Magnetic Response of Unstable Moments Measured by Neutron Scattering

Abstract

The instability of magnetic moments in metallic systems is due to the strong coupling (or even hybridization) between the local (3d, 4f or 5f) electrons and the conduction electrons. The magnetic response and its temperature dependence as measured by inelastic magnetic neutron scattering will be used as a guide to classify the different behavior of stable, Kondo, mixed valent or spin fluctuation systems. Ideally, a stable moment would be a local moment (built up by intra-atomic correlations of a partly filled 3d, 4f or 5f shell) without any interactions with its surroundings. Its response would be a δ-function at zero frequency (ω = o) and possible sharp excitations at finite frequencies (e.g. transitions to crystal field levels in a 4f system). A more realistic definition, however, would be a local moment with some weak interactions to its surroundings. The sharp structure of the magnetic response should be conserved; the meaning of “sharp”, however, depends now on the type of experiment and the corresponding frequency or energy resolution. In neutron scattering the accessible energy transfer goes from μeV through the typical meV range up to the eV range available only at spallation sources, with resolutions of 1% to 10% of the incident neutron energy. In addition to the energy spectrum neutron scattering also gives information about the spatial extent and spatial correlations of the magnetic moments through its Q dependence (Q=momentum transfer). For a detailed description of the magnetic scattering cross-sections see the article by T.M. Holden in this book.