INTERNATIONAL JOURNAL OF MODERN PHYSICS B, vol.22, no.27, pp.4877-4898, 2008 (SCI-Expanded)
The bilayer spin-2 Ising model on the Bethe lattice is investigated by taking into account the intralayer coupling constants of the two layers J(1) and J(2), interlayer coupling constant between the layers J(3) and crystal field interaction Delta by using the exact recursion equations in a pairwise approach. The ground state (GS) phase diagrams of the model are obtained on the (J(2)/vertical bar J(1)vertical bar, J(3)/q vertical bar J(1)vertical bar) planes for given Delta values and on the (Delta/qJ, J(3)/qJ) plane when J(1) = J(2) = J, and thus 33 distinct GS configurations are found. The temperature-dependent phase diagrams are obtained for J(1) > 0, J(2) > 0, and for J(3) > 0 or J(3) < 0 on the (kT/J(1), J(3)/J(1)) planes for given Delta/qJ(1) and J(2)/J(1) and on the (Delta/J, kT/J) plane for given J(3)/J when J(1) = J(2) = J for the coordination number q = 3. It was found that the system exhibits both first- and second-order phase transitions and tricritical points. The paramagnetic phases are also classified by studying the thermal variations of the quadrupolar moments.