Article
Second to firstorder transition in two coupled antiferromagnetic rings
Tongji University, Shanghai, Shanghai Shi, China
Physics of Condensed Matter (Impact Factor: 1.35). 05/2006; 51(4):473476. DOI: 10.1140/epjb/e2006002551 ABSTRACT
We numerically investigate an S=1/2 spin model, in which two
dimerized antiferromagnetic rings interact with each other
ferromagnetically. It is shown that the order of the magnetoelastic
transition is strongly affected by the interring coupling J⊥ and
there may exist a critical J⊥* dividing the firstorder
transition and the continuous transition.
dimerized antiferromagnetic rings interact with each other
ferromagnetically. It is shown that the order of the magnetoelastic
transition is strongly affected by the interring coupling J⊥ and
there may exist a critical J⊥* dividing the firstorder
transition and the continuous transition.

 "El estudio de sistemas de Ising mixtos, con espines de valores altos, sigue siendo un tema relevante debido a que son atractivos por su rica variedad de fenómenos multicríticos (Albayrak e Yigit, 2007; Deviren et al., 2009b). Dentro de dichos sistemas, encontramos el modelo de espines mixtos (S = 3/2, σ = 5/2), que es uno de los menos estudiados (Li et al., 2006; Yessoufou et al., 2009), por ello resulta interesante analizarlo. Además de la importancia del modelo, con respecto al estudio y caracterización de magnetos moleculares (Albayrak e Yigit, 2007; Zhang et al., 2005), también parece que es responsable de las propiedades magnéticas de ciertas proteínas "
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ABSTRACT: Using Monte Carlo simulations, the magnetic properties of a mixed Ising ferrimagnetic model with spins S = ±3/2, ±1/2 y σ = ±5/2, ±3/2, ±1/2 distributed on a square lattice with different anisotropies was analyzed. It was assumed that the exchange interaction to nearest neighbors, J1, between spins S and σ, is antiferromagnetic (J1 < 0). Also, it was considered that the effect of the intensities of the singleion anisotropies, due to the crystalline fields of the sublattices S and σ, Ds and Dj respectively. The existence and dependence of the compensation temperature in the model with respect to the singleion anisotropies was also studied. By fixing the parameter Ds and varying the intensity of Dj it probable phase transitions of first order appear. The analysis of the critical temperatures is obtained through the maximum of the specific heat of the system. Phase diagrams at finite temperatures are obtained in the temperatureanisotropy plane.  [Show abstract] [Hide abstract]
ABSTRACT: We use exact recursion relations to study the magnetic properties of the halfinteger mixed spin5/2 and spin3/2 BlumeCapel Ising ferromagnetic system on the twofold Cayley tree that consists of two sublattices A and B. Two positive crystalfield interactions Δ1 and Δ2 are considered for the sublattice with spin5/2 and spin3/2 respectively. For different coordination numbers q of the Cayley tree sites, the phase diagrams of the model are presented with a special emphasis on the case q = 3, since other values of q reproduce similar results. First, the T = 0 phase diagram is illustrated in the (D A = Δ1/J,D B = Δ2/J) plane of reduced crystalfield interactions. This diagram shows triple points and coexistence lines between thermodynamically stable phases. Secondly, the thermal variation of the magnetization belonging to each sublattice for some coordination numbers q are investigated as well as the Helmoltz free energy of the system. Firstorder and secondorder phase transitions are found. The secondorder phase transitions become sharper and sharper when D A or D B increases. The firstorder transitions only exist for some appropriate nonzero values of D A and/or D B. The corresponding transition lines never connect to the secondorder transition lines. Thus, the nonexistence of tricritical points remains one of the key features of the present model. The magnetic exponent β 0 of the model is estimated and found to be ¼ at small values of D A = D B = D and β 0 = ½ at large values of D. At intermediate values of D, there is a crossover region where the magnetic exponent displays interesting behaviours. 
Article: Magnetoelastic Instability in RingShaped Molecule Magnets with NextNearest Neighbor Coupling
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ABSTRACT: In considering nextnearest neighbor (NNN) coupling, we numerically investigate the magnetoelastic instability in ringshaped mesoscopic antiferromagnetic Heisenberg spin 1/2 systems with spinphonon interaction. The results indicate that, for antiferromagnetic NNN coupling J2, there may be a critical value Jc2, at which the ground state is dimerized for arbitrary lattice spring constant and beyond and below which the magnetoelastic instability behavior is different from each other. The values of Jc2 are irrelevant to the system size. For ferromagnetic NNN coupling, only continuous transition is present from dimerized phase to uniform phase as lattice spring constant is increased.
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