Fundamental mechanisms of single-pulse NMR echo formation

Abstract

A theoretical investigation of the formation of the so-called single pulse NMR echo has been undertaken in the presence of both Larmor and Rabi frequency inhomogeneities. It is found that this echo phenomenon, together with weaker, equally spaced secondary echoes, evolves from multiple-pulse excitation under certain constraints with respect to interpulse time intervals compared with longitudinal and transverse relaxation times. These transient coherences arise from a complex superposition of oscillatory free induction decays, which extend further into the time domain because of cumulative dephasing within each pulse in a multiple-pulse sequence. The effect is present for large Larmor inhomogeneity in isolation, but is stronger in the simultaneous presence of both frequency inhomogeneities, characteristic of ferromagnets. Predictions of this model analysis are confirmed through experiment via conventional, zero-field, pulsed NMR on binary $^{51}\mathrm{VFe}$ and pure elementary (enriched) $^{57}\mathrm{FeFe}$ ferromagnets.