Neutron scattering studies of nanomagnetism and artificially structured materials

Nanostructured magnetic materials are intensively investigated due to their unusual properties and promise for possible applications. The key issue for these materials is to understand the limits between their physical properties (transport, magnetism, mechanical, etc.) and their chemical-physical structure. In principle, a detailed knowledge of the chemical and physical structures allows calculation of their physical properties. Theoretical and computational methods are rapidly evolving so that magnetic properties of nanostructured materials might soon be predicted. Success in this endeavor requires detailed quantitative understanding of magnetic structure at the microscopic level. Neutron scattering is a well-developed technique that can determine magnetic structure at the atomic length scale in samples of ever diminishing size. This has opened up the use of neutron scattering to nanostructured materials prepared by thin film and lithographic techniques. Many interesting and unexpected results have emerged from the application of elastic neutron scattering to nanostructured magnetic thin films such as superlattices and multilayers. These include, distinguishing between magnetic and chemical boundaries, observing the spatial dependence of the magnetization vector in nonuniform materials, unusual coupling mechanisms across nonmagnetic materials, unexpected magnetic phase diagrams, etc. Extension of elastic neutron scattering to nanostructured arrays and three-dimensional magnetic composites will allow future determination of magnetic structure with unprecedented resolution. In this review, we discuss the impact of neutron scattering to the study of magnetic nanostructures, i.e., magnetic materials that are artificially structured at nanometer length scales, such as magnetic thin films, multilayers and nanodot arrays. The basic interactions and different length scales relevant to these systems as well as the basic issues and phenomena of interest are briefly reviewed. We discuss examples where the techniques of magnetic neutron diffraction, small-angle scattering, reflectometry, grazing incidence diffraction and diffuse scattering have helped to elucidate some of these phenomena. We also discuss potentially fruitful future applications of such techniques to the field of nanomagnetism. Furthermore, we argue that the development of inelastic neutron scattering techniques useful for the study of small volumes of material would raise neutron scattering to a much higher level of applicability for nanostructured magnetic materials.