Spintronics (a
portmanteau
A portmanteau word, or portmanteau (, ) is a blend of words[spin
Spin or spinning most often refers to:
* Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning
* Spin, the rotation of an object around a central axis
* Spin (propaganda), an intentionally b ...](_blank)
of the
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
and its associated
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...
, in addition to its fundamental
electronic charge, in
solid-state devices.
The field of spintronics concerns spin-charge coupling in metallic systems; the analogous effects in insulators fall into the field of
multiferroics Multiferroics are defined as materials that exhibit more than one of the primary ferroic properties in the same phase:
* ferromagnetism – a magnetisation that is switchable by an applied magnetic field
* ferroelectricity – an electric polarisa ...
.
Spintronics fundamentally differs from traditional electronics in that, in addition to charge state, electron spins are exploited as a further degree of freedom, with implications in the efficiency of data storage and transfer. Spintronic systems are most often realised in
dilute magnetic semiconductors (DMS) and
Heusler alloy
Heusler compounds are magnetic intermetallics with face-centered cubic crystal structure and a composition of XYZ (half-Heuslers) or X2YZ (full-Heuslers), where X and Y are transition metals and Z is in the p-block. The term derives from the name ...
s and are of particular interest in the field of
quantum computing
Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...
and
neuromorphic computing
Neuromorphic engineering, also known as neuromorphic computing, is the use of electronic circuits to mimic neuro-biological architectures present in the nervous system. A neuromorphic computer/chip is any device that uses physical artificial ne ...
.
History
Spintronics emerged from discoveries in the 1980s concerning spin-dependent electron transport phenomena in solid-state devices. This includes the observation of
spin-polarized electron injection from a ferromagnetic metal to a normal metal by Johnson and Silsbee (1985) and the discovery of
giant magnetoresistance
Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter ...
independently by
Albert Fert
Albert Fert (; born 7 March 1938) is a French physicist and one of the discoverers of giant magnetoresistance which brought about a breakthrough in gigabyte hard disks. Currently, he is an emeritus professor at Paris-Saclay University in Orsay, ...
et al. and
Peter Grünberg
Peter Andreas Grünberg (; 18 May 1939 – 7 April 2018) was a German physicist, and Nobel Prize in Physics laureate for his discovery with Albert Fert of giant magnetoresistance which brought about a breakthrough in gigabyte hard disk d ...
et al. (1988). The origin of spintronics can be traced to the ferromagnet/superconductor tunneling experiments pioneered by Meservey and Tedrow and initial experiments on magnetic tunnel junctions by Julliere in the 1970s. The use of semiconductors for spintronics began with the theoretical proposal of a spin field-effect-transistor by
Datta and Das in 1990 and of the
electric dipole spin resonance
Electric dipole spin resonance (EDSR) is a method to control the magnetic moments inside a material using quantum mechanical effects like the spin–orbit interaction. Mainly, EDSR allows to flip the orientation of the magnetic moments through the ...
by
Rashba in 1960.
Theory
The
spin
Spin or spinning most often refers to:
* Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning
* Spin, the rotation of an object around a central axis
* Spin (propaganda), an intentionally b ...
of the electron is an intrinsic
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
that is separate from the angular momentum due to its orbital motion. The magnitude of the projection of the electron's spin along an arbitrary axis is
, implying that the electron acts as a
fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks an ...
by the
spin-statistics theorem. Like orbital angular momentum, the spin has an associated
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...
, the magnitude of which is expressed as
:
.
In a solid, the spins of many electrons can act together to affect the magnetic and electronic properties of a material, for example endowing it with a permanent magnetic moment as in a
ferromagnet
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 a ...
.
In many materials, electron spins are equally present in both the up and the down state, and no transport properties are dependent on spin. A spintronic device requires generation or manipulation of a spin-polarized population of electrons, resulting in an excess of spin up or spin down electrons. The polarization of any spin dependent property X can be written as
:
.
A net spin polarization can be achieved either through creating an equilibrium energy split between spin up and spin down. Methods include putting a material in a large magnetic field (
Zeeman effect
The Zeeman effect (; ) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman, who discovered it in 1896 and received a Nobel prize ...
), the exchange energy present in a ferromagnet or forcing the system out of equilibrium. The period of time that such a non-equilibrium population can be maintained is known as the spin lifetime,
.
In a diffusive conductor, a
spin diffusion
Spin diffusion describes a situation wherein the individual nuclear spins undergo continuous exchange of energy. This permits polarization differences within the sample to be reduced on a timescale much shorter than relaxation effects.
Spin ...
length
can be defined as the distance over which a non-equilibrium spin population can propagate. Spin lifetimes of conduction electrons in metals are relatively short (typically less than 1 nanosecond). An important research area is devoted to extending this lifetime to technologically relevant timescales.
![Spin Injection](https://upload.wikimedia.org/wikipedia/commons/7/78/Spin_Injection.svg)
The mechanisms of decay for a spin polarized population can be broadly classified as spin-flip scattering and spin dephasing. Spin-flip scattering is a process inside a solid that does not conserve spin, and can therefore switch an incoming spin up state into an outgoing spin down state. Spin dephasing is the process wherein a population of electrons with a common spin state becomes less polarized over time due to different rates of electron spin
precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...
. In confined structures, spin dephasing can be suppressed, leading to spin lifetimes of milliseconds in semiconductor
quantum dots
Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
at low temperatures.
Superconductors
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
can enhance central effects in spintronics such as magnetoresistance effects, spin lifetimes and dissipationless spin-currents.
The simplest method of generating a spin-polarised current in a metal is to pass the current through a
ferromagnetic
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 ...
material. The most common applications of this effect involve giant magnetoresistance (GMR) devices. A typical GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, the electrical resistance will be lower (so a higher current flows at constant voltage) than if the ferromagnetic layers are anti-aligned. This constitutes a magnetic field sensor.
Two variants of GMR have been applied in devices: (1) current-in-plane (CIP), where the electric current flows parallel to the layers and (2) current-perpendicular-to-plane (CPP), where the electric current flows in a direction perpendicular to the layers.
Other metal-based spintronics devices:
*
Tunnel magnetoresistance
Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a fe ...
(TMR), where CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers.
*
Spin-transfer torque
Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin ...
, where a current of spin-polarized electrons is used to control the magnetization direction of ferromagnetic electrodes in the device.
* Spin-wave logic devices carry information in the phase. Interference and spin-wave scattering can perform logic operations.
Spintronic-logic devices
Non-volatile spin-logic devices to enable scaling are being extensively studied. Spin-transfer, torque-based logic devices that use spins and magnets for information processing have been proposed. These devices are part of the
ITRS exploratory road map. Logic-in memory applications are already in the development stage. A 2017 review article can be found in ''Materials Today''.
A generalized circuit theory for spintronic integrated circuits has been proposed so that the physics of spin transport can be utilized by SPICE developers and subsequently by circuit and system designers for the exploration of spintronics for “beyond CMOS computing.”
Applications
Read heads of magnetic
hard drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
s are based on the GMR or TMR effect.
Motorola developed a first-generation 256
kb magnetoresistive random-access memory
Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
(MRAM) based on a single magnetic tunnel junction and a single transistor that has a read/write cycle of under 50 nanoseconds.
Everspin has since developed a 4
Mb version. Two second-generation MRAM techniques are in development:
thermal-assisted switching (TAS) and
spin-transfer torque
Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin ...
(STT).
Another design,
racetrack memory
Racetrack memory or domain-wall memory (DWM) is an experimental non-volatile memory device under development at IBM's Almaden Research Center by a team led by physicist Stuart Parkin. In early 2008, a 3-bit version was successfully demonstrated. ...
, encodes information in the direction of magnetization between domain walls of a ferromagnetic wire.
In 2012, persistent spin helices of synchronized electrons were made to persist for more than a nanosecond, a 30-fold increase over earlier efforts, and longer than the duration of a modern processor clock cycle.
Semiconductor-based spintronic devices
Doped semiconductor materials display dilute ferromagnetism. In recent years, dilute magnetic oxides (DMOs) including
ZnO based DMOs and
TiO2-based DMOs have been the subject of numerous experimental and computational investigations. Non-oxide ferromagnetic semiconductor sources (like manganese-doped gallium arsenide ), increase the interface resistance with a tunnel barrier, or using hot-electron injection.
Spin detection in semiconductors has been addressed with multiple techniques:
* Faraday/Kerr rotation of transmitted/reflected photons
* Circular polarization analysis of electroluminescence
* Nonlocal spin valve (adapted from Johnson and Silsbee's work with metals)
* Ballistic spin filtering
The latter technique was used to overcome the lack of spin-orbit interaction and materials issues to achieve spin transport in
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
.
Because external magnetic fields (and stray fields from magnetic contacts) can cause large
Hall effect
The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was disco ...
s and
magnetoresistance
Magnetoresistance is the tendency of a material (often ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance. Some occur in bulk ...
in semiconductors (which mimic
spin-valve effects), the only conclusive evidence of spin transport in semiconductors is demonstration of spin
precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...
and
dephasing
In physics, dephasing is a mechanism that recovers classical physics, classical behaviour from a quantum physics, quantum system. It refers to the ways in which coherence (physics), coherence caused by perturbation decays over time, and the syst ...
in a magnetic field non-collinear to the injected spin orientation, called the
Hanle effect Hanle can mean:
* Hanle (village), located in eastern Indian Ladakh, near the Chinese border
** The Indian Astronomical Observatory, adjacent to Hanle village, location of the highest major astronomical telescopes in the world and colloquially know ...
.
Applications
Applications using spin-polarized electrical injection have shown threshold current reduction and controllable circularly polarized coherent light output. Examples include semiconductor lasers. Future applications may include a spin-based
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
having advantages over
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
devices such as steeper sub-threshold slope.
Magnetic-tunnel transistor: The magnetic-tunnel transistor with a single base layer
has the following terminals:
* Emitter (FM1): Injects spin-polarized hot electrons into the base.
* Base (FM2): Spin-dependent scattering takes place in the base. It also serves as a spin filter.
* Collector (GaAs): A
Schottky barrier
A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifying characteristics, suitable for use as a diode. One of the primary char ...
is formed at the interface. It only collects electrons that have enough energy to overcome the Schottky barrier, and when states are available in the semiconductor.
The magnetocurrent (MC) is given as:
:
And the transfer ratio (TR) is
:
MTT promises a highly spin-polarized electron source at room temperature.
Storage media
Antiferromagnetic
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...
storage media have been studied as an alternative to
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 ...
, especially since with antiferromagnetic material the bits can be stored as well as with ferromagnetic material. Instead of the usual definition 0 ↔ 'magnetisation upwards', 1 ↔ 'magnetisation downwards', the states can be, e.g., 0 ↔ 'vertically-alternating spin configuration' and 1 ↔ 'horizontally-alternating spin configuration'.).
The main advantages of antiferromagnetic material are:
* insensitivity to data-damaging perturbations by stray fields due to zero net external magnetization;
* no effect on near particles, implying that antiferromagnetic device elements would not magnetically disturb its neighboring elements;
[
* far shorter switching times (antiferromagnetic resonance frequency is in the THz range compared to GHz ferromagnetic resonance frequency);]
* broad range of commonly available antiferromagnetic materials including insulators, semiconductors, semimetals, metals, and superconductors.[
Research is being done into how to read and write information to antiferromagnetic spintronics as their net zero magnetization makes this difficult compared to conventional ferromagnetic spintronics. In modern MRAM, detection and manipulation of ferromagnetic order by magnetic fields has largely been abandoned in favor of more efficient and scalable reading and writing by electrical current. Methods of reading and writing information by current rather than fields are also being investigated in antiferromagnets as fields are ineffective anyway. Writing methods currently being investigated in antiferromagnets are through ]spin-transfer torque
Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin ...
and spin-orbit torque from the spin Hall effect
The spin Hall effect (SHE) is a transport phenomenon predicted by Russian physicists Mikhail I. Dyakonov and Vladimir I. Perel in 1971. It consists of the appearance of spin accumulation on the lateral surfaces of an electric current-carrying samp ...
and the Rashba effect
The Rashba effect, also called Bychkov–Rashba effect, is a momentum-dependent splitting of spin bands in bulk crystalsMore specifically, uniaxial noncentrosymmetric crystals. and low-dimensional condensed matter systems (such as heterostructure ...
. Reading information in antiferromagnets via magnetoresistance effects such as tunnel magnetoresistance
Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a fe ...
is also being explored.
See also
* Electric dipole spin resonance
Electric dipole spin resonance (EDSR) is a method to control the magnetic moments inside a material using quantum mechanical effects like the spin–orbit interaction. Mainly, EDSR allows to flip the orientation of the magnetic moments through the ...
* Josephson effect
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
* Magnetoresistive random-access memory
Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
(MRAM)
* Magnonics Magnonics is an emerging field of modern magnetism, which can be considered a sub-field of modern solid state physics. Magnonics combines the study of waves and magnetism. Its main aim is to investigate the behaviour of spin waves in nano-structur ...
* Potential applications of graphene#Spintronics
* Rashba effect
The Rashba effect, also called Bychkov–Rashba effect, is a momentum-dependent splitting of spin bands in bulk crystalsMore specifically, uniaxial noncentrosymmetric crystals. and low-dimensional condensed matter systems (such as heterostructure ...
* Spin pumping
Spin pumping is the dynamical generation of pure spin current by the coherent precession of magnetic moments, which can efficiently inject spin from a magnetic material into an adjacent non-magnetic material. The non-magnetic material usually host ...
* Spin-transfer torque
Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin ...
* Spinhenge@Home
* Spinmechatronics
* Spinplasmonics
Spinplasmonics is a field of nanotechnology combining spintronics and plasmonics. The field was pioneered by Professor Abdulhakem Elezzabi at the University of Alberta in Canada. In a simple spinplasmonic device, light waves couple to Spin quantu ...
* Valleytronics
Valleytronics (from ''valley'' and ''electronics'') is an experimental area in semiconductors that exploits local extrema ("valleys") in the electronic band structure. Certain semiconductors have multiple "valleys" in the electronic band structure ...
* List of emerging technologies
This is a list of emerging technologies, in-development technical innovations with significant potential in their applications. The criteria for this list is that the technology must:
# Exist in some way; purely hypothetical technologies can ...
*Multiferroics Multiferroics are defined as materials that exhibit more than one of the primary ferroic properties in the same phase:
* ferromagnetism – a magnetisation that is switchable by an applied magnetic field
* ferroelectricity – an electric polarisa ...
References
Further reading
* "Introduction to Spintronics". Marc Cahay, Supriyo Bandyopadhyay, CRC Press,
*
*
*
*
*
*
"Spintronics Steps Forward."
University of South Florida
The University of South Florida (USF) is a public research university with its main campus located in Tampa, Florida, and other campuses in St. Petersburg and Sarasota. It is one of 12 members of the State University System of Florida. USF is ...
News
*
External links
23 milestones in the history of spin compiled by ''Nature''
Milestone 18: A Giant Leap for Electronics: Giant Magneto-resistance, compiled by ''Nature''
Milestone 20: Information in a Spin: Datta-Das, compiled by ''Nature''
*
Spintronics portal with news and resources
RaceTrack:InformationWeek (April 11, 2008)
Spintronics research targets GaAs.
* Lecture on Spin transport by S. Datta (from Datta Das transistor)
Part 1
an
Part 2
{{Authority control
Emerging technologies
Theoretical computer science