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

The field of magnetoelectronics has been growing in practical importance in recent years1 For example, devices that harness electronic spin—such as giant-magnetoresistive sensors and magnetoresistive memory cells—are now appearing on the market2 In contrast, magnetoelectronic devices based on spin-polarized transport in semiconductors are at a much earlier stage of development, largely because of the lack of an efficient means of injecting spin-polarized charge Much work has focused on the use of ferromagnetic metallic contacts3,4, but it has proved exceedingly difficult to demonstrate polarized spin injection More recently, two groups5,6 have reported successful spin injection from an NiFe contact, but the observed effects of the spin-polarized transport were quite small (resistance changes of less than 1%) Here we describe a different approach, in which the magnetic semiconductor BexMnyZn1-x-ySe is used as a spin aligner We achieve injection efficiencies of 90% spin-polarized current into a non-magnetic semiconductor device The device used in this case is a GaAs/AlGaAs light-emitting diode, and spin polarization is confirmed by the circular polarization state of the emitted light

Injection and detection of a spin-polarized current in a light-emitting diode

Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders. Some are ferroelectric and ferromagnetic and provide the opportunity to encode information in electric polarization and magnetization to obtain four logic states. However, such materials are rare and schemes allowing a simple electrical readout of these states have not been demonstrated in the same device. Here, we show that films of La0.1Bi0.9MnO3 (LBMO) are ferromagnetic and ferroelectric, and retain both ferroic properties down to a thickness of 2 nm. We have integrated such ultrathin multiferroic films as barriers in spin-filter-type tunnel junctions that exploit the magnetic and ferroelectric degrees of freedom of LBMO. Whereas ferromagnetism permits read operations reminiscent of magnetic random access memories (MRAM), the electrical switching evokes a ferroelectric RAM write operation. Significantly, our device does not require the destructive ferroelectric readout, and therefore represents an advance over the original four-state memory concept based on multiferroics.

Tunnel junctions with multiferroic barriers

Spin-polarized electron tunneling

A new field that has come to be called “spin‐polarized transport” is growing dramatically. Although its roots are in the quantum description of solids, only recently have new material fabrication techniques permitted widespread study of the phenomenon and the development of device applications (see figure 1).

Spin‐Polarized Transport

An electron-spin polarization $P$ of as much as 80% has been observed in the tunnel current in Au/EuS/Al tunnel junctions. $P$ can be explained by the different heights of the tunnel barriers for the two spin directions. The Zeeman splitting of the Al quasiparticle density of states is greatly enhanced by the exchange interaction at the EuS/Al interface. Spin polarization was even seen in zero applied field. The value of $P$ calculated from the tunneling theory using known barrier heights in EuS is consistent with the measured values.

Electron-spin polarization in tunnel junctions in zero applied field with ferromagnetic EuS barriers.

We have observed electron-spin polarization in the tunnel current in metal-EuS-metal junctions. The value of polarization can be as high as 85%. This polarization is attributed to the exchange splitting of the EuS conduction band as a result of the ferromagnetic transition occurring in EuS at about 15 K. This conduction-band splitting lowers the tunnel barrier height for spin-up electrons and raises it for spin-down electrons, giving rise to spin polarization in the tunnel current. Tunnel barrier lowering is also manifested in the observed decrease of junction resistance with temperature below 15 K. The exchange interaction between quasiparticles in Al and the magnetic ${\mathrm{Eu}}^{2+}$ ions in EuS greatly enhances the Zeeman splitting of the superconducting quasiparticle density of states; in fact, some junctions show Zeeman splitting corresponding to an internal field of ${\mathit{B}}_{\mathit{i}}^{\mathrm{*}}$=3 T before any external magnetic field is applied.

Spin-filter effect of ferromagnetic europium sulfide tunnel barriers.

A thin film of EuSe was used as a funnel barrier between the normal metal Ag and superconducting Al. Tunneling characteristics at 0.45 K showed the following: (1) the absence of exchange splitting of the EuSe conduction band in zero magnetic field, where EuSe is antiferromagnetic; (2) field-dependent spin polarization of the tunneling electrons as high as 97% at H≥1.2 T; (3) enhanced Zeeman spliffing of the Al quasiparficle states because of the exchange interaction at the Al-EuSe; (4) a decrease in tunnel resistance by as much as 75% in magnetic field ≥1.7 T

Variation of the Electron-Spin Polarization in EuSe Tunnel Junctions from Zero to Near 100% in a Magnetic Field

Using the technique of spin-polarized tunneling, we studied tunnel junctions with EuS/Al bilayer electrodes. We found a large effective internal field in the Al film, which gives rise to extra Zeeman splitting in the superconducting quasiparticle density of states and which is attributed to the exchange interaction between the Eu ions and the Al conduction electrons. This exchange field, acting only on the electron spins, is inversely proportional to the thickness of the Al, causes no observable orbital depairing in the Al, and leads to a first-order transition to the normal state.

Thin-film superconductor in an exchange field.

Thin films of the high-T/sub c/ material Bi-Sr-Ca-Cu-O were prepared by the RF-magnetron technique from a single reacted and sintered target in pure argon atmosphere on sapphire substrates. The samples were annealed for three minutes in pure oxygen at 810 degrees C. Their composition was studied by Rutherford backscattering. The superconducting transition was very broad, with R=O at 65 K for the best film. MgO substrates and nonsintered targets were also used. The behavior of the resistance in a magnetic field (up to 20 T) was studied. >

X. Hao

Papers

Electron-spin polarization in tunnel junctions in zero applied field with ferromagnetic EuS barriers.

Spin-filter effect of ferromagnetic europium sulfide tunnel barriers.

Variation of the Electron-Spin Polarization in EuSe Tunnel Junctions from Zero to Near 100% in a Magnetic Field

Thin-film superconductor in an exchange field.

RF magnetron sputtering of Bi-Sr-Ca-Cu-O thin films