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Small, movable part of a disk drive

A disk read and write head is the small part of a disk drive which moves above the disk platter and transforms the platter's magnetic field into electric current (reads the disk) or, vice versa, transforms electric current into magnetic field (writes the disk). The heads have gone through a number of changes over the years. In a hard drive, the heads fly above the disk surface with clearance of as little as 3 nanometres. The flying height has been decreasing with each new generation of technology to enable higher areal density. The flying height of the head is controlled by the design of an air bearing etched onto the disk facing surface of the slider. The role of the air bearing is to maintain the flying height constant as the head moves over the surface of the disk. The air bearings are carefully designed to maintain the same height across the entire platter, despite differing speeds depending on the head distance from the center of the platter. If the head hits the disk's surface, a catastrophic head crash can result. The heads often have a diamond like carbon coating.

Inductive heads use the same element for both reading and writing.

The heads themselves started out similar to the heads in tape recorders—simple devices made out of a tiny C shaped piece of highly magnetizable material such as permalloy or ferrite wrapped in a fine wire coil. When writing, the coil is energized, a strong magnetic field forms in the gap of the C, and the recording surface adjacent to the gap is magnetized. When reading, the magnetized material rotates past the heads, the ferrite core concentrates the field, and a current is generated in the coil. In the gap the field is very strong and quite narrow. That gap is roughly equal to the thickness of the magnetic media on the recording surface. The gap determines the minimum size of a recorded area on the disk. Ferrite heads are large, and write fairly large features. They must also be flown fairly far from the surface thus requiring stronger fields and larger heads.

Metal in gap (MIG) heads are ferrite heads with a small piece of metal in the head gap that concentrates the field. This allows smaller features to be read and written. MIG heads were replaced by thin film heads.

First introduced in 1979 on the IBM 3370 disk drive, thin film technology uses photolithographic techniques similar to those used on semiconductor devices to fabricate hard drive heads. At the time, these heads had smaller size and greater precision than the ferrite based heads then in use; they were electronically similar to them and used the same physics. Thin layers of magnetic (Ni–Fe), insulating, and copper coil wiring materials were built on ceramic substrates that were then physically separated into individual read/write heads integrated with their air bearing, significantly reducing the manufacturing cost per unit. Thin film heads were much smaller than MIG heads and therefore allowed smaller recorded features to be used. Thin film heads allowed 3.5 inch drives to reach 4 GB storage capacities in 1995. The geometry of the head gap was a compromise between what worked best for reading and what worked best for writing.

In 2004, the first drives to use tunneling MR (TMR) heads were introduced by Seagate allowing 400 GB drives with 3 disk platters. Seagate introduced TMR heads featuring integrated microscopic heater coils to control the shape of the transducer region of the head during operation. The heater can be activated prior to the start of a write operation to ensure proximity of the write pole to the disk/medium. This improves the written magnetic transitions by ensuring that the head's write field fully saturates the magnetic disk medium. The same thermal actuation approach can be used to temporarily decrease the separation between the disk medium and the read sensor during the readback process, thus improving signal strength and resolution. By mid 2006 other manufacturers have begun to use similar approaches in their products.