Study Of Physical Properties On Multilayer And Multilattice Thin Films

Due to the small size and the large specific surface area, the properties of magnetic thin films are higher than that of the traditional bulk materials. Therefore, it is significant to study the surface effects on magnetic thin films. In this paper, the magnetic thin films with honeycomb lattice or square lattice described by the transverse Ising model are investigated within the framework of the effective-field theory with correlations. The influences of the surface exchange coupling and the surface transverse fields on magnetic and thermodynamic properties are studied in detail.With a certain parameter, the magnetization curves for honeycomb lattice represent new types of magnetization which have not been predicted in the Néel theory, namely the magnetization exhibits two compensation temperatures. Film thickness N has a great effect on the saturation magnetization. The saturation magnetization will be zero when N is fixed a certain value. The surface exchange coupling and the surface transverse fields also influence on the magnetization. The initial susceptibility appears singularity phenomenon at the transition temperature T_c and has two extremum values when temperature T<T_c. The surface intra-layer exchange coupling J_s exhibits the critical value J_sc in the phase diagram when the values of N are changed. The transition temperature decreases (increases) with increasing the value of N when J_s < (>) J_sc. On the other hand, the transition temperature increase with increasing of the surface exchange coupling and decrease with increasing of the surface transverse fields. One, two or three compensation points appear in the system within a certain parameter. Internal energy increases rapidly with increasing temperature when the temperature is below the transition temperature and increases slowly when the temperature is higher than Tc. At low temperature, the film thickness N plays an important role for internal energy. When the surface exchange coupling and the surface transverse fields intensify, internal energy decreases. An interesting phenomenon is found in the study of specific heat, i.e. the specific heat exhibits an extremum near the bulk transition temperature T_c^bulk. But this phenomenon is not found when the thickness N=3. The specific heat exhibits a shape peak when J_s=J_sc and shows a shape transition at the transition temperature which is the typical second-order phase transition.In the square lattice, the magnetization curves of multilayer thin films do not exhibit the compensation point when the value of the surface exchange coupling and the surface transverse fields are the same as those of the honeycomb lattice. The behavior of the transition temperature is similar to the honeycomb lattice, but the transition temperature of the square lattice is higher than that of the honeycomb lattice. There is also a critical value J_sc^′in the thin films with the square lattice, which is bigger than that of the honeycomb lattice. Within the range of parameter, one/two compensation points appear, but no three compensation points in this system. The temperature dependence of internal energy and specific heat are analogous to honeycomb lattice, although the temperature is higher than the honeycomb lattice at which the curves of specific heat appear singularity phenomenon.