Resolving magnetic order and dynamics with polarized neutron scattering: a practical guide to longitudinal polarization analysis

Abstract

Neutron scattering provides microscopic information on magnetic order and spin dynamics. Most experiments, however, use unpolarized neutrons and do not fully exploit the information carried by the neutron spin. Longitudinal polarization analysis (LPA), established in the pioneering work of Moon, Riste, and Koehler at Oak Ridge National Laboratory in the late 1960s, separates magnetic and nonmagnetic scattering contributions through their different effects on the neutron spin. In this review, we illustrate how LPA can distinguish weak magnetic scattering from nonmagnetic contributions, constrain small moment components, and resolve the spin-space anisotropy of magnetic excitations. We introduce the formalism connecting measured polarization channels to magnetic quantities and discuss scattering geometry, crystallographic projections, and corrections for finite polarization. Our aim is to provide a practical guide for neutron scatterers using LPA to determine magnetic structures and characterize spin excitations in quantum materials. We conclude with opportunities in altermagnets, unconventional superconductors, and kagome magnets.