21. Nuclear Magnetism

Abstract

This chapter discusses the nuclear magnetism. The chapter discusses the analysis of thermal neutron scattering by solid targets whose nuclear spins are not disordered, and of its physical applications. The order of the nuclear spins will refer as a rule to the case when their polarization is nonzero and may in some cases approach unity. The origin of the nuclear polarization can be many folds: (i) Thermal equilibrium in an external magnetic field, brought about by the thermal contact between the nuclear Zeeman interaction and the phonons. (ii) Forced polarization along an external field by a process known as dynamic nuclear polarization (DNP). (iii) Cooperative ordering of the nuclear spins at sufficiently low temperature under the effect of the nuclear spin-spin interactions. (iv) Cooperative ordering of nuclear spins “dressed” with electronic magnetization in Van Vleck ions. (v) Thermal equilibrium in the hyperfine field of electronic spins that are in an ordered state. The chapter discusses the influence on neutron scattering of short-range order between nuclear spins with no net nuclear polarization. Neutron scattering by targets of polarized nuclear spins provides information relevant to different domains of physics. First of all, it makes it possible to measure both in magnitude and sign, the spin-dependent part of the neutron scattering amplitude on various nuclear isotopes, which is useful information for practitioners of neutron scattering. Second, once this spin-dependent part is known, neutron scattering can be used to measure the nuclear polarization under conditions where this information cannot be obtained by magnetic resonance and to perform studies of solid-state physics.