GaMnAs
Encyclopedia
Gallium manganese arsenide is a magnetic semiconductor
. It is based on the world's second favorite semiconductor
, GaAs
, and as such is readily compatible with existing semiconductor
technologies. Differently from other dilute magnetic semiconductors (DMSs)
, such as the majority of those based on II-VI semiconductors, it is not paramagnetic
.
but ferromagnetic
, and hence exhibits hysteretic magnetization behavior. 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
, making it a p-type material. The presence of carriers
allows the material to be used for spin-polarized
currents. In contrast, many other ferromagnetic
DMSs
are strongly insulating
and so do not possess free carriers
. (Ga,Mn)As is therefore a candidate as a spintronic material.
, (Ga,Mn)As is formed by doping
a standard semiconductor
with magnetic elements. This is done using the growth technique 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 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°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.
This problem was overcome using the technique of low-temperature MBE. It was found, first in (In,Mn)As
and then later used for (Ga,Mn)As,
that by utilising non-equilibrium crystal growth techniques larger dopant
concentrations could be successfully incorporated. At lower temperatures, around 250°C, there is insufficient thermal energy for surface segregation to occur but still sufficient for a good quality single crystal alloy to form.
In addition to the substitutional incorporation of manganese, low-temperature MBE also causes the inclusion of other impurities. The two other common impurities are interstitial manganese
and arsenic antisites.
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 holes
provided by the substitutional manganese, and as such they are known as compensating defects. The interstitial manganese also bond antiferromagnetically
to substitutional manganese, removing the magnetic moment. Both these defects are detrimental to the ferromagnetic
properties of the (Ga,Mn)As, and so are undesired.
The temperature below which the transition from paramagnetism
to ferromagnetism
occurs is known as the Curie temperature
, TC. Theoretical predictions based on the Zener model suggest that the Curie temperature
scales with the quantity of manganese, so TC above 300 K is possible if manganese doping
levels as high as 10% can be achieved.
After its discovery by Ohno et al., the highest reported Curie temperatures
in (Ga,Mn)As rose from 60 K to 110 K. However, despite the predictions of room-temperature ferromagnetism
, no improvements in TC 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 hole
concentrations, preventing further gains in TC.
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.
These improvements have been attributed to the removal of the highly mobile interstitial manganese.
Currently, the highest reported values of TC in (Ga,Mn)As are around 173 K,
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 DMS
that does not share this limitation.
In addition to this, as 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 temperature
of (Ga,Mn)As.
has not yet been achieved, DMS
materials such as (Ga,Mn)As, have shown considerable success. Thanks to the rich interplay of physics inherent to 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 DMS
technology is gateable
ferromagnetism
, where an electric field is used to control the ferromagnetic
properties. This was achieved by Ohno et al.
using an insulating-gate field-effect transistor
with (In,Mn)As as the magnetic channel. The magnetic properties were inferred from magnetization dependent Hall measurements
of the channel. Using the gate
action to either deplete or accumulate holes
in the channel it was possible to change the characteristic of the Hall
response to be either that of a paramagnet
or of a ferromagnet
. When the temperature of the sample was close to its TC it was possible to turn the ferromagnetism
on or off by applying a gate
voltage which could change the TC by ±1 K.
A similar (In,Mn)As transistor device was used to provide further examples of gateable
ferromagnetism
.
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 gate bias
the electric field could be used to assist magnetization reversal or even demagnetize the 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 DMSs
is that of spin injection
. This is where the high spin polarization
inherent to these magnetic materials is used to transfer spin polarized
carriers
into a non-magnetic material.
In this example, a fully epitaxial heterostructure was used where spin polarized
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 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
mechanics because the domains can have a size of the order of 100 µm.
Several studies have been done in which lithographically
defined lateral constrictions
or other pinning points
are used to manipulate domain wall
s. These experiments are crucial to understanding domain wall
nucleation and propagation which would be necessary for the creation of complex logic circuits based on domain wall
mechanics.
Many properties of 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
s. Both positive
and negative
values of 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
s is provided by reference.
This experiment consisted of a 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
s, resulting in a giant magnetoresistance (GMR) signal. When the device operates in a tunnelling regime another magnetoresistance (MR)
effect is observed, discussed below.
A further interesting property of domain wall
s is that of current induced domain wall
motion. This reversal is believed to occur as a result of the spin-transfer torque
exerted by a spin polarized
current.
It was demonstrated in reference
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
. 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. Interestingly, 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.
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 MRs
of several orders of magnitude. This was demonstrated first in vertical tunnelling structures
and then later in lateral devices.
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 MR
effect in a (Ga,Mn)As device, the so-called Coulomb blockade
anisotropic magnetoresistance (CBAMR).
There are many excellent review articles about the properties and applications of DMSs
and (Ga,Mn)As in particular. If further information is required on the topic, the following reviews are recommended:.
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
. It is based on the world's second favorite semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
, GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
, and as such is readily compatible with existing semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
technologies. Differently from other dilute magnetic semiconductors (DMSs)
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
, such as the majority of those based on II-VI semiconductors, it is not paramagnetic
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
.
but ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
, and hence exhibits hysteretic magnetization behavior. 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
Acceptor (semiconductors)
In semiconductor physics, an acceptor is a dopant atom that when added to a semiconductor can form p-type regions.For example, when silicon , having four valence electrons, needs to be doped as a p-type semiconductor, elements from group III like boron or aluminium , having three valence...
, making it a p-type material. The presence of carriers
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
allows the material to be used for spin-polarized
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 ferromagnetic metals, such as iron, giving rise to...
currents. In contrast, many other ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
DMSs
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
are strongly insulating
and so do not possess free carriers
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
. (Ga,Mn)As is therefore a candidate as a spintronic material.
Growth
Like other DMSsMagnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
, (Ga,Mn)As is formed by doping
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
a standard semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
with magnetic elements. This is done using the growth technique 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
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
crystal and provide a magnetic moment. Because manganese has a low solubility in GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
, incorporating a sufficiently high concentration for ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
to be achieved proves challenging. In standard 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°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.
This problem was overcome using the technique of low-temperature MBE. It was found, first in (In,Mn)As
and then later used for (Ga,Mn)As,
that by utilising non-equilibrium crystal growth techniques larger dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
concentrations could be successfully incorporated. At lower temperatures, around 250°C, there is insufficient thermal energy for surface segregation to occur but still sufficient for a good quality single crystal alloy to form.
In addition to the substitutional incorporation of manganese, low-temperature MBE also causes the inclusion of other impurities. The two other common impurities are interstitial manganese
and arsenic antisites.
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 holes
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
provided by the substitutional manganese, and as such they are known as compensating defects. The interstitial manganese also bond antiferromagnetically
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...
to substitutional manganese, removing the magnetic moment. Both these defects are detrimental to the ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
properties of the (Ga,Mn)As, and so are undesired.
The temperature below which the transition from paramagnetism
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
to ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
occurs is known as the Curie temperature
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
, TC. Theoretical predictions based on the Zener model suggest that the Curie temperature
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
scales with the quantity of manganese, so TC above 300 K is possible if manganese doping
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
levels as high as 10% can be achieved.
After its discovery by Ohno et al., the highest reported Curie temperatures
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
in (Ga,Mn)As rose from 60 K to 110 K. However, despite the predictions of room-temperature ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
, no improvements in TC 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 hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
concentrations, preventing further gains in TC.
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.
These improvements have been attributed to the removal of the highly mobile interstitial manganese.
Currently, the highest reported values of TC in (Ga,Mn)As are around 173 K,
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 DMS
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
that does not share this limitation.
In addition to this, as 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 temperature
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
of (Ga,Mn)As.
Properties
Regardless of the fact that room-temperature ferromagnetismFerromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
has not yet been achieved, DMS
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
materials such as (Ga,Mn)As, have shown considerable success. Thanks to the rich interplay of physics inherent to DMSs
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
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 DMS
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
technology is gateable
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
, where an electric field is used to control the ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
properties. This was achieved by Ohno et al.
using an insulating-gate field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
with (In,Mn)As as the magnetic channel. The magnetic properties were inferred from magnetization dependent Hall measurements
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...
of the channel. Using the gate
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
action to either deplete or accumulate holes
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
in the channel it was possible to change the characteristic of the Hall
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...
response to be either that of a paramagnet
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
or of a ferromagnet
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
. When the temperature of the sample was close to its TC it was possible to turn the ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
on or off by applying a gate
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
voltage which could change the TC by ±1 K.
A similar (In,Mn)As transistor device was used to provide further examples of gateable
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
.
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
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
on the gate bias
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
the electric field could be used to assist magnetization reversal or even demagnetize the ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
material.
The combining of magnetic and electronic functionality demonstrated by this experiment is one of the goals of spintronics
Spintronics
Spintronics , also known as magnetoelectronics, is an emerging technology that exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.An additional effect occurs when a spin-polarized current is...
and may be expected to have a great technological impact.
Another important spintronic functionality that has been demonstrated in DMSs
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
is that of spin injection
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
. This is where the high spin polarization
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 ferromagnetic metals, such as iron, giving rise to...
inherent to these magnetic materials is used to transfer spin polarized
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 ferromagnetic metals, such as iron, giving rise to...
carriers
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
into a non-magnetic material.
In this example, a fully epitaxial heterostructure was used where spin polarized
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 ferromagnetic metals, such as iron, giving rise to...
holes
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
were injected from a (Ga,Mn)As layer to an (In,Ga)As quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
where they combine with unpolarized electrons from an n-type substrate. A polarization of 8% was measured in the resulting electroluminescence
Electroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
. This is again of potential technological interest as it shows the possibility that the spin states
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
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
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
mechanics because the domains can have a size of the order of 100 µm.
Several studies have been done in which lithographically
Nanolithography
Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm...
defined lateral constrictions
or other pinning points
are used to manipulate domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
s. These experiments are crucial to understanding domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
nucleation and propagation which would be necessary for the creation of complex logic circuits based on domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
mechanics.
Many properties of domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
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
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
s. Both positive
and negative
values of domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
resistance have been reported, leaving this an open area for future research.
An example of a simple device that utilizes pinned domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
s is provided by reference.
This experiment consisted of a lithographically
Nanolithography
Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm...
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
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
s, resulting in a giant magnetoresistance (GMR) signal. When the device operates in a tunnelling regime another magnetoresistance (MR)
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
effect is observed, discussed below.
A further interesting property of domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
s is that of current induced domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
motion. This reversal is believed to occur as a result of the spin-transfer torque
Spin torque transfer
Spin-transfer torque is an effect in which the orientation of a magnetic layer in a tunnel magnetoresistance or spin valve can be modified using a spin-polarized current....
exerted by a spin polarized
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 ferromagnetic metals, such as iron, giving rise to...
current.
It was demonstrated in reference
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
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
. 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. Interestingly, 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.
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 MRs
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
of several orders of magnitude. This was demonstrated first in vertical tunnelling structures
and then later in lateral devices.
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 MR
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
effect in a (Ga,Mn)As device, the so-called Coulomb blockade
Coulomb blockade
In physics, a Coulomb blockade , named after Charles-Augustin de Coulomb's electrical force, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the resistances of devices are not constant at low bias...
anisotropic magnetoresistance (CBAMR).
There are many excellent review articles about the properties and applications of DMSs
Magnetic semiconductor
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction...
and (Ga,Mn)As in particular. If further information is required on the topic, the following reviews are recommended:.