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'''砷化镓锰'''是一种基于[[砷化镓]]的[[磁性半导体]]磁性半导体材料。与其他稀磁半导体不同，如II-VI族半导体，砷化镓锰并非[[顺磁性|顺磁体]]，<ref name=furdyna_diluted_1988>{{Cite journal
| volume = 64
| issue = 4
| pages = R29–R64
| last = Furdyna
| first = J. K.
| title = Diluted magnetic semiconductors
| journal = Journal of Applied Physics
| year = 1988
| url = http://link.aip.org/link/?JAP/64/R29/1
| doi = 10.1063/1.341700
| bibcode = 1988JAP....64...29F
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223085636/http://link.aip.org/link/?JAP/64/R29/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref>
而是[[铁磁性|铁磁体]]，因而具有[[磁滞现象]]。This memory effect is of importance for the creation of persistent devices. In (Ga,Mn)As, the manganese atoms provide a magnetic moment, and each also acts as an [[Acceptor (semiconductors)|acceptor]], making it a ''p''-type material. The presence of [[Charge carrier|carriers]] allows the material to be used for [[Spin polarization|spin-polarized]] currents. In contrast, many other [[Ferromagnetism|ferromagnetic]] [[Magnetic semiconductor|DMSs]] are strongly insulating<ref name=ohno_magnetotransport_1992>
{{Cite journal
| doi = 10.1103/PhysRevLett.68.2664
| volume = 68
| issue = 17
| pages = 2664–2667
| last = Ohno
| first = H.
|author2=H. Munekata |author3=T. Penney |author4=S. von Molnár |author5=L. L. Chang
| title = Magnetotransport properties of p-type (In,Mn)As diluted magnetic III-V semiconductors
| journal = Physical Review Letters
| date = 1992-04-27
| pmid = 10045456
| bibcode=1992PhRvL..68.2664O
}}</ref><ref name=pinto_magnetic_2005>{{Cite journal
| volume = 72
| issue = 16
| pages = 165203
| last = Pinto
| first = N.
|author2=L. Morresi |author3=M. Ficcadenti |author4=R. Murri |author5=F. D'Orazio |author6=F. Lucari |author7=L. Boarino |author8=G. Amato
| title = Magnetic and electronic transport percolation in epitaxial Ge<sub>1−x</sub>Mn<sub>x</sub> films
| journal = Physical Review B
| date = 2005-10-15
| doi = 10.1103/PhysRevB.72.165203
|arxiv = cond-mat/0509111 |bibcode = 2005PhRvB..72p5203P | s2cid = 119477528
}}</ref>
and so do not possess [[Charge carrier|free carriers]]. (Ga,Mn)As is therefore a candidate as a [[spintronic]] material.

==Growth==

Like other [[Magnetic semiconductor|DMSs]], (Ga,Mn)As is formed by [[Doping (semiconductor)|doping]] a standard [[semiconductor]] with magnetic elements. This is done using the growth technique [[Molecular-beam epitaxy|molecular beam epitaxy (MBE)]], whereby crystal structures can be grown with atom layer precision. In (Ga,Mn)As the manganese substitute into gallium sites in the [[GaAs]] crystal and provide a magnetic moment. Because manganese has a low solubility in [[GaAs]], incorporating a sufficiently high concentration for [[ferromagnetism]] to be achieved proves challenging. In standard [[Molecular-beam epitaxy|MBE]] growth, to ensure that a good structural quality is obtained, the temperature the substrate is heated to, known as the growth temperature, is normally high, typically ~600&nbsp;°C. However, if a large flux of manganese is used in these conditions, instead of being incorporated, segregation occurs where the manganese accumulate on the surface and form complexes with elemental arsenic atoms.<ref name=desimone_manganese_1982>{{Cite journal
| volume = 53
| issue = 7
| pages = 4938–4942
| last = DeSimone
| first = D.
| author2 = C. E. C. Wood
| author3 = Jr. Evans
| title = Manganese incorporation behavior in molecular beam epitaxial gallium arsenide
| journal = Journal of Applied Physics
| date = July 1982
| url = http://link.aip.org/link/?JAP/53/4938/1
| doi = 10.1063/1.331328
| bibcode = 1982JAP....53.4938D
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223195641/http://link.aip.org/link/?JAP/53/4938/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref>
This problem was overcome using the technique of low-temperature [[Molecular-beam epitaxy|MBE]]. It was found, first in (In,Mn)As<ref name =munekata_diluted_1989>
{{Cite journal
| doi = 10.1103/PhysRevLett.63.1849
| volume = 63
| issue = 17
| pages = 1849–1852
| last = Munekata
| first = H.
|author2=H. Ohno |author3=S. von Molnar |author4=Armin Segmüller |author5=L. L. Chang |author6=L. Esaki
| title = Diluted magnetic III-V semiconductors
| journal = Physical Review Letters
| date = 1989-10-23
| pmid = 10040689
| bibcode=1989PhRvL..63.1849M
}}</ref>
and then later used for (Ga,Mn)As,<ref name=ohno_(gamn)as_1996>{{Cite journal
| volume = 69
| issue = 3
| pages = 363–365
| last = Ohno
| first = H.
| author2 = A. Shen
| author3 = F. Matsukura
| author4 = A. Oiwa
| author5 = A. Endo
| author6 = S. Katsumoto
| author7 = Y. Iye
| title = (Ga,Mn)As: A new diluted magnetic semiconductor based on GaAs
| journal = Applied Physics Letters
| date = 1996-07-15
| url = http://link.aip.org/link/?APL/69/363/1
| doi = 10.1063/1.118061
| bibcode = 1996ApPhL..69..363O
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223092401/http://link.aip.org/link/?APL/69/363/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref>
that by utilising non-equilibrium crystal growth techniques larger [[dopant]] concentrations could be successfully incorporated. At lower temperatures, around 250&nbsp;°C, there is insufficient thermal energy for surface segregation to occur but still sufficient for a good quality single crystal alloy to form.<ref name=ohno_making_1998>
{{Cite journal
| doi = 10.1126/science.281.5379.951
| volume = 281
| issue = 5379
| pages = 951–956
| last = Ohno
| first = H.
| title = Making Nonmagnetic Semiconductors Ferromagnetic
| journal = Science
| date = 1998-08-14
| url = http://www.sciencemag.org/cgi/content/abstract/281/5379/951
| pmid=9703503
|bibcode = 1998Sci...281..951O }}</ref>

In addition to the substitutional incorporation of manganese, low-temperature [[Molecular-beam epitaxy|MBE]] also causes the inclusion of other impurities. The two other common impurities are interstitial manganese<ref name=yu_effect_2002>
{{Cite journal
| volume = 65
| issue = 20
| pages = 201303
| last = Yu
| first = K. M.
|author2=W. Walukiewicz |author3=T. Wojtowicz |author4=I. Kuryliszyn |author5=X. Liu |author6=Y. Sasaki |author7=J. K. Furdyna
| title = Effect of the location of Mn sites in ferromagnetic Ga<sub>1−x</sub>Mn<sub>x</sub>As on its Curie temperature
| journal = Physical Review B
| date = 2002-04-23
| doi = 10.1103/PhysRevB.65.201303
|bibcode = 2002PhRvB..65t1303Y | url = https://zenodo.org/record/1233743
}}</ref>
and arsenic antisites.<ref name=grandidier_atomic-scale_2000>{{Cite journal
| volume = 77
| issue = 24
| pages = 4001–4003
| last = Grandidier
| first = B.
| author2 = J. P. Nys
| author3 = C. Delerue
| author4 = D. Stievenard
| author5 = Y. Higo
| author6 = M. Tanaka
| title = Atomic-scale study of GaMnAs/GaAs layers
| journal = Applied Physics Letters
| date = 2000-12-11
| url = http://link.aip.org/link/?APL/77/4001/1
| doi = 10.1063/1.1322052
| bibcode = 2000ApPhL..77.4001G
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223120914/http://link.aip.org/link/?APL/77/4001/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref>
The former is where the manganese atom sits between the other atoms in the zinc-blende lattice structure and the latter is where an arsenic atom occupies a gallium site. Both impurities act as double donors, removing the [[Electron hole|holes]] provided by the substitutional manganese, and as such they are known as compensating defects. The interstitial manganese also bond [[Antiferromagnetism|antiferromagnetically]] to substitutional manganese, removing the magnetic moment. Both these defects are detrimental to the [[Ferromagnetism|ferromagnetic]] properties of the (Ga,Mn)As, and so are undesired.<ref name=sadowski_influence_2004>
{{Cite journal
| doi = 10.1103/PhysRevB.69.075206
| volume = 69
| issue = 7
| pages = 075206
| last = Sadowski
| first = J.
|author2=J. Z. Domagala
| title = Influence of defects on the lattice constant of GaMnAs
| journal = Physical Review B
| date = 2004-02-19
|arxiv = cond-mat/0309033 |bibcode = 2004PhRvB..69g5206S | s2cid = 118891611
}}</ref>

The temperature below which the transition from [[paramagnetism]] to [[ferromagnetism]] occurs is known as the [[Curie point|Curie temperature]], ''T<sub>C</sub>''. Theoretical predictions based on the Zener model suggest that the [[Curie point|Curie temperature]] scales with the quantity of manganese, so ''T<sub>C</sub>'' above 300 K is possible if manganese [[Doping (semiconductor)|doping]] levels as high as 10% can be achieved.<ref name=dietl_zener_2000>
{{Cite journal
| doi = 10.1126/science.287.5455.1019
| volume = 287
| issue = 5455
| pages = 1019–1022
| last = Dietl
| first = T.
|author2=H. Ohno |author3=F. Matsukura |author4=J. Cibert |author5=D. Ferrand
| title = Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors
| journal = Science
| date = 2000-02-11
| url = http://www.sciencemag.org/cgi/content/abstract/287/5455/1019
| pmid = 10669409
| bibcode=2000Sci...287.1019D
}}</ref>
After its discovery by Ohno ''et al.'',<ref name="ohno_(gamn)as_1996"/> the highest reported [[Curie point|Curie temperatures]] in (Ga,Mn)As rose from 60 K to 110 K.<ref name=ohno_making_1998 /> However, despite the predictions of room-temperature [[ferromagnetism]], no improvements in ''T<sub>C</sub>'' were made for several years.

As a result of this lack of progress, predictions started to be made that 110 K was in fact a fundamental limit for (Ga,Mn)As. The self-compensating nature of the defects would limit the possible [[Electron hole|hole]] concentrations, preventing further gains in ''T<sub>C</sub>''.<ref name=yu_curie_2003>
{{Cite journal
| doi = 10.1103/PhysRevB.68.041308
| volume = 68
| issue = 4
| pages = 041308
| last = Yu
| first = K. M.
|author2=W. Walukiewicz |author3=T. Wojtowicz |author4=W. L. Lim |author5=X. Liu |author6=U. Bindley |author7=M. Dobrowolska |author8=J. K. Furdyna
| title = Curie temperature limit in ferromagnetic Ga<sub>1−x</sub>Mn<sub>x</sub>As
| journal = Physical Review B
| date = 2003-07-25
|arxiv = cond-mat/0303217 |bibcode = 2003PhRvB..68d1308Y | s2cid = 117990317
}}</ref>
The major breakthrough came from improvements in post-growth annealing. By using annealing temperatures comparable to the growth temperature it was possible to pass the 110 K barrier.<ref name=edmonds_high-curie-temperature_2002>{{Cite journal
| volume = 81
| issue = 26
| pages = 4991–4993
| last = Edmonds
| first = K. W.
| author2 = K. Y. Wang
| author3 = R. P. Campion
| author4 = A. C. Neumann
| author5 = N. R. S. Farley
| author6 = B. L. Gallagher
| author7 = C. T. Foxon
| title = High-Curie-temperature Ga<sub>1−x</sub>Mn<sub>x</sub>As obtained by resistance-monitored annealing
| journal = Applied Physics Letters
| date = 2002-12-23
| url = http://link.aip.org/link/?APL/81/4991/1
| doi = 10.1063/1.1529079
| arxiv = cond-mat/0209554
| bibcode = 2002ApPhL..81.4991E
| s2cid = 117381870
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223103921/http://link.aip.org/link/?APL/81/4991/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref><ref name=chiba_effect_2003>{{Cite journal
| volume = 82
| issue = 18
| pages = 3020–3022
| last = Chiba
| first = D.
| author2 = K. Takamura
| author3 = F. Matsukura
| author4 = H. Ohno
| title = Effect of low-temperature annealing on (Ga,Mn)As trilayer structures
| journal = Applied Physics Letters
| date = 2003-05-05
| url = http://link.aip.org/link/?APL/82/3020/1
| doi = 10.1063/1.1571666
| bibcode = 2003ApPhL..82.3020C
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223065849/http://link.aip.org/link/?APL/82/3020/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref><ref name=ku_highly_2003>{{Cite journal | volume = 82 | issue = 14 | pages = 2302–2304 | last1 = Ku | first1 = K. C. | first2 = S. J. | last2 = Potashnik | first3 = R. F. | last3 = Wang | first4 = S. H. | last4 = Chun | first5 = P. | last5 = Schiffer | first6 = N. | last6 = Samarth | first7 = M. J. | last7 = Seong | first8 = A. | last8 = Mascarenhas | first9 = E. | last9 = Johnston-Halperin | first10 = R. C. | last10 = Myers | first11 = A. C. | last11 = Gossard | first12 = D. D. | last12 = Awschalom | title = Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers | journal = Applied Physics Letters | date = 2003-04-07 | url = http://link.aip.org/link/?APL/82/2302/1 | doi = 10.1063/1.1564285 | arxiv = cond-mat/0210426 | bibcode = 2003ApPhL..82.2302K | s2cid = 119470957 | access-date = 2019-12-23 | archive-url = https://archive.today/20130223115931/http://link.aip.org/link/?APL/82/2302/1 | archive-date = 2013-02-23 | url-status = dead }}</ref>
These improvements have been attributed to the removal of the highly mobile interstitial manganese.<ref name=edmonds_mn_2004>
{{Cite journal
| volume = 92
| issue = 3
| pages = 037201–4
| last1 = Edmonds
| first1 = K. W.
| first2 = P. | last2 = Boguslawski | first3 = K. Y. | last3 = Wang | first4 = R. P. | last4 = Campion | first5 = S. N. | last5 = Novikov | first6 = N. R. S. | last6 = Farley | first7 = B. L. | last7 = Gallagher | first8 = C. T. | last8 = Foxon | first9 = M. | last9 = Sawicki | first10 = T. | last10 = Dietl | first11 = M. | last11 = Buongiorno Nardelli | first12 = J. | last12 = Bernholc
| title = Mn Interstitial Diffusion in (Ga,Mn)As
| journal = Physical Review Letters
| date = 2004-01-23
| doi = 10.1103/PhysRevLett.92.037201
| pmid=14753901
| bibcode=2004PhRvL..92c7201E
|arxiv = cond-mat/0307140 | s2cid = 26218929
}}</ref>

Currently, the highest reported values of ''T<sub>C</sub>'' in (Ga,Mn)As are around 173 K,<ref name=wang_magnetism_2005>
{{Cite conference
| publisher = AIP
| conference = PHYSICS OF SEMICONDUCTORS: 27th International Conference on the Physics of Semiconductors&nbsp;– ICPS-27
| volume = 772
| pages = 333–334
| last1 = Wang
| first1 = K. Y.
| first2 = R. P. | last2 = Campion | first3 = K. W. | last3 = Edmonds | first4 = M. | last4 = Sawicki | first5 = T. | last5 = Dietl | first6 = C. T. | last6 = Foxon | first7 = B. L. | last7 = Gallagher
| title = Magnetism in (Ga,Mn)As Thin Films With T<sub>C</sub> Up To 173K
| booktitle = Proceedings of the 27th International Conference on the Physics of Semiconductors
| location = Flagstaff, Arizona (USA)
| date = 2005-06-30
| doi = 10.1063/1.1994124
| arxiv = cond-mat/0411475}}</ref><ref name=jungwirth_prospects_2005>
{{Cite journal
| doi = 10.1103/PhysRevB.72.165204
| volume = 72
| issue = 16
| pages = 165204–13
| last1 = Jungwirth
| first1 = T.
| first2 = K. Y. | last2 = Wang | first3 = J. | last3 = Masek | first4 = K. W. | last4 = Edmonds | first5 = Jurgen | last5 = Konig | first6 = Jairo | last6 = Sinova | first7 = M. | last7 = Polini | first8 = N. A. | last8 = Goncharuk | first9 = A. H. | last9 = MacDonald | first10 = M. | last10 = Sawicki | first11 = A. W. | last11 = Rushforth | first12 = R. P. | last12 = Campion | first13 = L. X. | last13 = Zhao | first14 = C. T. | last14 = Foxon | first15 = B. L. | last15 = Gallagher
| title = Prospects for high temperature ferromagnetism in (Ga,Mn)As semiconductors
| journal = Physical Review B
| date = 2005-10-15
|arxiv = cond-mat/0505215 |bibcode = 2005PhRvB..72p5204J | hdl = 1969.1/146812
| s2cid = 21715086
}}</ref>
still well below the much sought room-temperature. As a result, measurements on this material must be done at cryogenic temperatures, currently precluding any application outside of the laboratory. Naturally, considerable effort is being spent in the search for an alternative [[Magnetic semiconductor|DMS]] that does not share this limitation.<ref name=matsumoto_room-temperature_2001>
{{Cite journal
| doi = 10.1126/science.1056186
| volume = 291
| issue = 5505
| pages = 854–856
| last = Matsumoto
| first = Yuji |author2=Makoto Murakami |author3=Tomoji Shono |author4=Tetsuya Hasegawa |author5=Tomoteru Fukumura |author6=Masashi Kawasaki |author7=Parhat Ahmet |author8=Toyohiro Chikyow |author9=Shin-ya Koshihara |author10=Hideomi Koinuma
| title = Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide
| journal = Science
| date = 2001-02-02
| url = http://www.sciencemag.org/cgi/content/abstract/291/5505/854
| pmid = 11228146
|bibcode = 2001Sci...291..854M | s2cid = 7529257
}}</ref><ref name=reed_room_2001>{{Cite journal
| volume = 79
| issue = 21
| pages = 3473–3475
| last = Reed
| first = M. L.
| author2 = N. A. El-Masry
| author3 = H. H. Stadelmaier
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| title = Room temperature ferromagnetic properties of (Ga, Mn)N
| journal = Applied Physics Letters
| date = 2001-11-19
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}}</ref><ref name=han_key_2002>{{Cite journal
| volume = 81
| issue = 22
| pages = 4212–4214
| last1 = Han
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| first2 = J. W.
| last2 = Song
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| title = A key to room-temperature ferromagnetism in Fe-doped ZnO: Cu
| journal = Applied Physics Letters
| date = 2002-11-25
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| archive-url = https://archive.today/20130223093539/http://link.aip.org/link/?APL/81/4212/1
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| arxiv = cond-mat/0208399
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}}</ref><ref name=saito_room-temperature_2003>
{{Cite journal
| doi = 10.1103/PhysRevLett.90.207202
| volume = 90
| issue = 20
| pages = 207202
| last = Saito
| first = H.
|author2=V. Zayets |author3=S. Yamagata |author4=K. Ando
| title = Room-Temperature Ferromagnetism in a II-VI Diluted Magnetic Semiconductor Zn<sub>1−x</sub>Cr<sub>x</sub>Te
| journal = Physical Review Letters
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{{Cite journal
| doi = 10.1038/nmat984
| volume = 2
| issue = 10
| pages = 673–677
| last = Sharma
| first = Parmanand
|author2=Amita Gupta |author3=K. V. Rao |author4=Frank J. Owens |author5=Renu Sharma |author6=Rajeev Ahuja |author7=J. M. Osorio Guillen |author8=Borje Johansson |author9=G. A. Gehring
| title = Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO
| journal = Nature Materials
| date = October 2003
| pmid = 14502276
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}}</ref>
In addition to this, as [[Molecular-beam epitaxy|MBE]] techniques and equipment are refined and improved it is hoped that greater control over growth conditions will allow further incremental advances in the [[Curie point|Curie temperature]] of (Ga,Mn)As.

==Properties==

Regardless of the fact that room-temperature [[ferromagnetism]] has not yet been achieved, [[Magnetic semiconductor|DMS]] materials such as (Ga,Mn)As, have shown considerable success. Thanks to the rich interplay of physics inherent to [[Magnetic semiconductor|DMSs]] a variety of novel phenomena and device structures have been demonstrated. It is therefore instructive to make a critical review of these main developments.

A key result in [[Magnetic semiconductor|DMS]] technology is [[Field-effect transistor|gateable]] [[ferromagnetism]], where an electric field is used to control the [[Ferromagnetism|ferromagnetic]] properties. This was achieved by Ohno ''et al.''<ref name=ohno_electric-field_2000>
{{Cite journal
| volume = 408
| pages = 944–946
| last = Ohno
| first = H.
|author2=D. Chiba |author3=F. Matsukura |author4=T. Omiya |author5=E. Abe |author6=T. Dietl |author7=Y. Ohno |author8=K. Ohtani
| title = Electric-field control of ferromagnetism
| journal = Nature
| date = 2000-12-01
| bibcode = 2000Natur.408..944O
| doi = 10.1038/35050040
| pmid = 11140674
| issue = 6815
| s2cid = 4397543
}}</ref>
using an insulating-gate [[field-effect transistor]] with (In,Mn)As as the magnetic channel. The magnetic properties were inferred from magnetization dependent [[Hall effect|Hall measurements]] of the channel. Using the [[Field-effect transistor|gate]] action to either deplete or accumulate [[Electron hole|holes]] in the channel it was possible to change the characteristic of the [[Hall effect|Hall]] response to be either that of a [[Paramagnetism|paramagnet]] or of a [[Ferromagnetism|ferromagnet]]. When the temperature of the sample was close to its ''T<sub>C</sub>'' it was possible to turn the [[ferromagnetism]] on or off by applying a [[Field-effect transistor|gate]] voltage which could change the ''T<sub>C</sub>'' by ±1 K.

A similar (In,Mn)As transistor device was used to provide further examples of [[Field-effect transistor|gateable]] [[ferromagnetism]].<ref name=chiba_electrical_2003>
{{Cite journal
| doi = 10.1126/science.1086608
| volume = 301
| issue = 5635
| pages = 943–945
| last = Chiba
| first = D.
|author2=M. Yamanouchi |author3=F. Matsukura |author4=H. Ohno
| title = Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor
| journal = Science
| date = 2003-08-15
| url = http://www.sciencemag.org/cgi/content/abstract/301/5635/943
| pmid = 12855816
|bibcode = 2003Sci...301..943C | s2cid = 29083264
}}</ref>
In this experiment the electric field was used to modify the coercive field at which magnetization reversal occurs. As a result of the dependence of the magnetic [[hysteresis]] on the [[Field-effect transistor|gate bias]] the electric field could be used to assist magnetization reversal or even demagnetize the [[Ferromagnetism|ferromagnetic]] material.
The combining of magnetic and electronic functionality demonstrated by this experiment is one of the goals of [[spintronics]] and may be expected to have a great technological impact.

Another important [[spintronic]] functionality that has been demonstrated in [[Magnetic semiconductor|DMSs]] is that of [[Spin (physics)|spin injection]]. This is where the high [[spin polarization]] inherent to these magnetic materials is used to transfer [[Spin polarization|spin polarized]] [[Charge carrier|carriers]] into a non-magnetic material.<ref name=ohno_electrical_1999>
{{Cite journal
| doi = 10.1038/45509
| volume = 402
| issue = 6763
| pages = 790–792
| last = Ohno
| first = Y.
|author2=D. K. Young |author3=B. Beschoten |author4=F. Matsukura |author5=H. Ohno |author6=D. D. Awschalom
| title = Electrical spin injection in a ferromagnetic semiconductor heterostructure
| journal = Nature
| date = 1999-12-16
|bibcode = 1999Natur.402..790O | s2cid = 4428472
}}</ref>
In this example, a fully [[epitaxial]] [[heterostructure]] was used where [[Spin polarization|spin polarized]] [[Electron hole|holes]] were injected from a (Ga,Mn)As layer to an (In,Ga)As [[quantum well]] where they combine with unpolarized electrons from an ''n''-type substrate. A polarization of 8% was measured in the resulting [[electroluminescence]]. This is again of potential technological interest as it shows the possibility that the [[Spin (physics)|spin states]] in non-magnetic [[semiconductors]] can be manipulated without the application of a magnetic field.

(Ga,Mn)As offers an excellent material to study [[Domain wall (magnetism)|domain wall]] mechanics because the domains can have a size of the order of 100&nbsp;μm.<ref name=fukumura_magnetic_2001>
{{Cite journal
| doi = 10.1016/S1386-9477(01)00068-6
| volume = 10
| issue = 1–3
| pages = 135–138
| last = Fukumura
| first = T.
|author2=T. Shono |author3=K. Inaba |author4=T. Hasegawa |author5=H. Koinuma |author6=F. Matsukura |author7=H. Ohno
| title = Magnetic domain structure of a ferromagnetic semiconductor (Ga,Mn)As observed with scanning probe microscopes
| journal = Physica E
| date = May 2001
|bibcode = 2001PhyE...10..135F }}</ref>
Several studies have been done in which [[Nanolithography|lithographically]] defined lateral constrictions<ref name=honolka_domain-wall_2005>{{Cite journal
| volume = 97
| issue = 6
| pages = 063903–063903–4
| last = Honolka
| first = J.
| author2 = S. Masmanidis
| author3 = H. X. Tang
| author4 = M. L. Roukes
| author5 = D. D. Awschalom
| title = Domain-wall dynamics at micropatterned constrictions in ferromagnetic (Ga,Mn)As epilayers
| journal = Journal of Applied Physics
| date = 2005-03-15
| url = http://link.aip.org/link/?JAP/97/063903/1
| archive-url = https://archive.today/20130223185836/http://link.aip.org/link/?JAP/97/063903/1
| url-status = dead
| archive-date = 2013-02-23
| doi = 10.1063/1.1861512
| bibcode = 2005JAP....97f3903H
}}</ref>
or other pinning points<ref name=holleitner_manipulating_2005>{{Cite journal
| volume = 97
| issue = 10
| pages = 10D314
| last = Holleitner
| first = A. W.
| author2 = H. Knotz
| author3 = R. C. Myers
| author4 = A. C. Gossard
| author5 = D. D. Awschalom
| title = Manipulating a domain wall in (Ga,Mn)As
| journal = J. Appl. Phys.
| date = 2005-05-15
| url = http://link.aip.org/link/?JAP/97/10D314/1
| doi = 10.1063/1.1849055
| bibcode = 2005JAP....97jD314H
| access-date = 2019-12-23
| archive-url = https://archive.today/20130223071205/http://link.aip.org/link/?JAP/97/10D314/1
| archive-date = 2013-02-23
| url-status = dead
}}</ref>
are used to manipulate [[Domain wall (magnetism)|domain wall]]s. These experiments are crucial to understanding [[Domain wall (magnetism)|domain wall]] nucleation and propagation which would be necessary for the creation of complex logic circuits based on [[Domain wall (magnetism)|domain wall]] mechanics.<ref name=allwood_magnetic_2005>
{{Cite journal
| doi = 10.1126/science.1108813
| volume = 309
| issue = 5741
| pages = 1688–1692
| last = Allwood
| first = D. A.
|author2=G. Xiong |author3=C. C. Faulkner |author4=D. Atkinson |author5=D. Petit |author6=R. P. Cowburn
| title = Magnetic Domain-Wall Logic
| journal = Science
| date = 2005-09-09
| pmid = 16151002
|bibcode = 2005Sci...309.1688A | s2cid = 23385116
}}</ref>
Many properties of [[Domain wall (magnetism)|domain wall]]s are still not fully understood and one particularly outstanding issue is of the magnitude and size of the resistance associated with current passing through [[Domain wall (magnetism)|domain wall]]s. Both positive<ref name=chiba_domain-wall_2006>
{{Cite journal
| volume = 96
| issue = 9
| pages = 096602
| last = Chiba
| first = D.
|author2=M. Yamanouchi |author3=F. Matsukura |author4=T. Dietl |author5=H. Ohno
| title = Domain-Wall Resistance in Ferromagnetic (Ga,Mn)As
| journal = Physical Review Letters
| date = 2006-03-10
| doi = 10.1103/PhysRevLett.96.096602
| pmid = 16606291
| bibcode=2006PhRvL..96i6602C
|arxiv = cond-mat/0601464 | s2cid = 32575691
}}</ref>
and negative<ref name=tang_negative_2004>
{{Cite journal
| doi = 10.1038/nature02809
| volume = 431
| issue = 7004
| pages = 52–56
| last = Tang
| first = H. X.
|author2=S. Masmanidis |author3=R. K. Kawakami |author4=D. D. Awschalom |author5=M. L. Roukes
| title = Negative intrinsic resistivity of an individual domain wall in epitaxial (Ga,Mn)As microdevices
| journal = Nature
| year = 2004
| pmid = 15343329
|bibcode = 2004Natur.431...52T | s2cid = 4418295
}}</ref>
values of [[Domain wall (magnetism)|domain wall]] resistance have been reported, leaving this an open area for future research.

An example of a simple device that utilizes pinned [[Domain wall (magnetism)|domain wall]]s is provided by reference.<ref name=ruster_very_2003>
{{Cite journal
| volume = 91
| issue = 21
| pages = 216602
| last = Ruster
| first = C.
|author2=T. Borzenko |author3=C. Gould |author4=G. Schmidt |author5=L. W. Molenkamp |author6=X. Liu |author7=T. J. Wojtowicz |author8=J. K. Furdyna |author9=Z. G. Yu |author10=M. E. Flattý
| title = Very Large Magnetoresistance in Lateral Ferromagnetic (Ga,Mn)As Wires with Nanoconstrictions
| journal = Physical Review Letters
| date = 2003-11-20
| doi = 10.1103/PhysRevLett.91.216602
| pmid = 14683324
| bibcode=2003PhRvL..91u6602R
|arxiv = cond-mat/0308385 | s2cid = 13075466
}}</ref>
This experiment consisted of a [[Nanolithography|lithographically]] defined narrow island connected to the leads via a pair of nanoconstrictions. While the device operated in a diffusive regime the constrictions would pin [[Domain wall (magnetism)|domain wall]]s, resulting in a [[giant magnetoresistance|giant magnetoresistance (GMR)]] signal. When the device operates in a tunnelling regime another [[magnetoresistance|magnetoresistance (MR)]] effect is observed, discussed below.

A furtherproperty of [[Domain wall (magnetism)|domain wall]]s is that of current induced [[Domain wall (magnetism)|domain wall]] motion. This reversal is believed to occur as a result of the [[Spin torque transfer|spin-transfer torque]] exerted by a [[Spin polarization|spin polarized]] current.<ref name=slonczewski_current-driven_1996>
{{Cite journal
| doi = 10.1016/0304-8853(96)00062-5
| volume = 159
| issue = 1–2
| pages = L1–L7
| last = Slonczewski
| first = J. C.
| title = Current-driven excitation of magnetic multilayers
| journal = Journal of Magnetism and Magnetic Materials
| date = June 1996
|bibcode = 1996JMMM..159L...1S }}</ref>
It was demonstrated in reference<ref name=yamanouchi_current-induced_2004>
{{Cite journal
| doi = 10.1038/nature02441
| volume = 428
| issue = 6982
| pages = 539–542
| last = Yamanouchi
| first = M.
|author2=D. Chiba |author3=F. Matsukura |author4=H. Ohno
| title = Current-induced domain-wall switching in a ferromagnetic semiconductor structure
| journal = Nature
| date = 2004-04-01
| pmid = 15057826
|bibcode = 2004Natur.428..539Y | s2cid = 4345181
}}</ref>
using a lateral (Ga,Mn)As device containing three regions which had been patterned to have different coercive fields, allowing the easy formation of a [[Domain wall (magnetism)|domain wall]]. The central region was designed to have the lowest coercivity so that the application of current pulses could cause the orientation of the magnetization to be switched. This experiment showed that the current required to achieve this reversal in (Ga,Mn)As was two orders of magnitude lower than that of metal systems. It has also been demonstrated that current-induced magnetization reversal can occur across a (Ga,Mn)As/GaAs/(Ga,Mn)As vertical tunnel junction.<ref name=chiba_current-driven_2004>
{{Cite journal
| doi = 10.1103/PhysRevLett.93.216602
| volume = 93
| issue = 21
| pages = 216602
| last = Chiba
| first = D.
|author2=Y. Sato |author3=T. Kita |author4=F. Matsukura |author5=H. Ohno
| title = Current-Driven Magnetization Reversal in a Ferromagnetic Semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As Tunnel Junction
| journal = Physical Review Letters
| date = 2004-11-18
| pmid = 15601045
| bibcode=2004PhRvL..93u6602C
|arxiv = cond-mat/0403500 | s2cid = 10297317
}}</ref>

Another novel [[spintronic]] effect, which was first observed in (Ga,Mn)As based tunnel devices, is tunnelling anisotropic magnetoresistance (TAMR). This effect arises from the intricate dependence of the tunnelling density of states on the magnetization, and can result in [[magnetoresistance|MRs]] of several orders of magnitude. This was demonstrated first in vertical tunnelling structures<ref name=ruster_very_2003 /><ref name=gould_tunneling_2004>
{{Cite journal
| volume = 93
| issue = 11
| pages = 117203
| last = Gould
| first = C.
|author2=C. Ruster |author3=T. Jungwirth |author4=E. Girgis |author5=G. M. Schott |author6=R. Giraud |author7=K. Brunner |author8=G. Schmidt |author9=L. W. Molenkamp
| title = Tunneling Anisotropic Magnetoresistance: A Spin-Valve-Like Tunnel Magnetoresistance Using a Single Magnetic Layer
| journal = Physical Review Letters
| year = 2004
| doi = 10.1103/PhysRevLett.93.117203
| pmid = 15447375
| bibcode=2004PhRvL..93k7203G
|arxiv = cond-mat/0407735 | s2cid = 222508
}}</ref>
and then later in lateral devices.<ref name=giddings_large_2005>
{{Cite journal
| volume = 94
| issue = 12
| pages = 127202–4
| last1 = Giddings
| first1 = A. D.
| first2 = M. N. | last2 = Khalid | first3 = T. | last3 = Jungwirth | first4 = J. | last4 = Wunderlich | first5 = S. | last5 = Yasin | first6 = R. P. | last6 = Campion | first7 = K. W. | last7 = Edmonds | first8 = J. | last8 = Sinova | first9 = K. | last9 = Ito | first10 = K.-Y. | last10 = Wang | first11 = D. | last11 = Williams | first12 = B. L. | last12 = Gallagher | first13 = C. T. | last13 = Foxon
| title = Large Tunneling Anisotropic Magnetoresistance in (Ga,Mn)As Nanoconstrictions
| journal = Physical Review Letters
| date = 2005-04-01
| doi = 10.1103/PhysRevLett.94.127202
| bibcode=2005PhRvL..94l7202G
|arxiv = cond-mat/0409209 | pmid=15903954| s2cid = 119470467
}}</ref>
This has established TAMR as a generic property of ferromagmetic tunnel structures. Similarly, the dependence of the single electron charging energy on the magnetization has resulted in the obersvation of another dramatic [[magnetoresistance|MR]] effect in a (Ga,Mn)As device, the so-called [[Coulomb blockade]] anisotropic magnetoresistance (CBAMR).

==Further reading==
There are many excellent review articles about the properties and applications of [[Magnetic semiconductor|DMSs]] and (Ga,Mn)As in particular. If further information is required on the topic, several reviews are recommended:

*{{Cite journal
| doi = 10.1016/S0038-1098(03)00337-5
| volume = 127
| issue = 2
| pages = 99–107
| last = Das Sarma
| first = S.
|author2=E. H. Hwang |author3=A. Kaminski
| title = How to make semiconductors ferromagnetic: a first course on spintronics
| journal = Solid State Communications
| date = July 2003
| bibcode=2003SSCom.127...99D
|arxiv = cond-mat/0304219 | s2cid = 97033263
}}
*{{Cite journal
| volume = 78
| issue = 3
| pages = 809–864
| last = Jungwirth
| first = T.
|author2=Jairo Sinova |author3=J. Masek |author4=J. Kucera |author5=A. H. MacDonald
| title = Theory of ferromagnetic (III,Mn)V semiconductors
| journal = Reviews of Modern Physics
| date = 2006-07-01
| bibcode = 2006RvMP...78..809J
| doi = 10.1103/RevModPhys.78.809
|arxiv = cond-mat/0603380 | s2cid = 119070905
}}
*{{Cite journal
| volume = 19
| issue = 3
| pages = 323–340
| last = Gould
| first = C.
|author2=K. Pappert |author3=G. Schmidt |author4=L. W. Molenkamp
| title = Magnetic Anisotropies and (Ga,Mn)As-based Spintronic Devices
| journal = Advanced Materials
| year = 2007
| doi = 10.1002/adma.200600126
}}

==References==
{{Reflist|2}}

[[Category:Semiconductor materials]]
[[Category:Ferromagnetic materials]]
[[Category:Gallium compounds]]
[[Category:Arsenides]]
[[Category:Manganese(III) compounds]]

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