Here we present results of our investigation of coherent effects in polarized spin systems on the basis of a microscopic model but not using the phenomenological Bloch equations. A computer simulation is realized for a system of nuclear or electron spins interacting with realistic dipole-dipole forces. Different initial and external conditions are analyzed for a spin system coupled with a resonator or one without the resonator and in the presence of an external oscillating field or without this pumping. Insofar as the phenomenological equations presuppose the uniformity of a system, it requires a microscopic model to show accurately when the coherence does appear. To this end, we introduce the coherence coefficients and consider their time behavior in detail. Some peculiarities due to dipole spin interactions, which are not present in the Bloch equations, are noted.

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