Polarised neutrons and complex antiferromagnets: an overview

Abstract

Determination of magnetic structure from measurement of the intensities of neutrons (or X-rays) scattered by magnetic Bragg reflections is in some ways a more difficult problem than determination of the crystal structure. This is because the magnetic structure factor is a vector quantity so that directional as well as phase information is lost in measuring just the intensity. In this situation, the vector properties of the neutron's polarisation provide a way of recovering some of this lost information. The changes in direction of the neutron spin which take place on scattering by the fourier components of the magnetic induction in a crystal are highly dependent on their relative orientation. These changes can be measured using a neutron polarimeter which can determine the direction of scattered neutron polarisation for any chosen incident polarisation direction. This technique has been named spherical polarimetry. Using spherical polarimetry, it has been possible to solve a number of magnetic structure problems which were intractable by other means. The polarimetric data obtained from just a very few Bragg reflections may impose sufficient constraints on the possible magnetic structures to allow a unique model to be established. The technique has proved particularly valuable in the study of non-collinear magnetic structures where it has already shown that certain previously determined structures were incorrect.