Editing Disk read-and-write head

{{Short description|Small, movable  part of a disk drive}}
{{more citations needed|date=July 2011}}

[[File:Seagate ST33232A hard disk head and platters detail.jpg|thumb|A hard disk head and arm on a platter]]
[[Image:Rwheadmicro.JPG|thumb|Microphotograph of a hard disk head. The size of the front edge is about 0.3 * 1.2 mm. The functional part of the head is the round, orange structure in the middle. Also note the connection wires bonded to gold-plated pads.]]
[[File:HDD read-write head.jpg|thumb|Read–write head of a {{val|3|u=TB}} hard disk drive manufactured in 2013. The dark rectangular component is the ''slider'' and is {{val|1.25|u=mm}} long.  The platter surface moves past the head from right to left.]]

A '''disk read-and-write head''' is the small part of a [[disk drive]] that 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).<ref>{{Cite book |author1=Mee, C. |author2=Daniel, Eric D. | title=Magnetic recording technology | date=1996 | publisher=McGraw-Hill | location=New York  | isbn=978-0-07-041276-7 | page=7.1}}</ref>  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]]s.  The [[flying height]] has been decreasing with each new generation of technology to enable higher [[Areal density (computer storage)|areal density]]. The flying height of the head is controlled by the design of an [[Fluid bearing|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.<ref>{{Cite web|last=August 2011|first=Bestofmedia Team 31|title=Hard Drives 101: Magnetic Storage|url=https://www.tomshardware.com/reviews/hard-drive-magnetic-storage-hdd,3005.html|access-date=2021-06-09|website=Tom's Hardware|date=31 August 2011 |language=en}}</ref> If the head hits the disk's surface, a catastrophic [[head crash]] can result. The heads often have a [[diamond-like carbon]] coating.<ref>https://www.fujitsu.com/global/documents/about/resources/publications/fstj/archives/vol42-1/paper13.pdf {{Bare URL PDF|date=August 2024}}</ref>

== 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]]s—simple devices made out of a tiny C-shaped piece of highly magnetizable material such as [[permalloy]] or [[ferrite (magnet)|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 [[magnetic core|ferrite core]] concentrates the field, and a [[current (electricity)|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.<ref name=":0">{{Cite web|url=https://www.tomshardware.com/reviews/hard-drive-magnetic-storage-hdd,3005-2.html|title=Read/Write Head Designs: Ferrite, Metal-In-Gap, And Thin-Film - Hard Drives 101: Magnetic Storage|date=2011-08-30|website=Tom's Hardware|language=en|access-date=2019-04-13}}</ref>

=== Metal-in-gap (MIG) heads===
Metal-in-gap (''MIG'') heads are [[Ferrite (magnet)|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.

===Thin-film heads===
First introduced in 1979 on the [[IBM 3370]] disk drive, [[thin film|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.<ref>{{cite web  |url=https://www.computerhistory.org/storageengine/thin-film-heads-introduced-for-large-disks/ |title=1979: Thin-film heads introduced for large disks  |date=December 2, 2015 |website=Computer History Museum |access-date= June 19, 2019}}</ref> Thin-film heads were much smaller than MIG heads and therefore allowed smaller recorded features to be used. Thin-film heads allowed 3.5&nbsp;inch drives to reach 4&nbsp;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.<ref name=":0" />

==Magnetoresistive heads (MR heads)==
{{Further|Giant magnetoresistance}}
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 thin-film head element for reading. The separate read element uses the [[magnetoresistance|magnetoresistive]] (MR) effect which changes the resistance of a material in the presence of a 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]]) 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.<ref>{{Cite journal|last=IWASAKI|first=Shun-ichi|date=February 2009|title=Perpendicular magnetic recording—Its development and realization—|journal=Proceedings of the Japan Academy. Series B, Physical and Biological Sciences|volume=85|issue=2|pages=37–54|doi=10.2183/pjab.85.37|issn=0386-2208|pmc=3524294|pmid=19212097|bibcode=2009PJAB...85...37I}}</ref>

=== AMR heads ===
The introduction of the AMR head in 1990 by IBM<ref name=comphist-museum /> 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.<ref name=comphist-museum>{{cite journal|author-link1=Christopher H. Bajorek |author=Christopher H. Bajorek |date=November 2014 |title=Magnetoresistive (MR) Heads and the Earliest MR Head-Based Disk Drives: Sawmill and Corsair |journal=Computer History Museum, Mountain View, CA |url=http://www.computerhistory.org/groups/storagesig/media/docs/Magnetoresistive_Heads.pdf |access-date=2015-09-25 |url-status=dead |archive-url=https://web.archive.org/web/20151220202441/http://www.computerhistory.org/groups/storagesig/media/docs/Magnetoresistive_Heads.pdf |archive-date=2015-12-20 }}</ref>

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.<ref>{{Cite news|url=https://www.abdn.ac.uk/news/5726/|title=Chemists exploring new material with 'next generation' computer hard drive possibilities|date=27 January 2014|work=The University of Aberdeen News}}</ref><ref>{{Cite book|title=Nanoscale Phase Separation and Colossal Magnetoresistance: The Physics of Manganites and Related Compounds|volume=136|last=Dagotto|first=Elbio|publisher=Springer Science & Business Media|date=14 March 2013|isbn=9783662052440|pages=395–396|chapter=Brief Introduction to Giant Magnetoresistance (GMR)|doi=10.1007/978-3-662-05244-0_21|series=Springer Series in Solid-State Sciences}}</ref>

=== TMR heads===
In 2004, the first drives to use [[magnetic tunnel effect|tunneling MR]] (''TMR'') heads were introduced by [[Seagate Technology|Seagate]]<ref name=comphist-museum/> 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 and medium. This improves the written magnetic transitions by ensuring that the head's write field fully [[saturation (magnetic)|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==
* [[Applied Magnetics Corporation]], once the largest supplier of disk heads
* [[Tape head]]

==References==
{{Reflist}}

==External links==
{{Commons category|Hard disk heads}}
*The PC Guide: [http://www.pcguide.com/ref/hdd/op/heads/opFunction-c.html Function of the Read/Write Heads]
*IBM Research: [http://www.research.ibm.com/research/gmr.html GMR introduction, animations] {{Webarchive|url=https://web.archive.org/web/20120111051257/http://www.research.ibm.com/research/gmr.html |date=2012-01-11 }}
*Hitachi Global Storage Technologies: [http://www1.hitachigst.com/hdd/research/recording_head/headmaterials/ Recording Head Materials] {{Webarchive|url=https://web.archive.org/web/20120330165531/http://www1.hitachigst.com/hdd/research/recording_head/headmaterials/ |date=2012-03-30 }}

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[[Category:Computer storage devices]]
[[Category:Hard disk computer storage]]
[[Category:Magnetic devices]]
[[Category:Rotating disc computer storage media]]

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