Section 2.9.1. Introduction

The neutron reflectivity of a surface is defined as the ratio of the number of neutrons elastically and specularly reflected to the number of incident neutrons. When measured as a function of neutron wave-vector transfer, the reflectivity curve contains information regarding the profile of the in-plane average of the scattering-length density (or simply scattering density) normal to that surface. The concentration of a given atomic species at a particular depth can then be inferred. Although similar information can often be extracted from X-ray reflectivity data, an additional sensitivity is gained by using neutrons in those cases where elements with nearly the same number of electrons or different isotopes (especially hydrogen and deuterium) need to be distinguished. Furthermore, if the incident neutron beam is polarized and the resultant polarization of the reflected beam analysed, it is possible to determine, in both magnitude and direction, the in-plane magnetic moment depth profile. This latter capability is greatly facilitated by the simple correlation of the relative counts of neutron spin-flip and non-spin-flip scattering to magnetic moment orientation and by the relatively high instrumental polarization and spin-flipping efficiencies possible with neutrons. These properties make neutron reflectivity a powerful tool for the study of surface layers and interfaces.