Disk read-and-write head
Small component that reads and writes data on disk platters.
A disk drive's read-write head is a tiny component that glides above the spinning platter. It either converts the platter's magnetic field into an electrical signal to read data, or does the reverse—turning an electrical current into a magnetic field to write data. Over time, head designs have evolved significantly.
In hard drives, the head flies just 3 nanometres above the disk surface, and this gap shrinks with each new generation to pack more data into the same area. The flying height is managed by an air bearing etched onto the slider's underside, which keeps the height constant even as the head moves across the platter at varying speeds. If the head ever touches the disk, a catastrophic head crash can occur. Heads are often coated with diamond-like carbon for protection.
**Inductive heads** use a single element for both reading and writing.
**Traditional heads** resembled tape recorder heads: a tiny C-shaped piece of magnetizable material (like permalloy or ferrite) wrapped in a fine wire coil. To write, the coil is energized, creating a strong magnetic field in the C's gap that magnetizes the adjacent recording surface. To read, the rotating magnetized material passes the head; the ferrite core concentrates the field, generating a current in the coil. The gap is very narrow—roughly the thickness of the magnetic media—and determines the smallest recordable area. Ferrite heads are large, write relatively big features, and must fly farther from the surface, requiring stronger fields.
**Metal-in-gap (MIG) heads** are ferrite heads with a small metal piece in the gap to concentrate the field, enabling smaller features. MIG heads were eventually replaced by thin-film heads.
**Thin-film heads**, first used in the IBM 3370 drive in 1979, are made with photolithographic techniques similar to semiconductor fabrication. They are smaller and more precise than ferrite heads, though they work on the same physics. Thin layers of magnetic (Ni–Fe), insulating, and copper coil materials are built on ceramic substrates, then cut into individual heads integrated with their air bearing, lowering manufacturing costs. Their smaller size allowed smaller recorded features, and by 1995, 3.5-inch drives could reach 4 GB capacities. The head gap geometry was a compromise between optimal reading and writing.
- Minimum flying height
- 3 nanometres
- First thin film head introduction
- 1979 on the IBM 3370 disk drive
- Amr head introduction
- 1990 by IBM
- Gmr head introduction
- 1997
- First tmr head drives introduction
- 2004 by Seagate
- Tmr head drive capacity
- 400 GB with 3 disk platters
Lore & Background
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. Ferrite heads are large, write fairly large features, and must be flown fairly far from the surface. Metal-in-gap (MIG) heads are ferrite heads with a small piece of metal in the head gap that concentrates the field, allowing 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. Thin-film heads were much smaller than MIG heads and allowed smaller recorded features. The next improvement was to separate the writing element from the reading element, allowing optimization of a thin-film element for writing and a separate thin-film head element for reading. The separate read element uses the magnetoresistive (MR) effect, which changes the resistance of a material in the presence of a magnetic field. The introduction of the AMR head in 1990 by IBM led to a period of rapid areal density increases of about 100% per year. In 1997, GMR heads started to replace AMR heads. In 2004, the first drives to use tunneling MR (TMR) heads were introduced by Seagate, featuring integrated microscopic heater coils to control the shape of the transducer region during operation.
Reader's Guide
The evolution of disk read-and-write heads has been central to the dramatic increases in storage capacity over decades. From early ferrite heads that wrote fairly large features and required larger clearances, to metal-in-gap heads that concentrated the field for smaller features, each generation enabled higher areal density. Thin-film heads, introduced in 1979, used semiconductor-like photolithography to produce smaller, more precise heads at lower cost, allowing 3.5-inch drives to reach 4 GB storage capacities in 1995. The separation of read and write elements with magnetoresistive (MR) technology was a pivotal advance: AMR heads introduced in 1990 by IBM drove annual areal density increases of about 100%, and GMR heads replaced them in 1997. TMR heads, introduced by Seagate in 2004, enabled 400 GB drives with 3 platters and added microscopic heater coils to control head-to-disk spacing during operation. The transition to perpendicular magnetic recording (PMR) media has major implications for the write process and the write element but less so for the MR read sensor. Studies of colossal magnetoresistance (CMR) have been done since the 1990s but have not led to practical applications because it requires low temperatures and large equipment size.
Did You Know?
- The flying height of a hard drive head can be as little as 3 nanometres.
- AMR heads, introduced by IBM in 1990, led to areal density increases of about 100% per year.
- Seagate introduced TMR heads in 2004 with integrated microscopic heater coils to control head shape during operation.
Frequently Asked Questions
What is a disk read-and-write head?
It is a minuscule component that hovers just above a spinning platter inside a disk drive. Its sole purpose is to either sense the platter's magnetic field and turn it into an electrical signal (reading) or to convert an electrical current into a magnetic field (writing).
How does the read-write head actually read and write data?
During a read, the head detects the magnetic field on the rotating platter and translates it into an electrical signal the drive can interpret. During a write, it reverses the process, using an electrical current to generate a magnetic field that alters the platter's surface.
How close does the head fly to the disk surface?
In modern hard drives the minimum flying height is just 3 nanometres above the platter. An air bearing etched into the underside of the slider maintains that razor-thin gap and keeps the height stable even at high rotational speeds.
When was the first thin-film read-write head introduced?
IBM debuted the first thin-film head in 1979 on its 3370 disk drive. That milestone paved the way for the AMR head (1990), the GMR head (1997), and eventually Seagate's TMR-head drives in 2004.
Why does the flying height keep shrinking with each new generation?
A smaller head-to-platter gap lets engineers pack more data into the same physical area, directly increasing storage density. By 2004, Seagate's TMR-head drives were already delivering 400 GB on just three platters, a clear payoff of those tighter tolerances.
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