Spintronics

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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-state devices.

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 alloys and are of particular interest in the field of quantum computing.

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)^[5] and the discovery of giant magnetoresistance independently by Albert Fert et al.^[6] and Peter Grünberg et al. (1988). The origins 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.^[8] The use of semiconductors for spintronics began with the theoretical proposal of a spin field-effect-transistor by Datta and Das in 1990^[9] and of the electric dipole spin resonance by Rashba in 1960.^[10]

Theory

The spin of the electron is an intrinsic angular momentum 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 ${\displaystyle {\tfrac {1}{2}}\hbar }$, implying that the electron acts as a Fermion by the spin-statistics theorem. Like orbital angular momentum, the spin has an associated magnetic moment, the magnitude of which is expressed as

${\displaystyle \mu ={\tfrac {\sqrt {3}}{2}}{\frac {q}{m_{e}}}\hbar }$.

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.
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

${\displaystyle P_{X}={\frac {X_{\uparrow }-X_{\downarrow }}{X_{\uparrow }+X_{\downarrow }}}}$.

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 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, ${\displaystyle \tau }$.

In a diffusive conductor, a spin diffusion length ${\displaystyle \lambda }$ 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.

A plot showing a spin up, spin down, and the resulting spin polarized population of electrons. Inside a spin injector, the polarization is constant, while outside the injector, the polarization decays exponentially to zero as the spin up and down populations go to equilibrium.

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. In confined structures, spin dephasing can be suppressed, leading to spin lifetimes of milliseconds in semiconductor quantum dots at low temperatures.

Superconductors can enhance central effects in spintronics such as magnetoresistance effects, spin lifetimes and dissipationless spin-currents.^[11][12]

The simplest method of generating a spin-polarised current in a metal is to pass the current through a ferromagnetic 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 (TMR), where CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers.
• Spin-transfer torque, 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.^[13] Spin-transfer, torque-based logic devices that use spins and magnets for information processing have been proposed.^[14][15] These devices are part of the ITRS exploratory road map. Logic-in memory applications are already in the development stage.^[16][17] A 2017 review article can be found in Materials Today.^[18]

Applications

Read heads of magnetic hard drives are based on the GMR or TMR effect.

Motorola developed a first-generation 256 kb magnetoresistive random-access memory (MRAM) based on a single magnetic tunnel junction and a single transistor that has a read/write cycle of under 50 nanoseconds.^[19] Everspin has since developed a 4 Mb version.^[20] Two second-generation MRAM techniques are in development: thermal-assisted switching (TAS)^[21] and spin-transfer torque (STT).^[22]

Another design, racetrack memory, encodes information in the direction of magnetization between domain walls of a ferromagnetic wire.

Magnetic sensors can use the GMR effect.^{[citation needed]}

In 2012 persistent spin helices of synchronized electrons were made to persist for more than a nanosecond, a 30-fold increase, longer than the duration of a modern processor clock cycle.^[23]

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.^[24][25] Non-oxide ferromagnetic semiconductor sources (like manganese-doped gallium arsenide GaMnAs),^[26] increase the interface resistance with a tunnel barrier,^[27] or using hot-electron injection.^[28]

Spin detection in semiconductors has been addressed with multiple techniques:

• Faraday/Kerr rotation of transmitted/reflected photons^[29]
• Circular polarization analysis of electroluminescence^[30]
• Nonlocal spin valve (adapted from Johnson and Silsbee's work with metals)^[31]
• Ballistic spin filtering^[32]

The latter technique was used to overcome the lack of spin-orbit interaction and materials issues to achieve spin transport in silicon.^[33]

Because external magnetic fields (and stray fields from magnetic contacts) can cause large Hall effects and magnetoresistance in semiconductors (which mimic spin-valve effects), the only conclusive evidence of spin transport in semiconductors is demonstration of spin precession and dephasing in a magnetic field non-collinear to the injected spin orientation, called the Hanle effect.

Applications

Applications using spin-polarized electrical injection have shown threshold current reduction and controllable circularly polarized coherent light output.^[34] Examples include semiconductor lasers. Future applications may include a spin-based transistor having advantages over MOSFET devices such as steeper sub-threshold slope.

Magnetic-tunnel transistor: The magnetic-tunnel transistor with a single base layer^[35] 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 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:

${\displaystyle MC={\frac {I_{c,p}-I_{c,ap}}{I_{c,ap}}}}$

And the transfer ratio (TR) is

${\displaystyle TR={\frac {I_{C}}{I_{E}}}}$

MTT promises a highly spin-polarized electron source at room temperature.

Storage media

Antiferromagnetic storage media have been studied as an alternative to ferromagnetism,^[36] 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'.^[37]).

The main advantages of antiferromagnetic material are:

• non-sensitivity against data-damaging perturbations by stray fields due to zero net external magnetization^[38];
• no effect on near particles, implying that antiferromagnetic device elements would not magnetically disturb its neighboring elements;^[38]
• far shorter switching times (antiferromagnetic resonance frequency is in the THz range compared to GHz ferromagnetic resonance frequency);^[39]
• broad range of commonly available antiferromagnetic materials including insulators, semiconductors, semimetals, metals, and superconductors.^[39]

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 done away with 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 anyways. Writing methods currently being investigated in antiferromagnets are through spin-transfer torque and spin-orbit torque from the spin Hall effect and the Rashba effect. Reading information in antiferromagnets via magnetoresistance effects such as tunnel magnetoresistance is also being explored.