  The energy (magnetostatic, exchange and Zeeman terms) of a square array of
cylindrical sub-micron dots made of soft ferromagnetic material is calculated
analytically and minimized, taking into account quasi-uniformity of dots
magnetization. The dependence of the equilibrium energy of the array on the
direction of the externally applied magnetic field in the array plane is
recovered, exhibiting the four-fold anisotropy. The anisotropy constant is
calculated. Its values for different array geometries are in excellent
agreement to the recent independent experiments. New eight-fold anisotropy
effect is predicted. Theory involves no adjustable parameters.
