Magnetic semiconductors are
semiconductor material
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
s that exhibit both
ferromagnetism
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
(or a similar response) and useful
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of
charge carrier
In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...
s (
n- or
p-type), practical magnetic semiconductors would also allow control of quantum
spin state (up or down). This would theoretically provide near-total
spin polarization
Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum of elementary particles, is aligned with a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in fe ...
(as opposed to
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
and other metals, which provide only ~50% polarization), which is an important property for
spintronics
Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...
applications, e.g.
spin transistor
The magnetically sensitive transistor (also known as the spin transistor or spintronic transistor—named for spintronics, the technology which this development spawned), originally proposed in 1990 by Supriyo Datta and Biswajit Das, currently sti ...
s.
While many traditional magnetic materials, such as
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
, are also semiconductors (magnetite is a
semimetal
A semimetal is a material with a very small overlap between the bottom of the conduction band and the top of the valence band.
According to electronic band theory, solids can be classified as insulators, semiconductors, semimetals, or metals. ...
semiconductor with
bandgap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
0.14 eV), materials scientists generally predict that magnetic semiconductors will only find widespread use if they are similar to well-developed semiconductor materials. To that end, dilute magnetic semiconductors (DMS) have recently been a major focus of magnetic semiconductor research. These are based on traditional semiconductors, but are
doped with
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s instead of, or in addition to, electronically active elements. They are of interest because of their unique
spintronics
Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...
properties with possible technological applications. Doped
wide band-gap metal oxides such as
zinc oxide
Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...
(ZnO) and
titanium oxide
Titanium oxide may refer to:
* Titanium dioxide (titanium(IV) oxide), TiO2
* Titanium(II) oxide (titanium monoxide), TiO, a non-stoichiometric oxide
* Titanium(III) oxide (dititanium trioxide), Ti2O3
* Ti3O
* Ti2O
* δ-TiOx (x= 0.68–0.75)
* T ...
(TiO
2) are among the best candidates for industrial DMS due to their multifunctionality in
opticomagnetic applications. In particular, ZnO-based DMS with properties such as transparency in visual region and
piezoelectricity
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ''p ...
have generated huge interest among the scientific community as a strong candidate for the fabrication of
spin transistor
The magnetically sensitive transistor (also known as the spin transistor or spintronic transistor—named for spintronics, the technology which this development spawned), originally proposed in 1990 by Supriyo Datta and Biswajit Das, currently sti ...
s and
spin-polarized light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
s, while
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
doped TiO
2 in the
anatase
Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) ...
phase of this material has further been predicted to exhibit favorable dilute magnetism.
Hideo Ohno
Hideo Ohno ( ja, 大野 英男; Hideo Ōno; born 18 December 1954, Tokyo) is a Japanese physicist. He is the 22nd president of Tohoku University, succeeding Susumu Satomi in April 2018.
Biography
Ohno received B.S., M.S. and Ph.D. degrees fro ...
and his group at the
Tohoku University
, or is a Japanese national university located in Sendai, Miyagi in the Tōhoku Region, Japan. It is informally referred to as . Established in 1907, it was the third Imperial University in Japan and among the first three Designated National ...
were the first to measure
ferromagnetism
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
in transition metal
doped compound semiconductors
Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way.
Because of ...
such as
indium arsenide
Indium arsenide, InAs, or indium monoarsenide, is a narrow-bandgap semiconductor composed of indium and arsenic. It has the appearance of grey cubic crystals with a melting point of 942 °C.
Indium arsenide is similar in properties to galli ...
and
gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
doped with
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
(the latter is commonly referred to as
GaMnAs Gallium manganese arsenide, chemical formula is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide, (chemical formula ), and readily compatible with existing semiconductor technologies. D ...
). These materials exhibited reasonably high
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
s (yet below
room temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
) that scales with the concentration of
p-type charge carriers. Ever since, ferromagnetic signals have been measured from various semiconductor hosts doped with different transition atoms.
Theory
The pioneering work of Dietl ''et al.'' showed that a modified Zener model for magnetism
well describes the carrier dependence, as well as anisotropic properties of
GaMnAs Gallium manganese arsenide, chemical formula is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide, (chemical formula ), and readily compatible with existing semiconductor technologies. D ...
.
The same theory also
predicted that room-temperature
ferromagnetism
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
should exist in heavily
p-type doped ZnO and GaN doped by Co and Mn, respectively.
These predictions were followed of a flurry of theoretical and experimental studies of various oxide and nitride semiconductors,
which apparently seemed to confirm room temperature ferromagnetism in nearly any semiconductor or insulator material
heavily doped by
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
impurities.
However, early
Density functional theory
Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...
(DFT) studies were clouded by band gap errors and overly delocalized defect levels,
and more advanced DFT studies refute most of the previous predictions of ferromagnetism.
[
]
Likewise, it has been shown that for most of the oxide based materials studies for magnetic semiconductors
do not exhibit an intrinsic ''carrier-mediated'' ferromagnetism as postulated by Dietl ''et al.''
[
]
To date,
GaMnAs Gallium manganese arsenide, chemical formula is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide, (chemical formula ), and readily compatible with existing semiconductor technologies. D ...
remains the only semiconductor material with robust coexistence of ferromagnetism persisting up to rather high Curie temperatures around 100–200 K.
Materials
The manufacturability of the materials depend on the thermal equilibrium
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubil ...
of the
dopant
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
in the base material. E.g., solubility of many dopants in
zinc oxide
Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...
is high enough to prepare the materials in bulk, while some other materials have so low solubility of dopants that to prepare them with high enough dopant concentration thermal nonequilibrium preparation mechanisms have to be employed, e.g. growth of
thin film
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
s.
Permanent magnetization has been observed in a wide range of semiconductor based materials.
Some of them exhibit a clear correlation between
carrier concentration
Charge carrier density, also known as carrier concentration, denotes the number of charge carriers in per volume. In SI units, it is measured in m−3. As with any density, in principle it can depend on position. However, usually carrier concentr ...
and magnetization,
including the work of
T. Story and co-workers where they demonstrated that the ferromagnetic Curie temperature of
Mn2+-doped
Pb1−xSnxTe can be controlled by the
carrier concentration
Charge carrier density, also known as carrier concentration, denotes the number of charge carriers in per volume. In SI units, it is measured in m−3. As with any density, in principle it can depend on position. However, usually carrier concentr ...
.
[
]
The theory proposed by Dietl required
charge carriers
In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...
in the case of
holes
A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
to mediate the
magnetic coupling A magnetic coupling is a coupling that transfers torque from one shaft to another, but using a magnetic field rather than a physical mechanical connection.
Magnetic shaft couplings are most often used for liquid pumps and propeller systems, since ...
of manganese
dopant
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
s in the prototypical magnetic semiconductor, Mn
2+-doped
GaAs
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circui ...
. If there is an insufficient hole concentration in the magnetic semiconductor, then the
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
would be very low or would exhibit only
paramagnetism
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
. However, if the hole concentration is high (
>~10
20 cm
−3), then the
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
would be higher, between 100–200 K.
However, many of the semiconductor materials studied exhibit a permanent magnetization ''extrinsic''
to the semiconductor host material.
A lot of the elusive extrinsic ferromagnetism (or ''phantom ferromagnetism'')
is observed in thin films or nanostructured materials.
[
]
Several examples of proposed ferromagnetic semiconductor materials are listed below. Notice that many of the observations and/or predictions below remain heavily debated.
*
Manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
-doped
indium arsenide
Indium arsenide, InAs, or indium monoarsenide, is a narrow-bandgap semiconductor composed of indium and arsenic. It has the appearance of grey cubic crystals with a melting point of 942 °C.
Indium arsenide is similar in properties to galli ...
and
gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
(
GaMnAs Gallium manganese arsenide, chemical formula is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide, (chemical formula ), and readily compatible with existing semiconductor technologies. D ...
), with Curie temperature around 50–100 K and 100–200 K, respectively
* Manganese-doped
indium antimonide
Indium antimonide (InSb) is a crystalline compound made from the elements indium (In) and antimony (Sb). It is a narrow- gap semiconductor material from the III- V group used in infrared detectors, including thermal imaging cameras, FLIR systems ...
, which becomes ferromagnetic even at room temperature and even with less than 1% Mn.
* Oxide semiconductors
** Manganese- and
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
-doped
indium oxide
Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts p ...
, ferromagnetic at room temperature. The ferromagnetism appears to be mediated by carrier-electrons, in a similar way as the
GaMnAs Gallium manganese arsenide, chemical formula is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide, (chemical formula ), and readily compatible with existing semiconductor technologies. D ...
ferromagnetism is mediated by carrier-holes.
**
Zinc oxide
Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...
*** Manganese-doped
zinc oxide
Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...
***
n-type cobalt-doped
zinc oxide
Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...
*** Lanthanide-doped zinc oxide
**
Magnesium oxide
Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...
:
***
p-type transparent MgO films with cation vacancies, combining ferromagnetism and multilevel switching (
memristor
A memristor (; a portmanteau of ''memory resistor'') is a non-linear two-terminal electrical component relating electric charge and magnetic flux linkage. It was described and named in 1971 by Leon Chua, completing a theoretical quartet of fu ...
)
**
Titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...
:
***
Cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
-doped
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...
(both
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer Polymorphism (materials science), polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest ...
and
anatase
Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) ...
), ferromagnetic above 400
K
***
Chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
-doped
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer Polymorphism (materials science), polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest ...
, ferromagnetic above 400
K
***
Iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
-doped rutile and iron-doped anatase, ferromagnetic at room temperature
***
Copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
-doped
anatase
Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) ...
***
Nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
-doped
anatase
Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) ...
**
Tin dioxide
Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in tin ...
*** Manganese-doped
tin dioxide
Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in tin ...
, with Curie temperature at 340 K
*** Iron-doped
tin dioxide
Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in tin ...
, with Curie temperature at 340 K
*** Strontium-doped tin dioxide () – Dilute magnetic semiconductor. Can be synthesized an
epitaxial
Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
thin film on a silicon chip.
**
Europium(II) oxide
Europium(II) oxide (EuO) is a chemical compound which is one of the oxides of europium. In addition to europium(II) oxide, there is also europium(III) oxide and the mixed valence europium(II,III) oxide.
Preparation
Europium(II) oxide can be prepa ...
, with a Curie temperature of 69K. The curie temperature can be more than doubled by doping (e.g. oxygen deficiency, Gd).
*
Nitride
In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...
semiconductors
** Chromium doped
aluminium nitride
** Manganese doped gallium nitride and boron nitride
*(Ba,K)(Zn,Mn)
2As
2: Ferromagnetic semiconductor with tetragonal average structure and orthorhombic local structure.
References
External links
*
*
{{DEFAULTSORT:Magnetic Semiconductor
Semiconductor material types
Spintronics
Ferromagnetic materials
de:Halbleiter#Semimagnetische Halbleiter