Interfaces in the confined Ising system with competing surface fields

When a magnetic Ising film is confined in a L×M geometry (L⪡M) short-range competing magnetic fields (h1) are applied at opposite walls along the M-direction, a (weakly rounded) localization–delocalization transition of the interface between domains of different orientation that runs parallel to walls can be observed. This transition is the precursor of a wetting phase transition that occurs in the limit of infinite film thickness (L→∞) at the critical curve Tw(h1). For T<Tw(h1)(T>Tw(h1)) such an interface is bound to (unbound from) the walls, while right at Tw(h1) the interface is freely fluctuating around the center of the film. We present extensive Monte Carlo simulations of Ising stripes in the L×M geometry, in order to describe both the localization–delocalization transition and the properties of the delocalized interface. To this aim, we take advantage of several available theoretical results. We make use of a suitable algorithm to define the local position of the interface along the film, such that its probability distribution can be used to account for the transition itself and the fluctuations in the local position of the interface (capillary waves). After describing the interface localization–delocalization transition, we pay attention to the properties of the delocalized interface with an emphasis on the effects of confinement. We analyze several quantities of interest in terms of the film thickness L. The width of the capillary waves (s) can be related to the width of the magnetization profiles (w) by means of a simple approximation. From this relation we estimate a value for the intrinsic width (w0) of the interface which agrees with the theoretical one. Also the correlation length ξ∥ along the film is considered, and the behavior ξ∥∼L2 compares very well to available exact results. Additionally, the interfacial stiffness βΓ obtained from the Fourier spectrum of the capillary waves reproduces the asymptotic theoretical value.