  The three-dimensional axial-next-nearest-neighbor Ising (ANNNI) model is
studied by a modified tensor product variational approach (TPVA). A global
phase diagram is constructed with numerous commensurate and incommensurate
magnetic structures. The devil's stairs behavior for the model is confirmed.
The wavelength of the spin modulated phases increases to infinity at the
boundary with the ferromagnetic phase. Widths of the commensurate phases are
considerably narrower than those calculated by mean-field approximations.
