Superconductivity and magnetism at intermediate (“mesoscopic”) length scales between atomic and bulk have a long history of interesting science. New science emerges due to the presence of multiple length scales, especially when these become comparable to relevant geometric sizes. New phenomena may appear due to topological interactions, geometric confinement, proximity between dissimilar materials, dimensional crossover, and collective effects induced by periodicity. In this review, we select a few, recent highlights that illustrate the type of novel science that can be accomplished in superconducting and magnetic structures. These materials can serve as model systems and provide new ideas, which can be extended to other systems such as ferroelectrics and multiferroics. We also highlight general open questions and new directions in which the field may move.

Proximity effects and exchange coupling across interfaces of hybrid magnetic heterostructures present unique opportunities for functional material design. In this review, we present an overview of recent experiments on magnetic hybrid materials in which magnetism was controlled by proximity to an active material. In particular, we discuss interfacial strain coupling of ferromagnetic materials in contact with a material undergoing a structural deformation. Bilayers containing VO2 and V2O3 as active materials are shown to strongly affect the magnetization and coercivity of ferromagnetic materials due to stress anisotropy caused by a temperature-dependent structural displacement in the oxide. The possibilities of tuning the system by sample morphology and materials choice are discussed in detail. In addition, we highlight a length-scale competition between magnetic and structural domains which leads to a maximum change in the coercivity in a narrow temperature window of the vanadium oxide phase transition.

The following article is an edited transcript based on the MRS Medalist presentation given by Ivan K.Schuller of the University of California, San Diego, on December 3, 2003, at the Materials Research Society Fall Meeting in Boston.Schuller received the MRS Medal for “his innovative studies of exchange bias in magnetic heterostructures and nanostructures.” Magnetic nanostructures have received increasing attention in recent years, motivated by the interesting phenomena that are apparent when physical size becomes comparable with relevant magnetic length scales.In addition, a number of important potential applications in the sensors and storage industries have emerged. When magnetic nanostructures are in contact with dissimilar magnetic materials, and because their magnetic fields extend considerably outside the physical structure, they are very susceptible to interaction with the surrounding environment.A particularly interesting situation is a ferromagnetic nanostructure in contact with an anti-ferromagnetic substrate.In this “exchange-biased” configuration, a variety of unusual phenomena arise:The reversal mode of the ferromagnet changes considerably, the superparamagnetic transition temperature is affected, and there is a noticeable change in the microscopic spin configuration.A series of experiments will be described involving these phenomena in nanostructured ferromagnets prepared by electron-beam lithography and self-assembly.

The adsorption/absorption of hydrogen and the adsorption of carbon monoxide by niobium foils, at room temperature, was studied using thermal desorption spectroscopy. Two hydrogen desorption peaks were observed with a maximum at 404 and 471 K. The first hydrogen desorption peak is regarded as hydrogen desorbing from surface sites while the second peak, which represents desorption from surface sites stronger bound to the surface, also has a component—due to its tailing to higher temperatures—of hydrogen diffusing from subsurface sites. Carbon monoxide adsorption was used to determine the number of surface sites, since it does not penetrate below the surface. Two carbon monoxide desorption peaks are observed in these experiments: at 425 and 608 K. The first peak is regarded as the adsorption of molecular carbon monoxide, and the second, as carbon monoxide dissociated on the niobium surface. The crystallographic orientation of the foils was determined by x-ray diffraction and showed a preferential (110) orientation of the untreated foil due to the effect of cold rolling. This preferential orientation decreased after hydrogen/heat treatment, appearing strong also in the (200) and (211) orientations. This change in texture of the foils is mainly due to the effect of heat treatment and not to hydrogen adsorption/desorption cycling. The kinetics of hydrogen and CO desorption is compared with that of Pd and Pd alloys.

We discuss here the complete determination of the elastic properties of a polyimide film using two experimental techniques. One technique employs the polymer film as a vibrating membrane and allows a direct determination of the ’macroscopic’ biaxial modulus. Brillouin scattering, which measures the elastic properties on a ∼ 100 μ scale, allows for a complete characterization of the elastic behavior. The results obtained by the two techniques are in agreement within the reported error bars.

The experimental fact that measured elastic and structural properties of superlattices are strongly correlated can be understood on the basis of a simple model based on the packing of hard spheres. The model is consistent with features of many models that have been proposed to explain the supermodulus effect, but contrary to previous explanations, it allows predictions for a given pair of constituents to be made. For an arbitrary pair of elements, it predicts the existence or non-existence of an elastic anomaly, and a rough estimate of its magnitude.

Thin-film materials are playing an ever-increasing role in basic and applied research and are the basis for a large number of devices. In many cases, thin films are used in multilayered configurations so that the interfaces between different materials play a key role in their properties. The thicknesses of thin films being investigated approach interatomic spacing, so interfacial structure and chemistry become dominant. The field is now at a stage where it is no longer sufficient to only obtain qualitative information; it is also necessary to understand the structure and chemistry quantitatively and to determine limits on the accuracy of the various characterization tools. The present and next issues of the MRS Bulletin bring together a number of experts in the most commonly used characterization tools. To highlight and encourage international collaboration, the articles presented here are jointly written by scientific groups in the United States and Europe.

The first issue of this series is dedicated to the most commonly used techniques: electron microscopy, x-ray diffraction, and surface analysis. The article on electron diffraction by Ourmazd, Scheffler, Heinemann, and Rouviere presents studies of atomic resolution microscopy and their limitations. Aspects highlighted are the importance of comparing simulations to the images obtained in the electron microscope, and the importance of theoretical studies to obtain quantitative information. The article by Fullerton, Bruynseraede, and Schuller describes recent developments in the field of x-ray diffraction. This article focuses on a refinement technique capable of quantitative determination of interfacial roughness, interdiffusion, and crystal structure changes in a variety of novel multilayer configurations.

The physical properties of superlattices have been the subject of considerable interest because a wide range of phenomena associated with very thin films, interfaces, and coupling effects can be studied. Recent areas of activity in metallic superlattices include antiferromagnetic coupling of ferromagnetic layers across nonmagnetic spacer layers, giant magnetoresistance, magnetic surface anisotropy, low-dimensional superconductivity, and anomalous mechanical properties. All of these phenomena are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. Therefore, a detailed understanding of the properties of superlattices requires a nondestructive, quantitative determination of the superlattice structure.

Because superlattices are not in thermodynamic equilibrium, their structure is sensitive to preparation methods and growth conditions. A dramatic example of superlattice structural dependence on growth conditions is shown in Figure 1, for sputtered Nb/Si superlattices. Increasing the Ar pressure during sputtering decreases the kinetic energy of the deposited atoms, thereby changing their surface mobility, and thus altering growth dynamics. Figure 1 shows the low-angle x-ray diffraction and cross-sectional transmission electron microscopy (TEM) images of [Nb(35 Å)/Si(25 Å)]40, superlattices sputtered in, respectively, 3 and 15 mTorr of Ar. The TEM image of the 3 mTorr superlattice clearly shows the smooth and continuous layering across the entire cross section of the image (≈5 μm). This is characteristic of sputtered metal/semiconductor superlattices used for x-ray optics.

The strain profiles of individual layers of a selected group of stable bcc(110)/fcc(111) metallic superlattices which have been determined by x-ray diffraction methods are compared, and discussed in terms of a number of mechanisms proposed for explaining their anomalous properties. The superlattices in this group are distinguished in terms of a highly anisotropic lattice spacing mismatch of their adjoining bcc and fcc layers (∼20% vs. 3%). The most prominent structural feature of the bcc layers consists of a highly anisotropic in-plane contraction accompanied with a small out-of-plane strain. The fee layers are found to show large out-of-plane expansions which based on their small in-plane expansions cannot be explained within framework of standard elasticity theory.

The mechanical properties of metallic superlattices have been shown to exhibit anomalous properties. Several of the elastic constants are found to exhibit anomalies which are correlated with structural anomalies in lattice mismatched systems which do not form solid solutions. Lattice matched systems which form solid solutions in their thermodynamics phase diagram, show much smaller elastic anomalies and no structural anomalies. Anomalous plastic behavior, on the other hand, seems to be present in both types of superlattices, indicating that the plastic behavior is possibly defect induced. Detailed quantitative structural measurements combined with comprehensive mechanical properties hold the promise of determining the physical origins of the anomalous properties of metallic superlattices.

We present a general procedure for quantitative structural refinement of superlattice structures. To analyze a wide range of superlattices, we have derived a general kinematical diffraction formula that includes random, continuous and discrete fluctuations from the average structure. By implementing a non-linear fitting algorithm to fit the entire x-ray diffraction profile, refined parameters that describe the average superlattice structure, and deviations from this average are obtained. The structural refinement procedure is applied to a crystalline/crystalline Mo/Ni superlattices and crystalline/amorphous Pb/Ge superlattices. Roughness introduced artificially during growth in Mo/Ni superlattices is shown to be accurately reproduced by the refinement.

The presence and origin of elastic anomalies in superlattices, interfaces, composites, and nanocrystalline materials has been a subject of much interest and controversy in recent years. In particular, superlattices are being used as model systems to study the effect of interfaces on the mechanical properties of novel materials. Early claims of anomalously large enhancements of the biaxial and flexural moduli of Au/Ni and Cu/Ni superlattices (“supermodulus effect”) created considerable controversy and contradictory reports in the experimental and theoretical literature. To understand the mechanical properties of superlattices and their implication for other types of nanofabricated materials, it is important to look critically at the field.

Superlattices have been fabricated in different laboratories by a variety of preparation methods and have been characterized structurally and elastically to various degrees. Because of this, before addressing any sophisticated theoretical issues regarding elastic anomalies, it is important to understand in detail the experimental techniques and the possible pitfalls in the experimental determination of mechanical properties of thin films. Once the presence or absence of the effect is ascertained, the need arises to understand any possible experimental correlations with other structural and/or physical properties. These correlations can be used to address issues regarding the possible origins of the effect and their theoretical basis. It will be apparent from this article that slight changes in structural properties are correlated with relatively large elastic anomalies. Consequently, this points to the importance of precise, comprehensive, and detailed structural characterization. With our present understanding of the physics of superlattices, qualitative studies are no longer sufficient!

A comprehensive review and state of the art in the field of surface, interface, and thin-film magnetism is presented. New growth techniques which produce atomically engineered novel materials, special characterization techniques to measure magnetic properties of low-dimensional systems, and computational advances which allow large complex calculations have together stimulated the current activity in this field and opened new opportunities for research. The current status and issues in the area of material growth techniques and physical properties, characterization methods, and theoretical methods and ideas are reviewed. A fundamental understanding of surface, interface, and thin-film magnetism is of importance to many applications in magnetics technology, which is also surveyed. Questions of fundamental and technological interest that offer opportunities for exciting future research are identified.

The study of Artificially Layered Superconductors (ALS) started more than 20 years ago with the search for unusual and/or high temperature superconductivity in a variety of metal-semiconducting layers. Renewed interest was motivated by the advent of novel preparation techniques that allow control of layer thicknesses close to interatomic distances. In this way layered superconductors can be used as model systems to study a variety of physical phenomena, prepare structures with improved properties and discover novel metastable phases which do not exist in nature. Examples of these studies include: a diversity of dimensional transitions, interaction between superconductivity and magnetism, interaction between superconductivity and electron localization, enhancements of critical fields and critical currents, and the study of incommensurate systems.

Recent developments in high temperature ceramic superconductors further increase the importance of studies of Artificially Layered Superconductors. The newly discovered ceramic superconductors are structurally layered and therefore many of their properties will also be determined by this structure. Because of this, particularly in the search for the mechanism of superconductivity, it is important to understand which properties are a consequence of the layered nature of the material and which are due to the presence of some unusual, yet undetermined physical phenomena.

What makes Artificially Layered Superconductors especially attractive for investigation? The main reason rests on the fact that the characteristic lengths which determine the superconducting properties, i.e., the coherence length and penetration depth, are quite long in conventional low temperature superconductors.

Our understanding of the superconducting mechanism in the newly discovered oxide superconductors is closely connected with a detailed knowledge of the structural properties displayed by these materials. Moreover, it is important to explore possible connections between structural and superconducting properties and to establish general structural principles which can guide our search for new compounds.

We present here a survey of the structures of the recently discovered oxide superconductors. The common feature of these structures (with the exception of cubic Ba1-x Kx BiO3-y) is the presence of one or more planes of copper atoms with four strongly bonded oxygen atom neighbors in a square planar arrangement at a distance of approximately 1.90 Å (“CuO2 planes”). In some structures these planes occur in groups, with the individual planes inside a group being separated by one Y or Ca layer and the groups being intercalated by a variable number of LaO, BaO, CuO, TIO and/or BiO layers. (The intercalated CuO layer is crystallographically distinct from the “CuO2 plane”). A natural model for superconductivity which arises from these structures is that the conductivity occurs predominantly in the Cu02 planes, while the other (intercalated) layers provide in some fashion carriers and/or the coupling mechanism necessary for the superconductivity.

We have performed extensive structural and electronic transport measurements of Pb/Cu multilayered structures. The high-angle x-ray diffraction spectrum indicates the presence of continuous interfacial disorder. In the low-angle x-ray spectrum, pronounced minima occur at positions given by q = 2π/Λ, where Λ is the bilayer thickness. This is in sharp contrast to the results for Pb/Ge (crystalline/amorphous) multilayers, where minima are observed at positions q = 2π / tPb with tPb the Pb thickness. These low-angle x-ray spectra can be explained by taking into account lateral coherence and continuously distributed thickness fluctuations. The effect of this interfacial disorder on the electronic properties is investigated by measuring the low-temperature resistivity and the superconducting transition temperature of the multilayers. The calculated values for the elastic mean free path in the individual layers confirm the presence of strong interfacial scattering. The thickness dependence of the critical temperature is explained by taking into account an interfacial barrier with penetration probability σ ≃ 0.5.

The growth of thin films has been instrumental in the study of many areas of material science, physics, metallurgy, and chemistry and is an important ingredient in the development of many devices. Although experimental studies have been extensively pursued for many years, theoretical studies have only been performed using model calculations which rely on a number of unknown parameters a priori. Only recently have attempts been made to understand thin film growth using realtime numerical simulation. The main reason for the recent increase of such studies is the development of computers capable of tackling a problem of the magnitude required to understand thin film growth. The phenomena present in thin film growth occur for systems containing many particles (e.g., columnar growth) and long relaxation times, which strain the capabilities presently available in modern supercomputers. Further increases in computational power might bring a number of important problems within reach and improve our understanding of thin film growth at the microscopic level.

I will present a number of epitaxial growth studies we have performed using molecular dynamics (MD) techniques. I will show that a number of properties predicted by these calculations are in good agreement with experimental observations. These include the microcrystalline and epitaxial growth of metal films, the growth of amorphous films in mixtures of metals, and the vapor phase growth of silicon. Finally, I will outline several important studies yet to be implemented.

We present the possibilities that metallic superlattices offer for the study of materials at length scales ranging from a few to hundreds of angstroms. The materials problems being studied span practically all interesting solid-state phenomena, including superconductivity, magnetism, elastic behavior, development of novel materials, diffusion, ion beam mixing, crystallization, and amorphization. Several applications are also being pursued. We present a few examples of problems that can be studied at different length scales. Emphasis is made that for a proper study of materials properties, extensive structural characterization is imperative.

Magnetic superlattices serve as model systems for the study of thin film, interfacial, proximity, coupling and superlattice phenomena. Due to these phenomena, the physical properties of magnetic superlattices can be tuned in a reproducible fashion by proper control of the preparation process.

Magnetic measurements in conjunction with detailed structural characterization provide a fruitful area of research, especially in understanding basic phenomena in magnetism. We describe here briefly a few experimental examples from our work which illustrate the possibilities magnetic superlattices offer for the study of basic phenomena in magnetism.