Article

# Second- to first-order transition in two coupled antiferromagnetic rings

Physics of Condensed Matter (Impact Factor: 1.28). 05/2006; 51(4):473-476. DOI: 10.1140/epjb/e2006-00255-1

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**ABSTRACT:**We use exact recursion relations to study the magnetic properties of the half-integer mixed spin-5/2 and spin-3/2 Blume-Capel Ising ferromagnetic system on the two-fold Cayley tree that consists of two sublattices A and B. Two positive crystal-field interactions Δ1 and Δ2 are considered for the sublattice with spin-5/2 and spin-3/2 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 crystal-field 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. First-order and second-order phase transitions are found. The second-order phase transitions become sharper and sharper when D A or D B increases. The first-order transitions only exist for some appropriate non-zero values of D A and/or D B . The corresponding transition lines never connect to the second-order transition lines. Thus, the non-existence 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. Keywordsmixed spin model–two-fold cayley tree–recursion relations–magnetization–phase transitionCentral European Journal of Physics 01/2009; 7(3):555-567. · 0.91 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**This paper numerically investigates the magnetoelastic instability in the S = 1/2 XXZ rings containing finite spins N with antiferromagnetic nearest-neighbour (NN) and next-nearest neighbour (NNN) coupling. It finds that, as the NN anisotropy Δ1 equals the NNN anisotropy Δ2, there exists a critical NNN coupling strength Jc2 (≈ 0.5), at which the systems always locate in dimerized phase for arbitrary large spring constant. As Δ1 ≠ Δ2, the values of Jc2 are dependent on N and the difference of (Δ1 – Δ2).Chinese Physics B 02/2010; 19(2):027503. · 1.15 Impact Factor -
##### Article: Magnetoelastic Instability in Ring-Shaped Molecule Magnets with Next-Nearest Neighbor Coupling

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**ABSTRACT:**In considering next-nearest neighbor (NNN) coupling, we numerically investigate the magnetoelastic instability in ring-shaped mesoscopic antiferromagnetic Heisenberg spin 1/2 systems with spin-phonon 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.Communications in Theoretical Physics 12/2009; 52(6):1125. · 0.95 Impact Factor

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