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Hard Disk Read/Write Heads

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Firstly, unlike older hard disks, modern hard disks do not use electromagnetic coils in the read element of the head. Instead, they use Giant Magneto Resistance (GMR) heads. The GMR is made of a compound that when placed into a magnetic field, it's resistance changes. By measuring the resistance change, the flux direction of the platter section under the read head can be determined to be of [N-S] or [S-N] polarity and converted to its corresponding digital value i.e. a 0 or 1.

When data is being written, it converts the 1 or 0 to a voltage in the write head. According to this value, the voltage passed through the coil creates a magnetic field that in turn creates the flux patterns e.g. [N-S] within the platter, storing the binary data.

GMR heads therefore need two separate elements, a read element and a write element. Older hard drives used the same element to read and write the data, so GMR. has the added advantage that each element can be manufactured and fine-tuned precisely for its purpose to either read or write data. The older drives with a single element were tuned somewhere between the optimum read and optimum write values.

Above is a picture of a GMR head assembly, near the bottom you can see the write element. This can be identified by the gap between the two 'square' sections. It is in this gap where the magnetic field is created that alters the flux pattern in the platters.

Above the Write head, you can see the GMR element, this is positioned so it lies between the gap of the write element as this is the area where the data would be in relation to the write elements would have written the corresponding data.

Heads and Flying

Heads literally fly just above the surface of the platters, the 'flying height is constantly getting smaller, and the gap between head and platter varies depending upon the age of the drive and the technology used, and is within the range of less than a micron (0.001mm) to a few nanometers (0.001microns). Hence the need for a cleanroom environment in order to open the drives. If a single dust particle were to enter the drive and get between the head and the platter, it can cause irreparable damage.

In order for a head to fly they need ..

1. Moving air. Air movement is created by rotation of the platters and also by the design of the internal structure of the disk casing, often there are vanes inside disks the help to deflect the air under the heads.
2. Aerodynamics. Heads are manufactured with aero dynamic properties so that once the air inside the drive is moving at the correct speed, the heads physically take-off. For hard disk drives that use parking or 'landing' zones on the platters, when the power is turned off, the heads are moved over the landing zone and as the motor rotation slows down the heads gently come to rest and have 'Landed'.