The McCoy-Wu model in the mean-field approximation

We consider a system with randomly layered ferromagnetic bonds (McCoy-Wu model) and study its critical properties in the frame of mean-field theory. In the low-temperature phase there is an average spontaneous magnetization in the system, which vanishes as a power law at the critical point with the critical exponents beta approximate to 3.6 and beta(1) approximate to 4.1 in the bulk and at the surface of the system, respectively. The singularity of the specific heat is characterized by an exponent alpha approximate to -3,1. The samples reduced critical temperature t(c) = T-c(av) - T-c has a power law distribution P(t(c)) similar to t(c)(omega) and we show that the difference between the values of the critical exponents in the pure and in the random system is just omega approximate to 3.1. Above the critical temperature the thermodynamic quantities behave analytically, thus the system does not exhibit Griffiths singularities.