Thread: 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 in 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 magneto resistance, 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 magneto resistance 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.