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

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

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.

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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

In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Spin Hall effects

We propose a thermoelectric (TE) energy conversion device based on the surface of a three-dimensional topological insulator (TI) that is magnetically gap-opened and ionically disordered. A pair of top and bottom surfaces of a single TI film takes the role of a vertical p–n junction with ambipolar conduction, which can be considered as a TE module consisting of two dissimilar TE materials. By tuning the surface carrier screening by means of electric gating, we find that the figure of merit ZT of the device exceeds 1 in the low-temperature-regime below 300 K. Our model may represent one direction for the implementation of TE energy conversion and heat management in nanodevices.We propose a thermoelectric (TE) energy conversion device based on the surface of a three-dimensional topological insulator (TI) that is magnetically gap-opened and ionically disordered. A pair of top and bottom surfaces of a single TI film takes the role of a vertical p–n junction with ambipolar conduction, which can be considered as a TE module consisting of two dissimilar TE materials. By tuning the surface carrier screening by means of electric gating, we find that the figure of merit ZT of the device exceeds 1 in the low-temperature-regime below 300 K. Our model may represent one direction for the implementation of TE energy conversion and heat management in nanodevices.

Ambipolar Seebeck power generator based on topological insulator surfaces

The electron transport through a magnetic point contact is studied with special attention to the effect of an atomic scale domain wall. When the magnetizations of left and light electrodes are antiparallel, the atomic scale domain wall is created inside the contact. We show that the spin precession of conduction electron is forbidden in such an atomic scale domain wall and the sequence of quantized conductances differs from that of the single domain point contact. We also show that the magnetoresistance is strongly enhanced for the narrow point contact and oscillates with the conductance.

Effect of the quantum domain wall on conductance quantization and magnetoresistance in magnetic point contacts

We theoretically study the spin-polarized current flowing through a Josephson junction (JJ) in a spin injection device. When the spin-polarized current is injected from a ferromagnet in a superconductor (SC), the charge current is carried by the superconducting condensate (Cooper pairs), while the spin-up and spin-down currents flow in equal magnitude but in the opposite direction in a SC, because of no quasiparticle charge current in the SC. This indicates that not only the Josephson current but also the spin current flow across JJ at zero bias voltage, thereby generating Joule heating by the spin current. The result provides a new method for detecting the spin current by measuring Joule heating at JJ.

Joule heating generated by spin current through Josephson junctions

The spin Hall effect in heavy metals generates spin current that enables control of magnetization of an nearby magnetic layer Conversely, such flow of spin current can influence the transport properties of the system itself For example, the resistance of a Pt layer attached to a magnetic insulator changes as the direction of the magnetization is changed Such effect, now commonly referred to as the spin Hall magnetoresistance (SMR), has been reported to be, however, order of magnitude smaller than the conventional anisotropic magnetoresistance in magnetic materials Here we report giant spin Hall magnetoresistance in a metallic bilayer system We find nearly a ten-fold increase of SMR in W/CoFeB compared to the previously studied heavy metal/magnetic insulator systems The large SMR accounts for the unconventionally large transverse magnetoresistance found in this system, manifesting the profound effect of spin current generated via the spin Hall effect on the electrical transport properties We show that the SMR is defined at the heavy metal/magnetic layer interface even for metallic heterostructures in which current flows into the magnetic layer These results demonstrate that SMR can be used as a powerful tool to evaluate the flow and accumulation of spin current in metallic heterostructures

Saburo Takahashi

Papers

Ambipolar Seebeck power generator based on topological insulator surfaces

Effect of the quantum domain wall on conductance quantization and magnetoresistance in magnetic point contacts

Joule heating generated by spin current through Josephson junctions

Giant spin Hall magnetoresistance in metallic bilayers

Numerical analysis of spin accumulation due to a domain wall