Disk drives that are based on GMR head technology use these properties to help control a sensor that responds to very small rotations on the disk. The magnetic rotation yields a very large change in sensor resistance, which in turn provides a signal that can be picked up by the electric circuits in the drive.

GMR heads are made up of four layers of thin material that combine into a single structure:

• Free layer - The sensing layer. The free layer is passed over the surface of the data bits to be read. It is free to rotate in response to the magnetic patterns on the disk.
• Spacer - Typically made from copper, this is a nonmagnetic layer that separates the magnetization of the free and pinned layers.
• Pinned layer - A layer of cobalt material that is held in a fixed magnetic orientation by its proximity to the exchange layer.
• Exchange layer - A layer of antiferromagnetic material that fixes the pinned layer's magnetic orientation.

When the head passes over a magnetic field of one polarity, the electrons on the free layer turn to align with those on the pinned layer, creating a lower resistance in the head structure. When the head passes over a field of opposite polarity, the free layer electrons rotate so that they are not aligned with the electrons on the pinned layer. This causes an increase in the structure's resistance. Because the resistance changes are caused by changes to the spin characteristics of electrons in the free layer, GMR heads are also known as spin valves, a term coined by IBM.

GMR Technology 
This guide provides diagrams and a technical explanation of the technology.

The Era of Giant Magnetoresistive Heads
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The Giant Magnetoresistive Head: A Giant Leap for IBM Research
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