MAGNETIC ORDER OF 2-DIMENSIONAL ISOTROPIC DIPOLAR ANTIFERROMAGNETS

The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets on a square lattice is studied. The spin-wave dispersion is evaluated and it is shown that the dipolar interaction lifts the degeneracy of the two magnon branches. In addition the dipolar interaction leads to a I:ap in the spin-wave energy and a nonvanishing order parameter. The resulting finite Neel temperature is calculated for a square lattice by means of linear spin-wave theory and a high-temperature expansion via the method of Callen. The transition temperatures are in good agreement with measured values on the quasi two-dimensional antiferromagnets K2MnF4, Rb2MnF4, Rb2MnCl4 and (CH3NH3)(2)MnCl4. In addition we studied the effect of an external magnetic field on the ground state and observed an intermediate state.