This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

http://science.sciencemag.org/content/sci/294/5546/1488.full.pdf

Spintronics: a spin-based electronics vision for the future.

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

We propose an electron wave analog of the electro‐optic light modulator. The current modulation in the proposed structure arises from spin precession due to the spin‐orbit coupling in narrow‐gap semiconductors, while magnetized contacts are used to preferentially inject and detect specific spin orientations. This structure may exhibit significant current modulation despite multiple modes, elevated temperatures, or a large applied bias.

Electronic analog of the electro‐optic modulator

Conventional electronics is based on the manipulation of electronic charge. An intriguing alternative is the field of ‘spintronics’, wherein the classical manipulation of electronic spin in semiconductor devices gives rise to the possibility of reading and writing non-volatile information through magnetism1,2. Moreover, the ability to preserve coherent spin states in conventional semiconductors3 and quantum dots4 may eventually enable quantum computing in the solid state5,6. Recent studies have shown that optically excited electron spins can retain their coherence over distances exceeding 100 micrometres (ref. 7). But to inject spin-polarized carriers electrically remains a formidable challenge8,9. Here we report the fabrication of all-semiconductor, light-emitting spintronic devices using III–V heterostructures based on gallium arsenide. Electrical spin injection into a non-magnetic semiconductor is achieved (in zero magnetic field) using a p-type ferromagnetic semiconductor10 as the spin polarizer. Spin polarization of the injected holes is determined directly from the polarization of the emitted electroluminescence following the recombination of the holes with the injected (unpolarized) electrons.

Electrical spin injection in a ferromagnetic semiconductor heterostructure

In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Spin Hall effects

The strong inequivalence of spin-up and spin-down subbands in a ferromagnet causes a coupling between the charge and spin transport across the interface of a ferromagnetic and a contiguous paramagnetic metal. This allows the use of sensitive electronic measurements to probe spin transport. Application of small static magnetic fields results in a Hanle effect which permits determination of the spin-relaxation time ${\mathrm{T}}_{2}$. The unique features of the method should make it applicable to a wide range of studies.

Interfacial charge-spin coupling: Injection and detection of spin magnetization in metals.

Spintronics increases the functionality of information processing while seeking to overcome some of the limitations of conventional electronics. The spin-injected field effect transistor, a lateral semiconducting channel with two ferromagnetic electrodes, lies at the foundation of spintronics research. We demonstrated a spin-injected field effect transistor in a high-mobility InAs heterostructure with empirically calibrated electrical injection and detection of ballistic spin-polarized electrons. We observed and fit to theory an oscillatory channel conductance as a function of monotonically increasing gate voltage.

https://science.sciencemag.org/content/sci/325/5947/1515.full.pdf

Control of spin precession in a spin-injected field effect transistor.

Spin injection at a ferromagnet-semiconductor interface is observed by projecting the spin-polarized current in the ferromagnet onto the spin-split density of states of a high mobility two-dimensional electron gas (2DEG). For a given polarization of carriers in the 2DEG, reversing the magnetization orientation of the ferromagnet modulates the interface resistance. Equivalently, reversing the polarization of the 2DEG carriers by reversing the bias polarity gives the same resistance modulation. Interface resistance changes of order 1% at room temperature indicate interfacial current polarizations of order 20%.

Observation of Spin Injection at a Ferromagnet-Semiconductor Interface

Microscopic models are presented to elucidate the concept of interfacial charge-spin coupling. At the interface between a ferromagnet and a paramagnet, the spin subbands are loosely coupled, an interfacial conductance may be defined for each, and a result of their inequivalence is that an electric current flowing from a ferromagnetic metal into a paramagnetic metal will be partially spin polarized, i.e., will have an associated current of magnetization. The inverse is also true; nonequilibrium magnetization present in a paramagnetic metal can be detected as an open circuit voltage across an interface between the paramagnet and a ferromagnet. Using this effect, a new technique to measure conduction electron relaxation times is described.

Coupling of electronic charge and spin at a ferromagnetic-paramagnetic metal interface.

The spin injection technique has been used to study the transport of spin polarized conduction electrons in gold films, and has resulted in the unique observation of a large ``spin bottleneck'' effect. Furthermore, the measured spin diffusion length is surprisingly long.

Mark Johnson

Papers

Interfacial charge-spin coupling: Injection and detection of spin magnetization in metals.

Control of spin precession in a spin-injected field effect transistor.

Observation of Spin Injection at a Ferromagnet-Semiconductor Interface

Coupling of electronic charge and spin at a ferromagnetic-paramagnetic metal interface.

Spin accumulation in gold films