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

Recent developments in the controlled movement of domain walls in magnetic nanowires by short pulses of spin-polarized current give promise of a nonvolatile memory device with the high performance and reliability of conventional solid-state memory but at the low cost of conventional magnetic disk drive storage. The racetrack memory described in this review comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip. Individual spintronic reading and writing nanodevices are used to modify or read a train of ∼10 to 100 domain walls, which store a series of data bits in each nanowire. This racetrack memory is an example of the move toward innately three-dimensional microelectronic devices.

http://science.sciencemag.org/content/sci/320/5873/190.full.pdf

Magnetic Domain-Wall Racetrack Memory

Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. We start with a brief summary of necessary background, followed by a detailed discussion of the Berry phase effect in a variety of solid state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, we demonstrate a re-quantization method that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as a basic ingredient to our understanding of basic material properties.

/pdf/berry-phase-effects-on-electronic-properties-53dkbbi2er.pdf

Berry phase effects on electronic properties

Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

Spin-torque switching with the giant spin hall effect of tantalum

The interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films. This leads to a local increase of the Gilbert damping parameter which characterizes spin dynamics. When a dc current crosses this interface, stimulated emission of spin waves is predicted to take place. Beyond a certain critical current density, the spin damping becomes negative; a spontaneous precession of the magnetization is predicted to arise. This is the magnetic analog of the injection laser. An extra dc voltage appears across the interface, given by an expression similar to that for the Josephson voltage across a superconducting junction. \textcopyright{} 1996 The American Physical Society.

/pdf/emission-of-spin-waves-by-a-magnetic-multilayer-traversed-by-4q1cwurb7d.pdf

Emission of spin waves by a magnetic multilayer traversed by a current.

The center of mass of a wave packet undergoes a discontinuous and finite sideways displacement on scattering by a central potential, in the presence of spin-orbit interaction. This is the main Hall-effect mechanism (${\ensuremath{\rho}}_{H}\ensuremath{\propto}{\ensuremath{\rho}}^{2}$) for Fe, Ni, and their alloys above 100 K, while asymmetric scattering dominates below 100 K. Displacement $\ensuremath{\Delta}y$ per actual collision is calculated by partial waves. In the case of Born expansion, the leading term of $\ensuremath{\Delta}y or \frac{{\ensuremath{\rho}}_{H}}{{\ensuremath{\rho}}^{2}}$ is of zero order in the scattering potential. The magnitude is predicted correctly ($\ensuremath{\Delta}y\ensuremath{\approx}{10}^{\ensuremath{-}10}\ensuremath{-}{10}^{\ensuremath{-}11}$ m) when using the effective spin-orbit Hamiltonian derived by Fivaz from spin-orbit interband mixing. The calculation of ${\ensuremath{\rho}}_{H}$ is extended to arbitrary ${\ensuremath{\omega}}_{c}\ensuremath{\tau}$ for compensated and un-compensated metals. Other nonclassical physical mechanisms proposed by Karplus and Luttinger and by Doniach and by Fivaz are spurious for the dc Hall effect.

Side-Jump Mechanism for the Hall Effect of Ferromagnets

When a current density ${j}_{x}$ crosses a 180\ifmmode^\circ\else\textdegree\fi{} domain wall in a metallic ferromagnet, the spin s of each conduction electron exerts an s-d exchange torque on the localized wall spins. Hence, the wall moment of a Bloch wall is canted out of the wall plane by an angle \ensuremath{\psi}, given by ${j}_{x}$=(eC/\ensuremath{\Elzxh})sin(2\ensuremath{\psi}), where C is the maximum restoring torque at \ensuremath{\psi}=45\ifmmode^\circ\else\textdegree\fi{}. This equation is the exact analog of the dc Josephson effect, and 2\ensuremath{\psi} is the analog of the superconducting phase difference \ensuremath{\varphi} across a junction. For \ensuremath{\Vert}${j}_{x}$\ensuremath{\Vert}geC/\ensuremath{\Elzxh}\ensuremath{\simeq}${10}^{6}$ A/${\mathrm{cm}}^{2}$, the s-d exchange torque overcomes the restoring torque, and the wall moment precesses with a frequency \ensuremath{\omega}=d(2\ensuremath{\psi})/dt. A dc voltage \ensuremath{\delta}V is expected to appear across the wall, satisfying the famous ac Josephson relation 2e\ensuremath{\delta}V=-\ensuremath{\Elzxh}\ensuremath{\omega}. This wall precession can be described as a translation of Bloch lines, and the Bloch lines are the exact analog of superconducting vortices. The electric current exerts a transverse force on Bloch lines.

Possible existence of a Josephson effect in ferromagnets

Large dc current pulses, ≂2 μs long, are sent through 30–40‐nm‐thick Ni87Fe13 films containing Neel walls. Wall displacements are seen for current densities ≥1.2×107 A/cm2. Displacements reverse when current sense is reversed. Walls always move in direction of charge carriers in this electronlike material. Our results agree with a theory of s‐d exchange interaction between walls and 4s conduction electrons. Hydromagnetic ‘‘domain‐drag’’ forces are too small in such very thin films to explain our data.

Observation of s‐d exchange force between domain walls and electric current in very thin Permalloy films

We recently showed that an electron wave packet undergoes an abrupt, sideways jump $\ensuremath{\Delta}y$ during scattering in the presence of spin-orbit interaction. This causes the Hall effect in ferromagnets around room temperature (${R}_{s}\ensuremath{\propto}{\ensuremath{\rho}}^{2}$). The value of the side jump per collision ($\ensuremath{\Delta}y\ensuremath{\approx}{10}^{\ensuremath{-}10}$ m) seems the same for impurity and phonon scattering. A more complete justification of the side-jump model is given here. This model is used to derive the isothermal Nernst coefficient ${Q}_{s}^{\mathrm{is}}$, giving ${Q}_{s}^{\mathrm{is}}\ensuremath{\propto}\ensuremath{\rho}T$, where $\ensuremath{\rho}$ is the resistivity. If spin-disorder scattering is also introduced, then the Hall conductivity ${\ensuremath{\gamma}}_{\mathrm{Hs}}$ is not affected, but the Nernst coefficient becomes ${Q}_{s}^{\mathrm{is}}=\ensuremath{-}T(\ensuremath{\alpha}+\ensuremath{\beta}\ensuremath{\rho})$. This formula agrees with the data of Kondorskii and Vasileva on Fe, Ni, Co, Gd, and Fe-Ni. The side jump is assumed to have the same value for spin disorder as for impurity or phonon scattering. The constant $\ensuremath{\alpha}$ is predicted to exist even in pure metals, in agreement with the above data but not with the Kondorskii theory.

L. Berger

Papers

Emission of spin waves by a magnetic multilayer traversed by a current.

Side-Jump Mechanism for the Hall Effect of Ferromagnets

Possible existence of a Josephson effect in ferromagnets

Observation of s‐d exchange force between domain walls and electric current in very thin Permalloy films

Application of the Side-Jump Model to the Hall Effect and Nernst Effect in Ferromagnets