The effect of finite size of the system on correlation length behaviour at the presence of external field

The explicit analytical expression for the effective correlation length xi as a function of reduced temperature tau, external field h and system size L for the Ising-like system near the phase transition point T-c. is obtained. The role of these quantities in the formation of the correlation length value is ascertained. It is shown that the irregular increase of the correlation length exists only in the case of tau -> 0, h -> 0 and L -> infinity. With deviation from these values the essential slowing down exists for the increasing xi. The criterium of the permissible range of temperature values (field values), where the correlation length behaviour is defined by temperature (or field) variable, is established for the fixed size of the system. Beyond this range tau < tau(c), h < h(cr), the system size becomes crucial for forming the correlation length.