Editing Disk read-and-write head (section)

==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.