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

Chapter 1: The functional source of innovation general patterns economic explanation shifting and predicting the sources of innovation innovation as a distributed process. Chapter 2: Users as innovators. Chapter 3: Variations in the functional source of innovation. Chapter 4: Why does the functional source of innovation vary? How do innovators benefit from innovations? Do benefit expectations differ? Chapter 5: The hypothesis in testable form methods five empirical studies discussion. Chapter 6: Shifting the functional source of innovation. Chapter 7: Root of the problem: market research constrained by user experience Lead users as a solution testing the method discussion. Chapter 8: Innovation cooperation between competing firms applications for innovation management.

The sources of innovation

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

An ab initio gauge-invariant molecular orbital theory is developed for nuclear magnetic shielding. The molecular orbitals are written as linear combinations of gauge-invariant atomic orbitals, the wavefunctions in the presence of a uniform external magnetic field being determined by self-consistent field perturbation theory. The final magnetic shielding result is broken up into contributions which can be related to various features of electronic structure. Calculated magnetic shielding constants are presented using three sets of atomic orbitals, all of which are taken as contracted gaussian-type functions. The first two sets are minimal and the third is slightly extended. All three levels of theory give good descriptions of shielding at first row and hydrogen atoms. Carbon and hydrogen chemical shifts calculated at the extended level are in excellent agreement with experimental values.

Self-consistent perturbation theory of diamagnetism

Quantum computers promise to exceed the computational efficiency of ordinary classical machines because quantum algorithms allow the execution of certain tasks in fewer steps. But practical implementation of these machines poses a formidable challenge. Here I present a scheme for implementing a quantum-mechanical computer. Information is encoded onto the nuclear spins of donor atoms in doped silicon electronic devices. Logical operations on individual spins are performed using externally applied electric fields, and spin measurements are made using currents of spin-polarized electrons. The realization of such a computer is dependent on future refinements of conventional silicon electronics.

A silicon-based nuclear spin quantum computer

Multiple magnetic resonance lines have been observed for H1, F19, and P31 nuclei in compounds such as PH3, PF3, F2PO(OH), and BrF5, in the liquid state. The multiplets consisted of two to seven equally spaced narrow components, symmetrically placed about a central frequency, and with splittings from 0.02 to 0.8 gauss. These multiplets arise from a new variety of interaction among the nuclear moments in a molecule. Resonance lines were found to be multiple either when a nucleus interacted with a different species of nucleus or when there was interaction between nuclei of the same species with resonance frequencies separated by a chemical shift. No compounds exhibited multiplets attributable to interactions among structurally equivalent nuclei. Nor were multiplets caused by nuclei whose electric quadrupole moments were coupled to a direction fixed in the molecule.The number and relative intensities of the components of a multiplet were determined by the number and statistical weights of the various nuclear spin orientations of the nuclei causing the splitting. In a given molecule, the ratio of the multiplet splittings of the two different resonance lines was inversely proportional to the ratio of the gyromagnetic ratios of the interacting nuclei. The splittings were independent of applied magnetic field at 4180 and 6365 gauss; they were independent of temperature over ranges from 55°C to −130°C. In PF5, the one gas examined, the splitting of the doublet fluorine resonance was the same in the gas and liquid phases; also, the doublet was demonstrated to arise from coupling with the phosphorus nucleus, rather than from a chemical shift between the resonances from the apex and meridian fluorines in the bipyramidal structure, as proposed earlier.All of the above characteristics are accounted for theoretically by assuming the magnetic nuclei interact via magnetic fields inside the molecule. The qualitative aspects are predicted by coupling of the form A12μ1·μ2 between the nuclear moments μ1 and μ2. The coupling constant A12 depends upon the detailed mechanism, which must involve the molecular electrons. Second‐order perturbation theory was used to calculate the relative magnitudes of coupling via the electron orbital and the electron spin magnetic moments. The electron spin mechanism was found to give splittings ten to twenty times the orbital. Approximate calculation of the electron spin mechanism in several simpler cases gave good agreement with experiment. The influence upon the splittings of electric quadrupole coupling and spin‐lattice relaxation was considered and is discussed briefly.

Nuclear Magnetic Resonance Multiplets in Liquids

Magnetic susceptibility, Cu NQR, and high-filed magnetization have been measured in polycrystalline $\mathrm{SrCu}{}_{2}(\mathrm{BO}{}_{3}{)}_{2}$ having a two-dimensional (2D) orthogonal network of Cu dimers. This cuprate provides a new class of 2D spin-gap system $(\ensuremath{\Delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}30\mathrm{K})$ in which the ground state can be solved ``exactly.'' Furthermore, in the magnetization, two plateaus corresponding to $\frac{1}{4}$ and $\frac{1}{8}$ of the full Cu moment were first observed for 2D quantum spin systems.

Exact Dimer Ground State and Quantized Magnetization Plateaus in the Two-Dimensional Spin System SrCu 2 ( BO 3 ) 2

Polarization of Nuclear Spins in Metals

Nuclear spin-lattice relaxation times have been measured in normal and superconducting Al from 0.94\ifmmode^\circ\else\textdegree\fi{}K to 4.2\ifmmode^\circ\else\textdegree\fi{}K, and as a function of static field in the normal state. In the normal state the relaxation rate is proportional to temperature as predicted by Redfield and others. The field dependence is somewhat greater than predicted. Relaxation in the superconductor was studied by a field cycling method which allowed the measurements to be made in the normal state but relaxation to occur in the superconductor. The results disagree with a simple two-fluid model, but are explained by the theory of Bardeen, Cooper, and Schrieffer. The contrast between the temperature dependence of nuclear relaxation and ultrasonic absorption confirms the central feature of the Bardeen-Cooper-Schrieffer theory that electrons of opposite spin and momentum are correlated.

Nuclear spin relaxation in normal and superconducting aluminum

A theoretical analysis is given of the fluorine magnetic resonance shifts in molecules. It is shown that the large range of shifts among different fluorine compounds arises from the effects of binding on the magnetic fields at the nucleus produced by the orbital motion of the electrons in the unfilled valence shell. The simple dependence on ionic character of the bond is also explained. The chemical shift between F2 and HF is calculated to be 14×10—4 compared to the experimentally observed value 6.3×10—4.

Charles P. Slichter

Papers

Nuclear Magnetic Resonance Multiplets in Liquids

Exact Dimer Ground State and Quantized Magnetization Plateaus in the Two-Dimensional Spin System SrCu 2 ( BO 3 ) 2

Polarization of Nuclear Spins in Metals

Nuclear spin relaxation in normal and superconducting aluminum

A Note on the Fluorine Resonance Shifts