CONICET | Buscador de Institutos y Recursos Humanos

Fixed vacancies (non-magnetic impurities) are placed along the center of Ising strips in order to study the wetting behaviour in this confined system, by means of numerical simulations analysed with the aid of finite size scaling and thermodynamic integration methods.By considering strips of size L × M (L much minor M ) where short-range competitive surface fields (Hs) act along the M-direction, we observe localisation-delocalisation transitions of the interface between magnetic domains of different orientation (driven by the corresponding surface fields), which are the precursors of the wetting transitions that occur in the thermodynamiclimit. By placing vacancies or equivalently non-magnetic impurities along the center of the sample, we found that for low vacancy densities the wetting transitions are of second order, while by increasing the concentration of vacancies the transitions become of first order.Second- and first-order lines meet in tricritical wetting points (H_sw^tric , T_w^tric ), where H_sw^tric and T_w^tricare the magnitude of the surface field and the temperature, respectively. In the phase diagram, tricritical points shift from the high temperature and weak surface field regime at large vacancy densities to the T tending 0, H_sw^tric tending 1 limit for low vacancy densities. By comparing the locations of the tricritical points with those corresponding to the case of mobile impurities, we conclude that in order to observe similar effects, in the latter the required density ofimpurities is much smaller (e.g. by a factor 3-5). Furthermore, a proper density of non magnetic impurities placed along the center of a strip can effectively pin rather flat magnetic interfaces for suitable values of the competing surface fields and temperature.