ABSENCE OF PARTIALLY DISORDERED STATES IN THE AXIAL NEXT NEAREST-NEIGHBOR ISING (ANNNI) MODEL

A modified mean field theory is applied to the axial next nearest-neighbor Ising (ANNNI) model, where the model is treated exactly along the direction of competing axial interactions while the mean field approximation is made within the ferromagnetic layer. Particular attention is paid to the partially disordered states and the asymmetric states, which are predicted to be stabilized by the mean field theory if the intralayer interaction is weak. Both numerical and analytical calculations are made to find that neither the partially disordered states nor the asymmetric states are stabilized but only the conventional ordered states are stable.