Magnetic Neutron Scattering

Abstract

AbstractThe neutron is a spin‐particle that carries a magnetic dipole moment of −1.913 nuclear magnetons. Magnetic neutron scattering then originates from the interaction of the neutron's spin with the unpaired electrons in the sample, either through the dipole moment associated with an electron's spin or via the orbital motion of the electron. 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, which reveal the structure and dynamics of materials over wide ranges of length scale and energy. 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 the elucidation of the magnetic characteristics essential in technological applications.One traditional role of magnetic neutron scattering has been the measurement of magnetic Bragg intensities in the magnetically ordered regime. Such measurements can be 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 can be carried out on single crystals, powders, thin films, and artificially grown multilayers, and often the information collected can be obtained by no other experimental technique. For magnetic phenomena that occur over length scales that are large compared to atomic distances, the technique of magnetic small angle neutron scattering (SANS) can be applied, in analogy to structural SANS. This is an ideal technique to explore domain structures, long wavelength oscillatory magnetic states, vortex structures in superconductors, and other spatial variations of the magnetization density on length scales from 1 to 1000 nm. This particular technique has enjoyed dramatic growth during the last decade due to the rapid advancement of atomic deposition capabilities.Neutrons can also scatter inelastically, to reveal the magnetic fluctuation spectrum of a material over wide ranges of energy and over the entire Brillouin zone. Neutron scattering plays a truly unique role in that it is the only technique that can directly determine the complete magnetic excitation spectrum, whether it is in the form of the dispersion relations for spin wave excitations, wave vector and energy dependence of critical fluctuations, crystal field excitations, magnetic excitons, or moment fluctuations. In this overview we will discuss some of these possibilities are discussed.