Abstract
A formal theory of nonlinear response which is a generalization of Kubo's formalism is developed on the particular example of an isolated paramagnetic spin system. After resummation of the whole perturbation expansion an exact formal equation for the mean magnetic moment is derived. The relation of the type χ is = χ s is essentially used in the derivation. This equation contains correlation functions of all orders and it is capable to describe nonequilibrium behaviour of the system which is driven arbitrary fast and far from initial equilibrium. Four special cases of nonlinear response are considered: the sudden and adiabatic approximations, the long time generalization of the Kubo formula and the kinetic regime of nonlinear response. In the latter case the expression for nonlinear response in terms of the linear one is obtained which is valid even though the external time-dependent field is not small. This expression can be verified experimentally. The concept of spin temperature and its generalization is also discussed in some detail.
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