Editing Spintronics (section)

== Theory ==
{{main|Spin (physics)}}

The [[Spin (physics)|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 <math>\tfrac{1}{2}\hbar</math>, 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

:<math>\mu=\tfrac{\sqrt{3}}{2}\frac{q}{m_e}\hbar</math>.

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

:<math>P_X=\frac{X_{\uparrow}-X_{\downarrow}}{X_{\uparrow}+X_{\downarrow}}</math>.

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, <math>\tau</math>.

In a diffusive conductor, a [[spin diffusion]] length <math>\lambda</math> 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.

[[File:Spin Injection.svg|right|thumb|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.<ref>{{cite journal|last1=Linder|first1=Jacob|last2=Robinson|first2=Jason W. A.|title=Superconducting spintronics|journal=Nature Physics|date=2 April 2015|volume=11|issue=4|pages=307–315|issn=1745-2473|doi=10.1038/nphys3242|arxiv = 1510.00713 |bibcode = 2015NatPh..11..307L |s2cid=31028550}}</ref><ref>{{Cite journal | doi=10.1063/1.3541944|title = Spin-polarized supercurrents for spintronics| journal=Physics Today| volume=64|issue = 1| pages=43–49|year = 2011|last1 = Eschrig|first1 = Matthias|bibcode = 2011PhT....64a..43E}}</ref>

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.