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

Magnetic skyrmions are particle-like nanometre-sized spin textures of topological origin found in several magnetic materials, and are characterized by a long lifetime. Skyrmions have been observed both by means of neutron scattering in momentum space and microscopy techniques in real space, and their properties include novel Hall effects, current-driven motion with ultralow current density and multiferroic behaviour. These properties can be understood from a unified viewpoint, namely the emergent electromagnetism associated with the non-coplanar spin structure of skyrmions. From this description, potential applications of skyrmions as information carriers in magnetic information storage and processing devices are envisaged.

Topological properties and dynamics of magnetic skyrmions

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

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

A new technique is required which enables tailoring of the morphology of a metallic nanostructured material down to the 10 nm length scale. Using nanoporous nuclear track etched membranes as templates for electrodeposition, an assembly of wires with diameters as low as 30 nm could be obtained. Alternating the electrodeposition of two metals resulted in multilayers grown perpendicular to the wire axis. Layer thicknesses as low as 2 nm could be reached. Application is demonstrated by making wires 6 μm long, 80 nm in diameter, having a succession of either Co and Cu layers or of (Ni,Fe) and Cu layers. Wires containing layers of 5–10 nm in thickness exhibited a giant magnetoresistance. The current was naturally perpendicular to the layers. At ambient temperature, a magnetoresistance of 14% for Co/Cu and of 10% for (Fe,Ni)/Cu was observed.

Giant magnetoresistance of nanowires of multilayers

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.

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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

Studies of the magnetization switching of individual ferromagnetic cylinders at low temperatures (0.1--6 K) were performed using a niobium microbridge-dc superconducting quantum interference device. Cylinders of Ni with diameters ranging from 40 to 100 nm and lengths up to 5 \ensuremath{\mu}m were studied. For diameter values under 50 nm, the switching probability as a function of time can be described by a single exponential function. The mean waiting time \ensuremath{\tau} followed an Arrhenius law as originally proposed by N\'eel. Temperature and field sweeping rate dependence of the mean switching field could be described by the model of Kurkij\"arvi which is based on the assumption of N\'eel of thermally assisted magnetization reversal over a simple potential barrier. Small deviations from this model are evidenced below 1 K. Our measurements allow us to estimate an 'activation volume,' two orders of magnitude smaller than the wire volume. This confirms the idea of the reversal of the magnetization caused by a nucleation of a reversed fraction of the cylinder, rapidly propagating along the whole sample. A pinning of the propagation of the magnetization reversal occurs for a few samples, where several jumps are observed in the hysteresis curves. The dynamic reversal properties of depinning were quite different from those of nucleation of a domain wall. For example, the probability of depinning as a function of time does not follow a single exponential law. Similar deviations are found for aged samples, revealing the influence of surface oxidation. These deviations from a simple model of thermally assisted magnetization reversal are particularly important when discussing quantum effects in the magnetization reversal at very low temperatures.

Measurements of magnetization switching in individual nickel nanowires

Reference EPFL-ARTICLE-147569doi:10.1103/PhysRevB.73.052410View record in Web of Science Record created on 2010-03-23, modified on 2017-05-12

J.-Ph. Ansermet

Papers

Giant magnetoresistance of nanowires of multilayers

Current-induced magnetization reversal in magnetic nanowires

Magnetoresistance of Ferromagnetic Nanowires

Measurements of magnetization switching in individual nickel nanowires

Spin-dependent Peltier effect of perpendicular currents in multilayered nanowires