The correlation length of 3D Ising systems in the presence of an external field

The explicit dependence of correlation length as function of temperature and external field is found for the Ising model on a simple cubic lattice near the phase transition point. xi = xi(0)((h) over tilde (2)+h(c)(2))(-1/(d+2)), where (h) over tilde and h(c) are the renormalized values of the external field h and reduced temperature tau. The critical amplitude xi(0) is proportional to the lattice constant through the coefficient, which is determined by the parameter of the interaction potential. The size of the critical region is established by the temperature and field. It is shown that the essential increasing of the system correlation length takes place at tau < tau* (tau* similar to 10(-2)) with h -> 0 and at h' < h* (h* similar to 10(-4), h' = beta h) with tau -> 0.