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 recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. However, a central question remains unresolved: what type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, but existing measurement techniques have provided only indirect evidence for dynamical states. The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a d.c. spin-polarized current. We show that spin transfer can produce several different types of magnetic excitation. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a d.c. electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators.

Microwave oscillations of a nanomagnet driven by a spin-polarized current

https://ralphgroup.lassp.cornell.edu/papers/JMMM-320-1190-2008.pdf

Spin transfer torques

Methods to manipulate the magnetization of ferromagnets by means of local electric fields or current-induced spin transfer torque allow the design of integrated spintronic devices with reduced dimensions and energy consumption compared with conventional magnetic field actuation. An alternative way to induce a spin torque using an electric current has been proposed based on intrinsic spin-orbit magnetic fields and recently realized in a strained low-temperature ferromagnetic semiconductor. Here we demonstrate that strong magnetic fields can be induced in ferromagnetic metal films lacking structure inversion symmetry through the Rashba effect. Owing to the combination of spin-orbit and exchange interactions, we show that an electric current flowing in the plane of a Co layer with asymmetric Pt and AlO(x) interfaces produces an effective transverse magnetic field of 1 T per 10(8) A cm(-2). Besides its fundamental significance, the high efficiency of this process makes it a realistic candidate for room-temperature spintronic applications.

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Current-driven spin torque induced by the Rashba effect in a ferromagnetic metal layer

Current-induced switching in the orientation of magnetic moments is observed in cobalt/copper/cobalt sandwich structures, for currents flowing perpendicularly through the layers. Magnetic domains in adjacent cobalt layers can be manipulated controllably between stable parallel and antiparallel configurations by applying current pulses of the appropriate sign. The observations are in accord with predictions that a spin-polarized current exerts a torque at the interface between a magnetic and nonmagnetic metal, due to local exchange interactions between conduction electrons and the magnetic moments.

https://science.sciencemag.org/content/sci/285/5429/867.full.pdf

Current-Induced Switching of Domains in Magnetic Multilayer Devices

The effect of pulsed currents on magnetization reversal were studied on single ferromagnetic nanowires of diameter about 80 nm and 6000 nm length. The magnetization reversal in these wires occurs with a jump of the magnetization at the switching field Hsw, which corresponds to unstable states of the magnetization. A pulsed current of about 107 A/cm2 was injected at different values of the applied field close to Hsw. The injected current triggered the magnetization reversal at a value of the applied field distant from the switching field by as much as 20%. This effect of current-induced magnetization reversal is interpreted in terms of the action of the spin-polarized conduction electrons on the magnetization.

/pdf/current-induced-magnetization-reversal-in-magnetic-nanowires-42ib7rtpcq.pdf

Current-induced magnetization reversal in magnetic nanowires

Magnetoresistance of single Ni and Co nanowires, of about 60 nm in diameter and 6000 nm in length, was measured at room temperature. The full magnetoresistive hysteresis loops of single Ni nanowires, including the irreversible jump, are understood qualitatively, and major progress has been made towards their quantitative description, on the basis of anisotropic magnetoresistance. In contrast, the magnetoresistive hysteresis loops of single Co nanowires could not be described quantitatively, due to the presence of nucleation processes of domain walls or vortices.

Magnetoresistance of Ferromagnetic Nanowires

Magnetization reversal of individual Ni nanowires has been studied by anisotropic magnetoresistance (AMR) measurements. The wires are homogeneous cylinders 6/spl mu/ long and averaging 80 nm in diameter. Nanowires were produced by electrodeposition in track-etched membrane templates. Electrical contacts to as-deposited single nanowires were obtained by a novel in-situ method where contact to a single wire is accomplished automatically and reliably during electrodeposition. Steps of the magnetoresistance are detected, corresponding to irreversible switching of the magnetization in a single wire. The dependence of the switching field on the orientation of the applied field as seen by AMR is identical to that recently observed by using micro-SQUID techniques.

Anisotropic magnetoresistance as a probe of magnetization reversal in individual nono-sized nickel wires

Magnetization reversal triggered by spin injection is measured in electrodeposited Co/Cu/Co pillars (diameter about 60 nm). Two protocols are used. (i) switching of magnetization after a current pulse is monitored as a function of applied field. The maximum offset from the switching field at which irreversible switching occurs is a measure of the strength of the effect; and (ii) irreversible and reversible magnetization changes are observed while the current is ramped at fixed applied field. (i) and (ii) show that irreversible transitions occur only from antiparallel to parallel magnetic configurations and for electrons flow from the polarizer to the analyzer.

Exchange torque and spin transfer between spin polarized current and ferromagnetic layers

Using electrochemical deposition, $6\ensuremath{\mu}\mathrm{m}$ long Ni nanowires, with typical diameters of the order of 80 nm, are grown in ion-track etched membranes. Electrical contacts are established during the growth, allowing measurements of single wires. The full magnetoresistive hysteresis loop is studied as a function of the angle between the applied field and the wire direction. For each field sweep, a discontinuity is observed. It corresponds to the irreversible switching of the magnetization. A model of uniform reversal of the magnetization accounts for the reversible part of these magnetoresistive loops using the conventional ${\mathrm{cos}}^{2}\ensuremath{\omega}$ dependence of the resistance with the angle $\ensuremath{\omega}$ defined between magnetization and electric current. The angular dependence of the switching field is discussed.

D. Kelly

Papers

Current-induced magnetization reversal in magnetic nanowires

Magnetoresistance of Ferromagnetic Nanowires

Anisotropic magnetoresistance as a probe of magnetization reversal in individual nono-sized nickel wires

Exchange torque and spin transfer between spin polarized current and ferromagnetic layers

Uniform magnetization rotation in single ferromagnetic nanowires