Neutron-Spin-Echo Spectroscopy and Magnetism

This chapter discusses neutron-spin-echo (NSE) spectroscopy and magnetism. In NSE spectroscopy, the precessions of neutron spins in a magnetic field are used as a stop-watch, which is inherent in each individual neutron and measures the energy transfer with accuracy better than 10−5. Neutron-spin-echo spectroscopy is fundamentally different from all classical methods because it decouples the accuracy with which neutron velocity changes are measured from the velocity distribution of the incoming neutrons. NSE reaches the highest energy resolution while maintaining the high intensity advantage of a beam that is only 10–20% monochromatic and poorly collimated. The decoupling of energy resolution from the spectral width holds exactly for quasielastic and for inelastic nondispersive scattering. In inelastic neutron scattering spectroscopy the spectral response of the sample is given by the probability of a change of the neutron energy in the scattering process, as expressed by the dynamic structure factor. At a given wavelength an NSE spectrometer must be tuned for each of the magnetic field integrals used for the measurement. The superparamagnetic fluctuations in monodomain iron particles are also elaborated in the chapter.