Nuclear Magnetic Resonance Modes in Magnetic Material. I. Theory

In a magnetic medium the nuclear spins are coupled by the Suhl-Nakamura indirect interaction. This interaction is weak but has a very long range b. It is known to contribute to the nuclear spin relaxation. It also gives rise to a displacement of the nuclear magnetic resonance frequency which is important at temperatures in the helium range for materials with a large concentration of nuclei and large b. More generally the indirect interaction gives rise to a spectrum of nuclear spin waves. This spectrum may be shown to be meaningful even when the nuclei are far from order because of the long range of the interaction. Different methods to observe this spectrum are discussed.