We consider a Heisenberg model with ferromagnetic nearest‐neighbor and competing further‐neighbor exchange interactions in a small applied magnetic field at low temperature T. As a function of the exchange constants, the modulation vector is shown to have devil’s staircase behavior. We consider the effects of nonzero temperature and quantum effects. We find a special modulation wave vector at which the incommensurability energy vanishes for the classical system at T=0.