Chapter 4 - Polarized Neutrons and Polarization Analysis

Abstract

This chapter discusses polarized neutrons and polarization analysis. The neutron carries a spin, which is an internal angular momentum with a quantum number. In a constant magnetic field, the magnetic moment of the neutron, and therefore its spin, rotate around the field in a Larmor precession. It is found that if before the rotation of the field the neutron spin was aligned along the field, after the rotation of the field, the neutron spin is still aligned along the field. Polarized neutron scattering, without polarization analysis, has been used mainly in elastic scattering in order to determine with a very good accuracy the magnetic scattering amplitudes. It is observed that for ferromagnets or ferrimagnets, but also for paramagnets or antiferromagnets for which a ferromagnetic component is induced by an applied magnetic field, the magnetic reflections occur at the same positions as the nuclear ones. Polarized neutrons take advantage of the coupling that exists between the nuclear and the magnetic amplitudes of the Bragg reflections. The way to retrieve the spin-density distribution, while avoiding the problems due to Fourier inversion, is to model the spin density and to determine the parameters involved in the model by a refinement procedure from the experimental data. The investigation of noncollinear magnetic structures is also elaborated.