Correlation Effect on the Nuclear Magnetic Resonance Linewidth in Magnetic Materials

In a magnetic medium the nuclear spins are coupled by the indirect Suhl-Nakamura interaction. This interaction contributes to the nuclear magnetic resonance (NMR) linewidth and gives rise to a shift in the NMR frequency which is important in the helium-temperature range for materials with a large concentration of nuclear spins and a large interaction range, $b$. When this frequency pulling is appreciable, there exists a relative narrowing of the NMR line of the same order of magnitude as the relative frequency shift. This narrowing arises because the motions of the nuclear spins within the interaction range, $b$, are correlated.