An NMR view of nonlinear magnetization dynamics : in liquid and solid, at low and high polarizations

 Abstract

In Nuclear Magnetic resonance (NMR), it is well known that the interaction between the magnetization of a sample and the detection circuit in an NMR spectrometer gives rise to nonlinear magnetization dynamics. This phenomenon is commonly termed “radiation damping”). In the context of Dynamic Nuclear Polarization (DNP) experiments at low temperatures (< 4 K), it is possible to achieve very high nuclear spin polarization (typically ∼ 80 − 90%. In such experiments, the increased magnetization can generate sustained NMR masers and coherent radio frequency signals persistent for minutes or even hours (to be compared to the typical few μs long free induction decay signal in a nonrotating solid) with amplitude modulation. This behaviour can be ascribed to the combination of two competing mechanisms, namely the return of the nuclear magnetization by radiation damping towards its equilibrium direction, on the one hand, and the re-building of polarization of the proton spins, creating large magnetization pointing to the opposite direction, on the other hand. Fine experimental features of these experimentscan be interpreted by taking into account the presence of the associated collective effect of the distant dipolar field (DDF). Alternatively, for thermally polarized spins (P ∼ 10 −5 at ambient temperature), the use of an electronic feedback unit to control the radiation damping field allows for similar sustained NMR masers. Chaotic behaviour was also observed and analyzed in previous work. Recent experiments have shown the possibility to achieve multi-mode NMR masers in solution. Both these illustrations of unusual nonlinear magnetization dynamics can be rationalized in terms of classical Bloch-Maxwell equations, possibly augmented by the presence of collective dipolar field effects.

Talk by Daniel Abergel