Field effects on the magnetic properties of three-layer films

We study by extensive Monte Carlo (MC) simulations and Green function (GF) method some properties of a system composed of three thin ferromagnetic (FM) films coupled to each other by an antiferromagnetic (AF) interface exchange interaction in the presence of a magnetic field H perpendicular to the film surfaces. The films have a triangular lattice structure with Ising spins. In the simplest case, where all interactions except the interface ones are FM and taken to be equal to J=1, and the interface interactions between the second (middle) film and its neighboring first and third films, J12 and J23, are equal to -1, the (T,H)-phase diagram obtained by MC simulations shows several interesting features. While for H smaller than the critical value Ht there observed a second-order phase transition at a finite T from the AF ordering to paramagnetic phase, for H>Ht the transition becomes of first-order. The field Ht depends on the interface interactions and the film thickness. The hysteresis cycle measured in the transition region by field-heating and field-cooling shows a two-step cycle which may have important practical applications in magnetic recording. An analytical calculation by the GF method using an Ising-like model has been performed for the same system. The obtained self-consistent layer magnetizations are in excellent agreement with the MC ones