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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
nanometre 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
s. The
flying height The flying height or floating height or head gap is the distance between the disk read/write head on a hard disk drive and the platter. The first commercial hard-disk drive, the IBM 305 RAMAC, used forced air to maintain a 0.002 inch (51&n ...
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 i ...
. The flying height of the head is controlled by the design of an
air bearing Air bearings (also known as aerostatic or aerodynamic bearings) are fluid bearings that use a thin film of pressurized gas to provide a low friction load-bearing interface between surfaces. The two surfaces do not touch, thus avoiding the tradit ...
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 A head crash is a hard-disk failure that occurs when a read–write head of a hard disk drive makes contact with its rotating platter, slashing its surface and permanently damaging its magnetic media. It is most often caused by a sudden seve ...
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 Permalloy is a nickel–iron magnetic alloy, with about 80% nickel and 20% iron content. Invented in 1914 by physicist Gustav Elmen at Bell Telephone Laboratories, it is notable for its very high magnetic permeability, which makes it useful as ...
or ferrite wrapped in a fine wire coil. When writing, the coil is energized, a strong
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
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

Metal-in-gap (''MIG'') heads are ferrite heads with a small piece of
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
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 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 c ...
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 Magnetoresistance is the tendency of a material (often Ferromagnetism, ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance. So ...
(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 (''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 (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 con ...
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 A head crash is a hard-disk failure that occurs when a read–write head of a hard disk drive makes contact with its rotating platter, slashing its surface and permanently damaging its magnetic media. It is most often caused by a sudden seve ...


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