Magnetic Phase Transitions in Ultrathin Films of Different Crystal Structures

The properties of ultrathin films have been studied within the framework of Ising model and the method of random-field interactions. It is shown that the Curie temperature is inversely proportional to the number of layers. Critical exponent nu has been obtained and it is shown that it does not depend on the type of crystalline lattice. As a characteristic, that allows to evaluate the smallness of magnetic objects, one can take the length of the spin-spin correlation xi. It is used to describe the spin fluctuations near the critical temperature. In the case when the film thickness is less than the correlation length of bulk samples, there appear various size effects. In particular, the decrease of film thickness can lead to a reduction of phase transition temperature: according to measurements on films of iron, nickel and cobalt [1-8], T-c, decreases with decreasing of film thickness N (here N is the number of monolayers). The experimentally observed dependence of the Curie temperature on the thickness T-c(N) can be described by the relation introduced in [6-7]: epsilon(N) =(def) T-c(N ->infinity) - T-c(N)/T-c(N ->infinity) = c(0)(N - (N) over tilde)(-lambda) where T-c(N -> infinity) =(def) T-infinity - temperature of bulk sample, c(0) and (N) over tilde - constants. The argument lambda is related to the critical exponent of the spin-spin correlation v as follows: lambda = 1/v [9]. And, as was shown in [10-12] the constants and argument of the dependency (1) essentially depend on the type and symmetry of the crystal lattice of sprayed film and its substrate. In this paper we assess the influence of film thickness and its crystal structure on the temperature of magnetic phase transition. To solve this problem we use the following model.