Reentrance Phenomenon of a mixed spin Ising model

. A reentrant magnetic phase transition in molecular based magnet

, is investigated by the use of Mean-Field Theory(MFT) for a mixed spin-2 and spin-5/2 Ising system on a simple cubic lattice. It is found that a double-entrant ferrimagnetic behaviour in the mixed-spin system depends on the negative values of the anisotropies in the order-disorder system for the two sublattices of the mixture. It is shown using numerical calculations that this phenomenon is strongly affected by the magnetic anisotropy and lattice disorder.

INTRODUCTION
Recently, a lot of efforts has been directed to the ferrimagnetic materials, specially due to their great potential for technological applications [1]. Ferrimagnets are materials where ions on different sublattices have opposing magnetic moments and show paramagnetic behaviour above transition temperature ( c T ). J.Oitmaa et al introduced a mixed-spin Ising model on the simple cubic lattice which showed reentrant behaviours [2]. Other researchers in this respect observed that the effect of next-nearest-neighbour interaction between the spin sites on the lattice possibly leads to a reentrant transition as well the single-ion anisotropy may cause a reentrant one [3,4,5].  [6]. It is worth to note that the origin of reentrant phenomenon is competition between two opposites: ferromagnetic and antiferromagnetic interactions present in materials. Here, it has been studied a mixed-spin Ising ferrimagnetic system on a simple cubic lattice in which the two mixing sublattices have spins two ( 2 , 1 , 0   ) and spins five-half( ). In this work we have studied, within a mean-field approximation, the reentrant phenomenon of the model. We found that the system considered exhibits a double entrant behaviour at low temperatures.

THE MODEL
It has been studied a 3-D Ising model with spin 2 , are the anisotropies acting on the spins i  and j  respectively. J is the nearest neighbour exchange parameter. Within the framework of the mean-field method, it is necessary to evaluate the following expectation values for the considered spin problems in this work, that : The free energy of the system is obtained from a mean field calculation of the system Hamiltonian based on the Bogoliubov inequality [5,7,8,9] : where F is the free energy of H given by relation (1), that  The approximated free energy of the system considered is obtained by minimizing the right side of equation (4) ,and z(z=6) its nearest-neighbor coordination number of a simple cubic lattice.

RESULTS AND DISCUSSIONS
First, it has been evaluated the approximated magnetic moments of a mixed spin-2 and spin-5/2 on a simple cubic lattice with different single-ion anisotropies. The numerical results are presented in Figs.((1),(2)  The sublattice magnetization curves labeled from -3.98 to -3.90 are for the values of J D A / corresponding to the critical points. Fig.(1) and Fig.(2) show the sublattice magnetization curves at low temperature for anisotropy J D A / corresponding to the reentrant behaviour as in Fig.(3).

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ILCPA Volume 56  2)). At very low temperature the system will be disordered, since 0  A D favours that 0  S at A sites. As the temperature increases the system orders at a lower critical temperature, moves to an ordered phase passing through another two critical points or another critical one near 99 respectively, then disordered again at the transition temperature( C T ) [2,3,5]. Note that such a reentrant behaviour with three consecutive critical points has not been reported in the analogous mixed-spin system on the honeycomb lattice [10].These transitions of this kind found in the exact treatment of a mixed spin-1/2 and spin-1 Ising model on the diced lattice [3]. It is noteworthy that the results obtained can provide the analytical background for further investigations on the effects of crystal field and boundary conditions on the critical properties of the Ising model.

4.CONCLUSION
We have applied MFA to the study of a mixed spin-2 and spin-5/2 Ising model on a simple cubic lattice modeled by Eq. (1)  . In particular, we have studied the effect of a single-ion anisotropy on the existence of the reentrant ferrimagnetism phenomenon. We have shown that the system undergoes a double reentrant behaviour in the phase diagrams as shown in Fig.(1).It is also interesting to remark that the first-order transition lines always exhibit a reentrant International Letters of Chemistry, Physics and Astronomy Vol. 56 behaviour at low temperature as shown in Fig.(2), i.e., two phase transitions [2,5]. These behaviors started from the disordered ground state before entering the ordered phase at lower critical temperature. Thus the ferrimagnetic long-range order disappears due to the thermal fluctuations at the upper critical temperature C T . We hope that this work could stimulate experimental and theoretical works on the system which is considered.