![HDD read-write head](https://upload.wikimedia.org/wikipedia/commons/2/29/HDD_read-write_head.jpg)
A disk read-and-write head is the small part of a
disk drive
Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
which moves above the disk platter and transforms the platter's magnetic field into electrical current (reads the disk) or, vice versa, transforms electrical 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 area density (also known as areal density, surface density, superficial density, areic density, mass thickness, column density, or density thickness) of a two-dimensional object is calculated as the mass per unit area. The SI derived unit is ...
. 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.
Inductive heads
Inductive heads use the same element for both reading and writing.
Traditional head
The heads themselves started out similar to the heads in
tape recorder
An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present ...
s—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
In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability couple ...
concentrates the field, and a
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
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
Metal-in-gap (''MIG'') heads are
ferrite heads with a small piece of
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
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.
Thin-film heads
First introduced in 1979 on the
IBM 3370
IBM manufactured magnetic disk storage devices from 1956 to 2003, when it sold its hard disk drive business to Hitachi. Both the hard disk drive (HDD) and floppy disk drive (FDD) were invented by IBM and as such IBM's employees were responsible ...
disk drive,
thin-film technology use photolithographic techniques similar to those used on semiconductor devices to fabricate HDD heads with smaller size and greater precision than the ferrite-based designs then in use. Thin-film heads are electronically similar to ferrite heads and used the same physics. Thin layers of magnetic (Ni–Fe), insulating, and copper coil wiring materials are built on ceramic substrates that are 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
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ...
of the head gap was a compromise between what worked best for reading and what worked best for writing.
Magnetoresistive heads (MR heads)
The next head improvement in head design was to separate the writing element from the reading element allowing the optimization of a thin-film element for writing and a separate head element for reading. The separate read element uses the
magnetoresistive (MR) effect which changes the resistance of a material in the presence of magnetic field. These MR heads are able to read very small magnetic features reliably, but can not be used to create the strong field used for writing. The term ''AMR'' (Anisotropic MR) is used to distinguish it from the later introduced improvement in MR technology called ''GMR'' (
giant magnetoresistance
Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter G ...
) and "TMR" (tunneling magnetoresistance).
The transition to
perpendicular magnetic recording
Perpendicular recording (or perpendicular magnetic recording, PMR), also known as conventional magnetic recording (CMR), is a technology for data recording on magnetic media, particularly hard disks. It was first proven advantageous in 1976 by Sh ...
(''PMR'') media has major implications for the write process and the write element of the head structure but less so for the MR read sensor of the head structure.
AMR heads
The introduction of the AMR head in 1990 by IBM
led to a period of rapid areal density increases of about 100% per year.
GMR heads
In 1997 GMR, giant magnetoresistive heads started to replace AMR heads.
Since the 1990s, a number of studies have been done on the effects of
colossal magnetoresistance
Colossal magnetoresistance (CMR) is a property of some materials, mostly manganese-based perovskite oxides, that enables them to dramatically change their electrical resistance in the presence of a magnetic field. The magnetoresistance of conventio ...
(CMR), which may allow for even greater increases in density. But so far it has not led to practical applications because it requires low temperatures and large equipment size.
TMR heads
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
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and cont ...
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.
See also
* Head crash
References
External links
*The PC Guide
Function of the Read/Write Heads
*IBM Research
*Hitachi Global Storage Technologies
Recording Head Materials
{{Authority control
Computer storage devices
Hard disk computer storage
Magnetic devices
Rotating disc computer storage media
it:Disco rigido#Descrizione