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

Fabrication techniques developed for graphene research allow the disassembly of many layered crystals (so-called van der Waals materials) into individual atomic planes and their reassembly into designer heterostructures, which reveal new properties and phenomena. Andre Geim and Irina Grigorieva offer a forward-looking review of the potential of layering two-dimensional materials into novel heterostructures held together by weak van der Waals interactions. Dozens of these one-atom- or one-molecule-thick crystals are known. Graphene has already been well studied but others, such as monolayers of hexagonal boron nitride, MoS2, WSe2, graphane, fluorographene, mica and silicene are attracting increasing interest. There are many other monolayers yet to be examined of course, and the possibility of combining graphene with other crystals adds even further options, offering exciting new opportunities for scientific exploration and technological innovation. Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

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Van der Waals heterostructures

In the previous paper Ralph Brodd and Martin Winter described the different kinds of batteries and fuel cells. In this paper I will describe lithium batteries in more detail, building an overall foundation for the papers that follow which describe specific components in some depth and usually with an emphasis on the materials behavior. The lithium battery industry is undergoing rapid expansion, now representing the largest segment of the portable battery industry and dominating the computer, cell phone, and camera power source industry. However, the present secondary batteries use expensive components, which are not in sufficient supply to allow the industry to grow at the same rate in the next decade. Moreover, the safety of the system is questionable for the large-scale batteries needed for hybrid electric vehicles (HEV). Another battery need is for a high-power system that can be used for power tools, where only the environmentally hazardous Ni/ Cd battery presently meets the requirements. A battery is a transducer that converts chemical energy into electrical energy and vice versa. It contains an anode, a cathode, and an electrolyte. The anode, in the case of a lithium battery, is the source of lithium ions. The cathode is the sink for the lithium ions and is chosen to optimize a number of parameters, discussed below. The electrolyte provides for the separation of ionic transport and electronic transport, and in a perfect battery the lithium ion transport number will be unity in the electrolyte. The cell potential is determined by the difference between the chemical potential of the lithium in the anode and cathode, ∆G ) -EF. As noted above, the lithium ions flow through the electrolyte whereas the electrons generated from the reaction, Li ) Li+ + e-, go through the external circuit to do work. Thus, the electrode system must allow for the flow of both lithium ions and electrons. That is, it must be both a good ionic conductor and an electronic conductor. As discussed below, many electrochemically active materials are not good electronic conductors, so it is necessary to add an electronically conductive material such as carbon * To whom correspondence should be addressed. Phone and fax: (607) 777-4623. E-mail: stanwhit@binghamton.edu. 4271 Chem. Rev. 2004, 104, 4271−4301

http://www.chemie.uni-regensburg.de/Anorganische_Chemie/Vortrag/StudentenWS06-07/Vortrag_Amereller.pdf

Lithium Batteries and Cathode Materials

Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM). The performance of these devices is currently limited by the modest (< approximately 70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to approximately 220% at room temperature and approximately 300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of approximately 85%, which rivals that previously observed only using half-metallic ferromagnets. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 degrees C) will accelerate the development of new families of spintronic devices.

Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers

II. Structure of Perovskites 1982 A. Crystal Structure 1982 B. Nonstoichiometry in Perovskites 1983 1. Oxygen Nonstoichiometry 1983 2. Cation Nonstoichiometry 1984 C. Physical Properties 1985 D. Adsorption Properties 1986 1. CO and NO Adsorption 1986 2. Oxygen Adsorption 1987 E. Specific Surface and Porosity 1987 F. Thermal Stability in a Reducing Atmosphere 1989 III. Acid−Base and Redox Properties 1990 A. Acidity and Basicity 1990 B. Redox Processes 1991 1. Kinetics and Mechanisms 1992 2. Reduction−Oxidation Cycles 1993 C. Ion Mobility 1993 1. Oxygen Transport 1993 2. Cation Transport 1994 IV. Heterogeneous Catalysis 1995 A. Oxidation Reactions 1995 1. CO Oxidation 1995 2. Oxidation of Hydrocarbons 1996 B. Pollution Abatement 2001 1. NOx Decomposition 2001 2. Exhaust Treatment 2002 3. Stability 2004 C. Hydrogenation and Hydrogenolysis Reactions 2004 1. Hydrogenation of Carbon Oxides 2004 2. Hydrogenation and Hydrogenolysis Reactions 2006

Chemical structures and performance of perovskite oxides.

An investigation designed to display the intrinsic properties of perovskite manganites was accomplished by comparing the behavior of bulk samples with that of thin films. Epitaxial 1500 \AA{} films of perovskite ${\mathrm{La}}_{0.67}$${\mathrm{Ca}}_{0.33}$Mn${\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ were grown by solid source chemical vapor deposition on LaAl${\mathrm{O}}_{3}$ and post annealed in oxygen at 950 \ifmmode^\circ\else\textdegree\fi{}C. Crystals were prepared by laser heated pedestal growth. The magnetic and electrical transport properties of the polycrystalline pellets, crystals, and annealed films are essentially the same. Below $\frac{{T}_{C}}{2}$ the intrinsic magnetization decreases as ${T}^{2}$ (as can be expected for itinerant electron ferromagnets) while the intrinsic resistivity increases proportional to ${T}^{2}$. The constant and ${T}^{2}$ coefficients of the resistivity are largely independent of magnetic field and alkaline earth element (Ca, Sr, or Ba). Hall effect measurements indicate that holes are mobile carriers in the metallic state. We identify three distinct types of negative magnetoresistance. The largest effect, observed near the Curie temperature, is 25% for the Sr and 250% [$\frac{\ensuremath{\Delta}R}{R(H)}$] for the Ca compound. There is also magnetoresistance associated with the net magnetization of polycrystalline samples which is not seen in films. Finally a small magnetoresistance linear in $H$ is observed even at low temperatures. The high temperature (above ${T}_{C}$) resistivity of ${\mathrm{La}}_{0.67}$${\mathrm{Ca}}_{0.33}$Mn${\mathrm{O}}_{3}$ is consistent with small polaron hopping conductivity with a slight transition at 750 K, while ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ does not exhibit activated conductivity until about 500 K, well above ${T}_{C}$. The limiting low and high temperature resistivities place a limit on the maximum possible magnetoresistance of these materials and may explain why the "colossal" magnetoresistance reported in the literature correlates with the suppression of ${T}_{C}$.

Intrinsic electrical transport and magnetic properties of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 MOCVD thin films and bulk material.

Using single‐target off‐axis sputter deposition, high quality superconducting films of YBa2Cu3O7−δ were made in situ. These films have properties which are distinctly different from those of bulk ceramics and of post‐deposition annealed films. Their superconducting resistive transitions remain sharp regardless of the value of Tc between 75 and 86 K. Normal‐state conductivities are as high or higher than the best single crystals. Critical current densities as high as 6×107 A/cm2 at 4.2 K. Tc (R=0) falls off with film thickness down to 10 K for 35–40 A films. All of the above properties are relatively insensitive to compositional variation. The results can be explained if the in situ growth results in well‐formed CuO2 planes with defects occurring elsewhere.

In situ grown YBa2Cu3O7−d thin films from single‐target magnetron sputtering

Exploration of the generality of the recently discovered reaction whereby certain organic molecules can be inserted between the metallic layers of the superconductors tantalum disulfide and niobium disulfide revealed that a large variety of organic and inorganic molecules can penetrate between the crystalline layers of a number of transition metal dichalcogenides and that the resulting complexes are superconducting if the layered chalcogenide from which they are formed is superconducting. The critical temperatures of the 50 new superconductors we report depend on the nature of the intercalate but are insensitive to a separation of the superconducting planes of up to 57 angstroms.

https://science.sciencemag.org/content/sci/174/4008/493.full.pdf

Intercalation Complexes of Lewis Bases and Layered Sulfides: A Large Class of New Superconductors

We report the growth of in-plane textured (100) MgO on amorphous Si3N4 substrates by ion beam assisted deposition (IBAD). The textured MgO can be used as a structural template for subsequent epitaxial thin film deposition. The results are compared with IBAD of (100) and (111) yttria-stabilized-zirconia (YSZ). Based on transmission electron microscopy (TEM) and in situ reflection high energy-electron diffraction (RHEED), we find that MgO texturing is a nucleation-controlled process and the alignment is a function of nuclei size and density. This differs greatly from the evolutionary-type texturing process observed for IBAD (100) YSZ. Consequently, we are able to make 100 A thick MgO films with 7° in-plane alignment, whereas IBAD (100) YSZ films need to be thicker than 5000 A to achieve in-plane alignment better than 13°. This has important implications for the economical application of IBAD induced alignment in real manufacturing processes, including high Tc superconductor (i.e., YBCO) coated tapes, photov...

Deposition of in-plane textured MgO on amorphous Si3N4 substrates by ion-beam-assisted deposition and comparisons with ion-beam-assisted deposited yttria-stabilized-zirconia

The upper critical field ${H}_{c2}$ and fluctuation conductivity were measured for highly oriented thin films of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$. The ${H}_{c2}$ results demonstrate the intrinsic anisotropy in this layered superconductor. The broadening of the resistive transition under fields is interpreted in terms of critical fluctuations. The fluctuation conductivity shows dimensional crossover as expected for quasi-two-dimensional material. Using this data we determine the intra- and interlayer coherence lengths ${\ensuremath{\xi}}_{c}$(0) and ${\ensuremath{\xi}}_{\mathrm{ab}}$(0), and discuss the dimensionality of the superconductivity in this material.

T. H. Geballe

Papers

Intrinsic electrical transport and magnetic properties of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 MOCVD thin films and bulk material.

In situ grown YBa2Cu3O7−d thin films from single‐target magnetron sputtering

Intercalation Complexes of Lewis Bases and Layered Sulfides: A Large Class of New Superconductors

Deposition of in-plane textured MgO on amorphous Si3N4 substrates by ion-beam-assisted deposition and comparisons with ion-beam-assisted deposited yttria-stabilized-zirconia

Upper critical field, fluctuation conductivity, and dimensionality of YBa2Cu3O7-x.