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INTRODUCTION.Investigation of vortex state stimulated dynamics in high-temperature superconductors (HTSC) is one of the most important problems from the point of view of practical applications in the new generation of electronic devices operating on the basis of HTSC materials, as well as, from the point of view of fundamental problems connected with hightemperature superconductivity [1]. It is clear that vortex states in superconductors are complicated and multiform, but they are exceptionally important for understanding the behavior of high-temperature superconductors in electromagnetic fields and under the current loads [2]. The critical temperature T c of the superconducting transition of HTSC cuprates is so high that they remain superconductive at temperatures, at which the thermal fluctuations play a noticeable role, as their energy become comparable with the elastic energy of vortices and also with the pinning energy [3]. It creates prerequisites for phase transitions [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. In the type-II HTSC superconductors the number of vortices, "particles" in the vortex matter may be changed in a very wide range, several orders of magnitude by varying the magnetic field, B. As the interaction between the vortices is changed, thermal fluctuations in the HTSC vortex matter can be observed in a very wide temperature interval, on the B-T phase diagram. This gives a good possibility to study the disordered media -one of the central problems of condensed state physics. The investigation of vortex matter dynamics is of particularly topical interest in the case of HTSCs, which, because of their high critical temperatures and layered structure are characterized by a much higher mobility of the vortex lattice, as compared with the ordinary type-II superconductors, which is hampered by pinning connected with various defects of the crystal structure. These defects are the pinning centers that prevent the flow of magnetic flux and the energy dissipation, accompanying it. In the strongly anisotropic high-temperature superconductors, as it was shown in recent works [16][17][18], the critical current at the transition of 3D vortices in the quasi-two-dimensional 2D vortices can be sharply increased, making such anisotropic high-temperature superconductors, e.g. BiPbSrCaCuO system, useful for technical applications. Furthermore, the upper critical field H c2 , at which the superconductivity in these materials is

The results of a NMR study of the nature of main multiple nuclear spin-echo components in lithium ferrite are presented. It is shown that the strongly relaxing echo component in lithium ferrite is formed by the stimulated mechanism similar to the one observed in this material earlier for a single-pulse echo.