Spintronics: Fundamentals and applications

TL;DR

<3-5 sentence high-level summary>

Abstract

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.

Figures (13)

• Figure 1: Scheme of the Datta-Das spin field-effect transistor (SFET). The source (spin injector) and the drain (spin detector) are ferromagnetic metals or semiconductors, with parallel magnetic moments. The injected spin-polarized electrons with wave vector ${\bf k}$ move ballistically along a quasi-one-dimensional channel formed by, for example, an InGaAs/InAlAs heterojunction in a plane normal to ${\bf n}$. Electron spins precess about the precession vector ${\bf \Omega}$, which arises from spin-orbit coupling and which is defined by the structure and the materials properties of the channel. The magnitude of ${\bf \Omega}$ is tunable by the gate voltage $V_G$ at the top of the channel. The current is large if the electron spin at the drain points in the initial direction (top row), for example, if the precession period is much larger than the time of flight, and small if the direction is reversed (bottom).
• Figure 2: Schematic illustration of electron tunneling in ferromagnet/insulator/ferromagnet (F/I/F) tunnel junctions: (a) Parallel and (b) antiparallel orientation of magnetizations with the corresponding spin-resolved density of the d states in ferromagnetic metals that have exchange spin splitting $\Delta_{ex}$. Arrows in the two ferromagnetic regions are determined by the majority-spin subband. Dashed lines depict spin-conserved tunneling.
• Figure 3: Schematic illustration of (a) the current in plane (CIP) (b) the current perpendicular to the plane (CPP) giant magnetoresistance geometry.
• Figure 4: Pedagogical illustration of the concept of electrical spin injection from a ferromagnet (F) into a normal metal (N). Electrons flow from F to N: (a) schematic device geometry; (b) magnetization M as a function of position. Nonequilibrium magnetization $\delta M$ (spin accumulation) is injected into a normal metal; (c) contribution of different spin-resolved densities of states to charge and spin transport across the F/N interface. Unequal filled levels in the density of states depict spin-resolved electrochemical potentials different from the equilibrium value $\mu_0$.
• Figure 5: Spin injection, spin accumulation, and spin detection: (a) two idealized completely polarized ferromagnets F1 and F2 (the spin down density of states ${\cal N}_\downarrow$ is zero at the energy of electrochemical potential $E=\mu_0$) with parallel magnetizations are separated by the nonmagnetic region N; (b) density-of-states diagrams for spin injection from F1 into N, accompanied by the spin accumulation--generation of the nonequilibrium magnetization $\delta M$. At F2 in the limit of low impedance ($Z$=0) spin is detected by measuring the spin-polarized current across the N/F2 interface. In the limit of high impedance ($Z=\infty$) spin is detected by measuring the voltage $V_s\sim \delta M$ developed across the N/F2 interface; (c) spin accumulation in a device in which a superconductor (with the superconducting gap $\Delta$) is occupying the region between F1 and F2.