SPIN-WAVE THEORY OF 2-DIMENSIONAL FERROMAGNETS IN THE PRESENCE OF DIPOLAR INTERACTIONS AND MAGNETOCRYSTALLINE ANISOTROPY

A spin-wave theory of two-dimensional ferromagnets taking into account both the long-range dipolar interactions and a uniaxial magnetocrystalline anisotropy is presented. We show that in the case of the perpendicular easy axis the stabilization of long-range ferromagnetic order at finite temperature is due mainly to the anisotropy-induced gap at the bottom of the spin-wave spectrum, whereas the dipolar interactions play a negligible role; on the other hand, in the case of easy-plane anisotropy, the spin-wave spectrum remains gapless, so that the stabilization of long-range ferromagnetic order is due in turn to the long-range character of the dipolar interactions. The temperature dependence of the magnetization is calculated in both cases. The results are discussed in connection with experimental data reported for fcc (001) cobalt monolayers epitaxially grown onto (001) copper single crystals.