Equilibrium dynamics of a paramagnetic cluster

Abstract
The transverse autocorrelation function at an arbitrary temperature is calculated rigorously for a system with an arbitrary number of quantum spins, each of which is coupled to all the other spins with equal exchange. It is shown that the cluster approximation, formed by a model for a real magnet, explains short- and intermediate-time (up to the time to reach the spin-diffusion regime) experimental measurements in the paramagnetic phase.