Formation mechanisms and relaxation of NMR spin-echo signals excited by   two arbitrary duration radio-frequency pulses in magnets
  The work is devoted to the problem of multiple signals of nuclear spin echoes in magnets, excited by a series of radio-frequency (RF) arbitrary duration pulses exceeding the free induction decay time. The quantum-statistical approach based on the Liouville equation solution for the statistical operator of system is developed for the investigation of echo-processes. The obtained theoretical results for the number of echo signals, time moments of their formation and their intensities are in good agreement with experiments carried out on magnets (ferrites, ferrometals, half metals, manganites). The pointed approach is general and could be applied to EPR and NQR, which is interesting also for its application for remote detection of explosives and narcotics. The application of wide RF pulses and their sequences makes it possible to accumulate weak signals, enriches the echo-response spectrum with clearly separated intensive lines, i.e. essentially increases the sensitivity of apparatus and, correspondingly, the possibilities to explore the fine details of dynamical and relaxation processes taking place in nuclear spin-systems with sufficiently long relaxation times. The work contains also results of new experiments on study of relaxation processes in these systems. 
M.D Zviadadze, G.I. Mamniashvili, T.O. Gegechkori, A.M. Akhalkatsi,   T.A. Gavasheli
cond-mat.other