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The phase diagrams and magnetization curves of a two-sublattice Ising metamagnet at finite temperature with longitudinal crystal field H are investigated by the use of an effective-field theory (EFT) with correlations. In addition to the second-order transition lines, the first-order transition lines are also presented, since a method to calculate the Gibbs free energy numerically at finite temperature within EFT is found in this work. The results show that there is no fourth-order critical point or reentrant phenomenon in the phase diagrams given by using EFT as found by using mean-field theory (MFT). 相似文献
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Jiajia Geng 《Journal of magnetism and magnetic materials》2009,321(13):1964-1970
The phase diagrams of a two-sublattice Ising metamagnet at finite temperature in a mixed longitudinal field and a transverse magnetic field are investigated by the use of an effective-field theory (EFT) with correlations. In addition to the second-order transition lines, the first-order transition lines are also presented in the phase diagrams, since the Gibbs free energy can be calculated numerically. The results show that there is no fourth-order critical line in the phase diagrams given by using EFT as found by using mean-field theory (MFT). The tricritical lines and their projection in the t−hx plane obtained by using EFT are also quite different from those by using MFT. Only one type of phase diagram is obtained by using EFT while three kinds of phase diagrams are obtained by using MFT, which indicates that only the first kind of phase diagrams obtained by using MFT is reliable. Furthermore, it is shown that the region of first-order transitions increases as the transverse magnetic field hx decreases. 相似文献
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The ground state magnetic properties of a two-sublattice Ising metamegnet in both external longitudinal and transverse fields are studied within the mean-field approach. A parameter α=(Z1J1+Z2J2)/(Z1J1−Z2J2) which reflects the strength ratio of spin coupling in the plane and in adjacent planes is introduced. The ground state energy, the longitudinal staggered magnetization, the longitudinal total magnetization and the transverse total magnetization are calculated. The ground state phase diagrams in Ω−h and Ω−α plane are presented. The results show that when Ω is given, the longitudinal critical magnetic field increases when α decreases; the phase transition changes always from first order to second order with increase in the longitudinal magnetic field h or decrease in α. The reentrant phenomenon occurs in the range α?−0.66, Ω?0.21, h?0.78. 相似文献
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The ground-state magnetic properties of a two-sublattice Ising metamagnet in a mixed longitudinal and transverse magnetic field are studied within the effective-field theory. A parameter j2=J2/J1 is introduced, which reflects the strength ratio of spin coupling between adjacent planes and in each plane. In addition to the second-order transition lines, the first-order transition lines are also presented, since the ground-state energy can be calculated numerically. The ground-state phase diagrams in hx–hz are presented. The results show that when j2<0 the phase transition of the system is always first-order for hx<2.751, and when −1000?j2<0 it is always second-order for hx>4.36. For the given hx (0<hx<14.71), the longitudinal critical magnetic field increases as j2 decreases. The reentrant phenomenon occurs in the range of j2<−11.89, hx>14.71. There is no fourth-order critical point in the phase diagrams given by using EFT as found by using mean field theory (MFT). 相似文献
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Ricardo L. Silva 《Solid State Communications》2005,134(5):313-317
We study static vacancies on a ferromagnetic spin-1/2 chain described by the transverse Ising model with second neighbor interactions at zero temperature. Using exact diagonalization techniques and applying a finite-size scaling approach, it is found that a strong magnetic field induces an effective potential of interaction between two vacancies that is attractive. 相似文献
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The anisotropy of two successive transitions of the spin-ladder compound LiCu2O2 is studied by the specific heat ( C) under magnetic fields with H / / c and H / / ab (written as H^c and H^ab in the following) up to 14 T. The peak of specific heat at 24.5 K in zero field shifts to lower temperature when the field is increased and the magnitude of the peak is suppressed by the field. On the contrary, the peak of 22.5 K shifts to higher temperature, especially at 14 T. Its magnitude increases in the field of H^c, whereas it decreases in the field of H^ab. We calculate the entropy change between 21 K and 63 K. The different influence of the spin ordering by fields of different direction is obtained. Our experimental results suggest a mixed state between the long range incommensurate helimagnetic ground state and the higher-T short-range dimer liquid state. The temperature range of mixed state is shrunk with the increasing field. Possible mechanism is discussed. 相似文献
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The phase diagrams of the spin-1 transverse Ising model with the presence of a crystal field is investigated by using an effective-field theory (EFT). We give a method to calculate the Gibbs free energy numerically at finite temperature within the EFT. The first-order transition lines are obtained by comparing the Gibbs free energy. The phase diagrams and the Gibbs free energy are also compared with those given using the mean-field theory (MFT). 相似文献
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P. F. Li Y. G. Chen H. Chen 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(4):473-476
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 first-order
transition and the continuous transition. 相似文献
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The spin current in the one-dimensional quantum XXZ spin chain is studied based on the exact solutions. It is found that the spin voltage controlled by the unparallel boundary magnetic fields can induce the pure longitudinal spin current in the system. By using Wiener–Hopf and generalized algebraic Bethe ansatz methods, the analytic expressions for the spin current and the spin conductance are obtained. The spin current is proportional to the inverse of the length of the spin chain. The magnitude of spin current can be manipulated by the strength and the twist angle of two boundary magnetic fields. The exact analysis also shows that there exist an Ohm law or London equation type relation between the spin current and the spin conductance. 相似文献
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The electronic structure and magnetic states for hexagonal-MnFeAs have been studied by a first-principles density functional theory (DFT) calculation. The ground state is ferromagnetic and the calculated magnetic moments for Fe and Mn are 1.1 and 3.1μB, respectively, leading to a total magnetization of 4.1μB per formula unit due to the small negative moments of As atoms. The exchange interaction between Fe and Mn layers () is positive and tends to form the ferromagnetic ordering. On the other hand, the exchange interaction at the Fe-As1 layer () is negative while that at the Mn-As2 layer () is positive. The field induced first order magnetic transition at TC is related to the competed exchange interaction in the compound. 相似文献
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E. Vallejo 《Journal of magnetism and magnetic materials》2009,321(6):640-643
The effect of the interplay between magnetism, charge ordering and lattice distortion within a like double and super-exchange model is studied in low-dimensional systems. An important magnetoelastic effect that leads to a lattice contraction is presented in conjunction with an analytical minimization for a three-site one-dimensional model. The model is discussed in connection with the magnetism, charge ordering and the contraction of the rungs experimentally observed within the three-leg ladders (3LL) present in the oxyborate Fe3O2BO3. 相似文献
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The phase diagram of mixed spin-3/2 and spin-2 Ising ferromagnetic model with different single-ion anisotropies is investigated by the use of a mean-field theory based on the Bogoliubov inequality for the Gibbs free energy. Global phase diagrams are obtained in the temperature-anisotropy plane. In particular, by changing values of the single-ion anisotropies, several different types of phase diagrams of first-order transition between two ordered phases, are studied in detail. A variety of multicritical points such as tricritical points, isolated critical points, and triple points are obtained. 相似文献
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A formalism is proposed to study the electronic and transport properties of graphene sheets with corrugations as the one recently synthesized. The formalism is based on coupling the Dirac equation that models the low energy electronic excitations of clean flat graphene samples to a curved space. A cosmic string analogy allows to treat an arbitrary number of topological defects located at arbitrary positions on the graphene plane. The usual defects that will always be present in any graphene sample as pentagon–heptagon pairs and Stone–Wales defects are studied as an example. The local density of states around the defects acquires characteristic modulations that could be observed in scanning tunnel and transmission electron microscopy. 相似文献
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S. Mahdavifar M. R. Soltani A. A. Masoudi 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(2):215-220
We have investigated the ground state phase diagram of the 1D AF spin-
Heisenberg model with the staggered Dzyaloshinskii-Moriya (DM) interaction in an external uniform magnetic field H. We have
used the exact diagonalization technique. In the absence of the uniform magnetic field (H=0), we have shown that the DM interaction
induces a staggered chiral phase. The staggered chiral phase remains stable even in the presence of the uniform magnetic field.
We have identified that the ground state phase diagram consists of four Luttinger liquid, staggered chiral, spin-flop, and
ferromagnetic phases. 相似文献
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S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):371-376
The energy gap of the 1D AF-Heisenberg model in the presence of both uniform (H) and staggered (h) magnetic fields is investigated
using the exact
diagonalization technique. The opening of the gap in the presence of a
staggered field is found to scale with hν, where ν=ν(H) is the
critical exponent, and depends on the uniform field. With respect to the range
of the staggered magnetic field, two regimes are identified through which the
dependence of the real critical exponent ν(H) on H can be calculated
numerically. Our numerical results are in good agreement with the results
obtained by other theoretical approaches. 相似文献
19.
A.V. Syromyatnikov 《Journal of magnetism and magnetic materials》2009,321(7):928-930
We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg ferromagnet (FM) with dipolar forces at TC?T?0 using 1/S expansion. A comprehensive analysis is carried out of the first 1/S corrections to the spin-wave spectrum. In particular, we obtain that the spin-wave interaction leads to the gap in the spectrum εk renormalizing greatly the bare gapless spectrum at small momenta k. Expressions for the spin-wave damping Γk are derived self-consistently and it is concluded that magnons are well-defined quasi-particles in both quantum and classical 2D FMs at small T. We observe thermal enhancement of both Γk and Γk/εk at small momenta. In particular, a peak appears in Γk and Γk/εk at small k and at any given direction of k. If S∼1 the height of the peak in Γk/εk is not larger than a value proportional to T/D?1, where D is the spin-wave stiffness. In the case of large spins S?1 the peak in Γk/εk cannot be greater than that of the classical 2D FM found at k=0 whose height is small only numerically: Γ0/ε0≈0.16 for the simple square lattice. 相似文献
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G. M. Rocha-Filho A. S.T. Pires M. E. Gouvêa 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(1):7-14
We study the one-dimensional isotropic spin-1 Heisenberg
magnet with antiferromagnetic nearest-neighbor (nn) and
next-nearest-neighbor (nnn) interactions by using the modified spin
wave theory (MSWT). The ground state energy and the singlet-triplet
energy gap are obtained for several values of j, defined as the
ratio of the nnn interaction constant to the nn one. We also compare
two different ways of implementing the MSWT currently found in the
literature, and show that, despite the remarkable differences between
the equations to be solved in each procedure, the results given by
both are equivalent, except for the predicted value of the
jmax, the maximum value of j accessible in each treatment.
Here, we suggest that jmax is related to the disorder point of
the first kind. Our results show that the ground state and the gap
energies increase with j, for j ≤jmax, in accordance to
previous numerical results. 相似文献