Monte Carlo study of the spatially modulated phase in an Ising model

The spatially modulated or "sinusoidal" phase which occurs in a three-dimensional spin-{} Ising model with competing ferromagnetic and antiferromagnetic interactions between adjacent and next-nearest xy layers is studied by Monte Carlo methods. The equilibrium wave vector, $q(T)$, changes, possibly with jumps, from q=π2a at $T=0$, to its near-critical value: for -J₂J₁=0.6, the change occurs rapidly in the interval 0.8 Tc to 0.9 Tc. The magnetization wave form appears to display distinct, although small, third harmonic components up to Tc.