With materials of ever-increasing complexity becoming key elements of the technologies underpinning industrial and economic development, there is an ongoing need for tools that reveal the microscopic origins of physical, electrical, magnetic, chemical, and biological properties. Neutron scattering is one such powerful tool for the study of the structure and dynamics of materials. Neutrons are well suited to this purpose for several reasons:

∎ Neutrons are electrically neutral, leading to penetration depths of centimeters and thereby enabling in situ studies.

∎ Neutron cross sections exhibit no regular dependence on atomic number and are similar in magnitude across the periodic table, giving rise to sensitivity to light elements in the presence of heavier ones.

∎ Certain large differences in isotopic scattering cross sections (e.g., hydrogen to deuterium, H/D) make neutrons especially useful for the study of light atoms in materials.

∎ The range of momentum transfer available allows probing of a broad range of length scales (0.1–105 Å), important in many different materials and applications.

∎ Thermal and “cold” (longer-wavelength) neutrons cover a range of energies sufficient to probe a wide range of lattice or magnetic excitations, “slow” dynamic processes such as polymer chain reptation, and so forth.

∎ Neutrons have magnetic moments and are thus uniquely sensitive probes of magnetic interactions.

∎ Neutrons can be polarized, allowing the cross sections (magnetic and non-magnetic) to be separated.

∎ The simplicity of the magnetic and nuclear interactions makes interpretation of results straightforward.

In the past, many unexpected failures of components were due to poor quality control or a failure to calculate—or to miscalculate—the stresses or fatigue stresses a component would experience in service. Today, improved manufacturing, fracture mechanics, and computational finite element methods combine to provide a solid framework for reducing safety factors, enabling leaner design. In this context, residual stress—that is, stress that equilibrates within the structure and is always present at some level due to manufacturing—presents a real problem. It is difficult to predict and as hard to measure. If unaccounted for in design, these stresses can superimpose upon in-service stresses to result in unexpected failures.

Neutron diffraction is one of the few methods able to provide maps of residual stress distributions deep within crystalline materials and engineering components. Neutron strain scanning, as the technique is called, is becoming an increasingly important tool for the materials scientist and engineer alike. Point, line-scan, area-scan, and full three-dimensional (3D) maps are being used to design new materials, optimize engineering processes, validate finite element modeis, predict component life, and diagnose engineering failures.

The discovery and development of new materials is the foundation of the science and technology “food chains.” Examples of new materials with novel properties that have stimulated new scientific questions and/or led to new technologies include liquid crystals, advanced batteries, structural ceramics, dielectrics, ferroelectrics, catalysts, high-temperature superconductors, har dmagnets, and magnetoresistive devices. Establishing the crystal structure of a newly discovered Compound is a mandatory first step, but the most important contribution of diffraction techniques is to provide an understanding of the relationships among chemical composition, crystal structure, and physical behavior. In this way, diffraction experiments provide critical Information for testing theories that explain novel behavior and guide the optimization of new materials to meet the demands of emerging technologies.

The first samples of newly discovered materials are often polycrystalline. With state-of-the-art neutron powder diffraction data and Rietveld refinement techniques, for structures of modest complexity, the precision for atom positions rivals that obtained by single-crystal diffraction. Rietveld refinement is a method of obtaining accurate values for atom positions and other structural parameters from powder diffraction data by least-squares fitting of a calculated model to the full diffraction pattern. As evidence of thi s success, the Inorganic Crystal Structure Database contains 6044 entries from neutron powder diffraction, 7096 from laboratory x-ray powder diffraction, an d 228 from Synchrotron x-ray powder diffraction. Other reasons for the rapidly growing impact of neutron diffraction include the favorable neutron-scattering cross sections for light elements, the sensitivity to magnetic moments, and the ability to penetrate special sample environments for in situ studies. These strengths are widely accepted and have been exploited for many years. Previous reviews have focused on these topics.

The neutron is a spin 1/2 particle with a well-specified magnetic dipole moment which interacts with magnetic fields in solids. Because typical neutron wavelengths are comparable to interatomic distances in solids, the fields that can be probed range from the dipoles of individual electron spins to those associated with flux lines in superconductors. In this article, we give a brief introduction to the use of magnetic neutron scattering in materials science.

Progress in materials science typically involves new materials or the improved fabrication of previously known materials, followed by measuring quantities such as electrical resistivity, magnetic susceptibility, or specific heat, typically as a function of variables such as temperature, magnetic field, chemical composition, or pressure. A material is deemed interesting if a simply defined bulk characteristic either undergoes a spectacular evolution as a function of one of the variables—superconductivity comes to mind—or has a range of values that makes it uniquely suited for some predetermined application. Once the material is thought to be interesting, there are natural drives to understand the origin of its interesting properties as well as to improve them. At this point, a microscopic tool such as magnetic neutron scattering provides valuable insights that bulk measurements cannot yield.

Neutron scattering had its origin in 1932, the year that marked the discovery of the neutron by Chadwick, and the first nuclear reactors were successfully operated in Chicago and Oak Ridge, Tenn., in the early 1940s. During its initial stages, neutron scattering was used mainly for the study of “hard” crystalline materials. For example, Shull and Wollan's pioneering research, which led to the 1994 Nobel Prize in physics, began with studies of iron, chromium, and iridium, and was followed by the development of polarization analysis to determine the structure of magnetic materials. Such studies continue to yield important structural information (see the articles on magnetism by Aeppli and Hayden and on crystallography by Radaelli and Jorgensen in this issue of MRS Bulletin), although during the last two decades, the technique has been increasingly used by scientists from other disciplines (chemistry, biology, polymer science), and many of these newer applications have involved “soft” matter such as polymers, colloids, and gels. By definition, these substances are “plastic” or “squishy,” and easy to mold into different shapes; because of this flexibility, they have become some of the most practical and widely used materials today.

The substantial power of both elastic and inelastic neutron-scattering techniques for the investigation of the structure and dynamics of biological systems and related biomolecular-based materials—as with soft matter in the previous article by Lindner and Wignall—arises primarily from the essentially isomorphous nature of the substitution of deuterium for selected hydrogen atoms in these systems, coupled with the exquisite sensitivity of neutron scattering to this isotopic substitution. Since these systems are comprised of large macromolecules and supramolecular assemblies thereof, their essential structures and dynamics extend from the atomic scale up to very large length scales of the Order of 101–104 Å. Hence neutron sources and neutron-scattering spectrometers optimized for longer wavelength (or “cold”) thermal neutrons are necessary in order to most effectively address the structure and dynamics at the longer length scales inherent to these Systems.

The large majority of previous neutron-scattering experiments on biological systems have been performed with reactor neutron sources. Some of the more significant of these are briefly summarized in the following sections. They may be categorized in terms of the nature of the intermolecular order, both orientational and positional, within the System of interest and either the elastic neutron-scattering technique employed to investigate their time-averaged structures or the inelastic neutron-scattering technique employed to investigate their dynamics.