR and GMR) heads don't use the induced current in the coil to read back the information; they function instead by using the principle of magnetoresistance, where certain materials change their resistance when subjected to different magnetic fields.
The heads are usually called "read/write heads", and older ones did both writing and reading using the same element. Newer MR and GMR heads however, are in fact composites that include a different element for writing and reading. This design is more complicated to manufacture, but is required because the magnetoresistance effect used in these heads only functions in the read mode. Having separate units for writing and reading also allows each to be tuned to the particular function it does, while a single head must be designed as a compromise between fine-tuning for the write function or the read function. See the discussion of MR read heads for more on this. These dual heads are sometimes called "merged heads".
Because of the tight packing of data bits on the hard disk, it is important to make sure that the magnetic fields don't interfere with one another. To ensure that this does not happen, the stored fields are very small, and very weak. Increasing the density of the disk means that the fields must be made still weaker, which means the read/write heads must be faster and more sensitive so they can read the weaker fields and accurately figure out which bits are ones and which bits are zeroes. This is the reason why MR and GMR heads have taken over the market: they are more sensitive and can be made very small so as not read adjacent tracks on the disk. Special amplification circuits are used to convert the weak electrical pulses from the head into proper digital signals that represent the real data read from the hard disk. Error detection and correction circuitry must also be used to compensate for the increased likelihood of errors as the signals get weaker and weaker on the hard disk. In addition, some newer heads employ magnetic "shields" on either side of the read head to ensure that the head is not affected by any stray magnetic energy.