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

We review the dynamical mean-field theory of strongly correlated electron systems which is based on a mapping of lattice models onto quantum impurity models subject to a self-consistency condition. This mapping is exact for models of correlated electrons in the limit of large lattice coordination (or infinite spatial dimensions). It extends the standard mean-field construction from classical statistical mechanics to quantum problems. We discuss the physical ideas underlying this theory and its mathematical derivation. Various analytic and numerical techniques that have been developed recently in order to analyze and solve the dynamical mean-field equations are reviewed and compared to each other. The method can be used for the determination of phase diagrams (by comparing the stability of various types of long-range order), and the calculation of thermodynamic properties, one-particle Green's functions, and response functions. We review in detail the recent progress in understanding the Hubbard model and the Mott metal-insulator transition within this approach, including some comparison to experiments on three-dimensional transition-metal oxides. We present an overview of the rapidly developing field of applications of this method to other systems. The present limitations of the approach, and possible extensions of the formalism are finally discussed. Computer programs for the numerical implementation of this method are also provided with this article.

Dynamical mean-field theory of strongly correlated fermion systems and the limit of infinite dimensions

An introductory review of the central ideas in the modern theory of dynamic critical phenomena is followed by a more detailed account of recent developments in the field. The concepts of the conventional theory, mode-coupling, scaling, universality, and the renormalization group are introduced and are illustrated in the context of a simple example---the phase separation of a symmetric binary fluid. The renormalization group is then developed in some detail, and applied to a variety of systems. The main dynamic universality classes are identified and characterized. It is found that the mode-coupling and renormalization group theories successfully explain available experimental data at the critical point of pure fluids, and binary mixtures, and at many magnetic phase transitions, but that a number of discrepancies exist with data at the superfluid transition of $^{4}\mathrm{He}$.

Theory of Dynamic Critical Phenomena

REVIEW Semiconductor devices generally take advantage of the charge of electrons, whereas magnetic materials are used for recording information involving electron spin. To make use of both charge and spin of electrons in semiconductors, a high concentration of magnetic elements can be introduced in nonmagnetic III-V semiconductors currently in use for devices. Low solubility of magnetic elements was overcome by low-temperature nonequilibrium molecular beam epitaxial growth, and ferromagnetic (Ga,Mn)As was realized. Magnetotransport measurements revealed that the magnetic transition temperature can be as high as 110 kelvin. The origin of the ferromagnetic interaction is discussed. Multilayer heterostructures including resonant tunneling diodes (RTDs) have also successfully been fabricated. The magnetic coupling between two ferromagnetic (Ga,Mn)As films separated by a nonmagnetic layer indicated the critical role of the holes in the magnetic coupling. The magnetic coupling in all semiconductor ferromagnetic/nonmagnetic layered structures, together with the possibility of spin filtering in RTDs, shows the potential of the present material system for exploring new physics and for developing new functionality toward future electronics.

https://science.sciencemag.org/content/sci/281/5379/951.full.pdf

Making Nonmagnetic Semiconductors Ferromagnetic

This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Spin glasses: Experimental facts, theoretical concepts, and open questions

The importance to the mechanism of ferromagnetim of exchange interaction between conduction electrons and unfilled inner shell electrons (called s-d interaction) has been pointed out by Zener. Especially for rare earth metals, this interaction seems to be the only mechanism which can cause ferroand antiferromagnetism. However Zener's works are unsatistisfactory because his model is phenomenological and moreover does not involve antiferromagnetism and spin wave mode. Our paper considers this s-d interaction on a more rigorous basis. By a certain approximation, there appear long range eychange type interactions between d-electron spins and, in certain conditions both ferroand antiferromagnetism appear. The excitations of spin wave modes are the same as those in the ordinary modes of the short range exchange force, viz, the energy of the spin wave excitations is proportional to q2 for ferromagnetism and q for antiferromagnetism in the region of small wave vector q. The T3/2 law for the temperature dependence of the magnetization of ferromagnetism is applicable up to very high temperatures, and this result is in good agreement with the results of experiments on metallic ferromagnetism.

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A Theory of Metallic Ferro- and Antiferromagnetism on Zener's Model

In ferromagnetic metals an anomalous electrical · resistance is observed which is characteristic of the magnetization. and is additive to the ordinary electrical resistance. The origin of this anomalous electrical resistance has been discussed by several authors, but all such attempts do not seem to have satisfactorily explained· this phenomenon either quantitatively or qualitatively. In this paper we calculate the anomalous electrical resistance from the standpoint of s-d interaction as developed by the author. It is our opinion that the anomalous electrical resistance occurs because the exchange energy between the conduction and the unfilled inner shell electrons depends on the relative direction of the spins of the electrons, and that this interaction is not periodic in finite temperatures. In numerical values as well as in the temperature dependence, our results are in good agreement with experimental results.

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Electrical Resistance of Ferromagnetic Metals

Magnetic phase diagram of CeB6

Various anomalous optical, magnetic, and transport phenomena have been found in Eu-chalcogenide alloys. Anomalous optical properties are explained by the magnetic exciton in which a hole in the $4f$ conduction band and an electron (for optically active magnetic exciton) in the $5d$ conduction band make a bound state. Strong $d\ensuremath{-}f$ exchange interaction between the magnetic-exciton electron of $5d$ character and the $4f$ electrons in the surrounding ${\mathrm{Eu}}^{2+}$ ions is the origin of the anomaly. Anomalous magnetic properties in dilute ${\mathrm{R}}^{3+}$ alloys are explained by the properties of the giant spin molecule which comes from the strong $s\ensuremath{-}f$ exchange interaction between the impurity electron and the $4f$ electrons at the central ${R}^{3+}$ and the surrounding ${\mathrm{Eu}}^{2+}$. The anomalous transition from the hopping type to the metallic-impurity band conduction in dilute ${\mathrm{R}}^{3+}$ alloys is also explained by the temperature dependence of the activation energy due to the $s\ensuremath{-}f$ exchange interaction in the magnetic-impurity state.

Tadao Kasuya

Papers

A Theory of Metallic Ferro- and Antiferromagnetism on Zener's Model

Electrical Resistance of Ferromagnetic Metals

Magnetic phase diagram of CeB6

Anomalous Transport Phenomena in Eu-Chalcogenide Alloys

Theory of Heavy Fermions and Valence Fluctuations