  We propose a microscopic mechanism of the long-range order in two-dimensional
space induced by random local fields of crystal defects. The anisotropic
distribution of defect-induced random local field directions in the
n-dimensional space of vector order parameter with the O(n) symmetry is shown
to give rise to the defect-induced effective anisotropy in the system. The
expression for the effective anisotropy constant is found. A weak anisotropy of
the "easy axis" type transforms the X-Y model and the Heisenberg model to the
class of Ising models, and brings into existence the long-range order in the
system.
