Critical Temperature of Site-Diluted Spin-1/2 Systems with Long-Range Ferromagnetic Interactions

In the paper the pair approximation (PA) method for studies of the site-diluted spin-1/2 systems of arbitrary dimensionality with the long-range ferromagnetic interactions is adopted. The method allows to take into account arbitrary anisotropy of the interactions in the spin space, so it is not limited to purely Ising couplings. Within this approach, the Gibbs free energy is obtained, which allows to derive all the further interesting thermodynamic properties. In particular, we obtain an equation for the critical temperature of the second-order phase transitions for the model in question. In the study we focus our attention on the systems with ferromagnetic interactions decaying with the distance according to the power law J(r) proportional to r(-n). We discuss the dependence of the critical temperature on the concentration of magnetic component and the index n for selected one-, two-, and three-dimensional lattices. We confirm the absence of the critical concentration for a diluted magnet with infinite interaction range. In the regime of the low concentrations of magnetic component, we find a non-linear increase of the critical temperature with the concentration in the form of T-c proportional to p(n/d), depending on the system dimensionality d and the index n, whereas n > d.