Magnetic structure determinations at NBS/NIST

Headnote

Magnetic neutron scattering plays a central role in determining and understanding the microscopic properties of a vast variety of magnetic systems, from the fundamental nature, symmetry, and dynamics of magnetically ordered materials to elucidating the magnetic characteristics essential in technological applications. From the early days of neutron scattering measurements at NBS/NIST, magnetic diffraction studies have been a central theme involving many universities, industrial and government labs from around the United States and worldwide. Such measurements have been used to determine the spatial arrangement and directions of the atomic magnetic moments, the atomic magnetization density of the individual atoms in the material, and the value of the ordered moments as a function of thermodynamic parameters such as temperature, pressure, and applied magnetic field. These types of measurements have been carried out on single crystals,

Headnote

powders, thin films, and artificially grown multilayers, and often the information collected can be obtained by no other experimental technique. This article presents, in an historical perspective, a few examples of work carried out at the NIST Center for Neutron Research (NCNR), and discusses the key role that the Center can expect to play in future magnetism research.

Key words: applied magnetic field; magnetic multilayers; magnetic order parameter; magnetic structure; magnetic superconductors; magnetic symmetry; neutron diffraction; polarization analysis; pressure dependence.

Headnote

Accepted: August 22, 2001

Available online: http://www.nist.gov/jres

1. Introduction

There have been hundreds of studies of magnetic structures and magnetic ordering at the NCNR, on wide classes of materials. A comprehensive review of this work is not possible within this context, so in the current article we simply discuss a few examples of the type of work that has been carried out at the NCNR, and provide some additional representative references to the wider distribution of work. The neutron instrumentation required to make such measurements is generally the same as needed for the determination of crystallographic structures on a variety of length scales, and the history of the available instrumentation is discussed elsewhere in this volume. Here we briefly note the neutron instrumentation presently available to the magnetism community at the NCNR, and mention plans for new instrumentation which will take the field in the United States into the next decade and beyond.

Magnetic neutron scattering originates from the interaction of the neutron's spin with the unpaired electrons in the sample. The strength of this magnetic dipole-dipole interaction is comparable to the neutron-- nuclear interaction, and thus there are magnetic cross-sections that are analogous to the nuclear ones that reveal the complete structure and full range of lattice dynamics of materials over wide ranges of length scale and energy. The traditional role of magnetic neutron scattering is the measurement of magnetic Bragg intensities in the magnetically ordered regime, which can be used to determine the spin configuration and directions of the atomic magnetic moments as a function of temperature, pressure, and applied magnetic field, on single crystals samples, powders, thin films and artificially grown multilayers [1]. Early studies addressed materials such as spinels and ferrites, followed by rare-earth intermetallics [2] and rare earth hydrides [3]. One topic that has sustained interest over the years, though, is the magnetic ordering that occurs in superconductors [4-7], and we will present some examples below. Other types of systems that have been investigated with magnetic diffraction include heavy fermion systems [8-13], ruthenates [14-15] and cobalates [16-17], amorphous [18] and nanocrystalline [19-21] systems, frustrated magnets [22-24], molecular magnets, [25-26] and colossal magnetoresistive oxides [27-33].

5. Future

In recent years the new suite of cold neutron instrumentation has developed into the best facilities available in the U.S., and these new world-class neutron spectrometers have dramatically improved our measurement capability for exploring the properties of magnetic materials. Presently we are developing a new suite of thermal neutron instrumentation that will be unparalleled in this country, and we anticipate that these new instruments will produce an equally important impact on future investigations of magnetic phenomena.

One of the advantages of working at a neutron facility with a suite of modern instruments is that one has the ability to explore a wide range of phenomena, from domain structures, ferrofluids, and magnetically active bio-organisms with SANS, to multilayer magnets with reflectometry, to magnetic diffraction studies as a function of temperature, pressure, and applied magnetic and electric fields. Magnetic neutron scattering presently plays a dominant role in addressing these kinds of problems, and this will no doubt continue for many years to come.

Acknowledgments

The authors have collaborated with many researchers over the years as indicated in the references, and we thank our collaborators for working with us on these very interesting and productive endeavors.

References

6. References

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References

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