Diatomic spin-1 molecules with ferromagnetic interactions on the Bethe lattice

The phase transition properties of spin-1 diatomic molecules withferromagnetic (FM) interactions are examined on the Bethe lattice(BL) in terms of the exact recursion relations (ERR). It is assumedthat the spins of each diatomic molecule interact with bilinearinteraction parameter (J>0), while the spins of nearest-neighbour(NN) molecules interact with (J ′ . 0). The system is also underthe influence of the crystal field, which drives the spins to thelowest ground state values as it becomes sufficiently negative.Each site of the BL, i.e. the coordination number, is set to three,with each spin having one + six neighbours, one from its hostmolecule and the six from its NN molecules. To determine thephase diagrams on the (D/J, T/J) and (D/J ′, T/J ′) planes forspecified values of J ′/J and J/J ′, respectively, thermal fluctuationsof magnetizations for each diatomic molecule are examined. Theyare also obtained on the (J ′/J, T/J) and (J/J ′, T/J ′) planes forgiven values of D/J and D/J ′. The stable solutions of the modeldisplay second- and first-order phase transitions and tricritical(TCP) points in agreement with the phase diagrams of usual spin-1 and spin-2 models.