Neutrons as a Probe of Magnetization on Atomic to Mesoscopic Scales
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Neutrons as a Probe of Magnetization on Atomic to Mesoscopic Scales G. Aeppli and S.M. Hayden
■ local magnetic-field distributions (e.g., the flux lattice of a superconductor), ■ moment distributions (e.g., the mag netization in ferromagnetic multilayers),
MRS BULLETIN/DECEMBER1999
An important feature of neutron scat tering is the simple and well-understood interaction of the neutron with matter. The neutron interacts with Condensed matter through two fundamental forces: the nuclear strong force and the electromagnetic force. These two contributions to the scattering have very different characteristics and can be easily separated. In this article, we will only consider the magnetic dipole interaction. The neutron p r o b e s the local m a g n e t i c field B(r) through the potential V(r) =
Introduction The neutron is a spin 1/2 particle with a well-specified magnetic dipole moment which interacts with magnetic fields in solids. 1 Because typical neutron wavelengths are comparable to interatomic distances in solids, the fields that can be probed ränge 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 mate rials, 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 comp o s i t i o n , or p r e s s u r e . A m a t e r i a l 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 ränge of values that makes it uniquely suited for some predetermined application. Once the ma terial 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. Examples of physical quantities that can be measured by magnetic neutron scatter ing are
What Can Be Measured?
■ magnetic structures (e.g., antiferromagnetism), ■ magnetization density (e.g., how the moment is distributed in iron), and ■ magnetic excitations (e.g., spin waves). Instead of aiming for exhaustive coverage of a huge field, we discuss three specific problems where magnetic neu tron scattering has provided key insights. The remainder of the article is devoted to a n e x p a n d e d discussion of what the technique measures, discussions of each of the problems, and how magnetic neu tron scattering contributed to their reso-
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• B(r).
(1)
Thus spatially varying magnetic fields can be measured directly. A microscopic magnetic moment, such as an electron spin, produces a corresponding micro scopic magnetic field. Neutro
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