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1.
The ground-state magnetic properties of the spin-2 transverse Ising model with a longitudinal crystal field are studied within the framework of mean-field theory (MFT) and effective-field theory (EFT), respectively. The phase diagrams and magnetization curves are examined in detail. It is found that the system exhibits a tricritical behavior in the ground-state phase diagrams. Some interesting phenomena have been found, especially the first-order phase transition from one ordered phase to the other ordered phase, which is due to the high spin. The spin correlation has important effect on the magnetic properties of the system. We also find that the ground-state phase diagrams of the spin-2 transverse Ising model are very different from those of the spin-3/2 transverse Ising model. 相似文献
2.
The Blume–Emery–Griffiths model, a spin-1 Ising model with bilinear, biquadratic, and crystal field interactions, provides a general system for the analysis of systems driven by fluctuations in density and magnetization. In this study, we consider an exactly solvable system in which frustration is present due to competing biquadratic and crystal-field interactions. Thus, this calculation models a dilute ferromagnetic material with two types of nearest-neighbor site pairs, distinguished by whether or not simultaneous occupation is energetically favored. To determine the effects of this competition, we have constructed exactly solvable frustrated hierarchical models similar to those introduced to study spin glasses. The resulting phase diagrams reveal two distinct paramagnetic phases separated by a plane in parameter space in which the biquadratic interaction and crystal-field strength rescale chaotically. Each paramagnetic phase has a ferromagnetic complement in which the unique distribution of occupied sites possesses a net magnetization. 相似文献
3.
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
4.
Daniel P. Snowman 《Journal of magnetism and magnetic materials》2009,321(19):3007-3013
Phase diagrams have been produced and critical exponents calculated for a Blume-Emery-Griffiths system with competing biquadratic and crystal-field interactions with uniform ferromagnetic bilinear interactions. This competition directly effects the clustering and density of nonmagnetic impurities. These results have been produced using renormalization-group methods with a hierarchical lattice. A series of planes of constant, repulsive biquadratic coupling have been probed while varying the temperature and concentration of annealed vacancies in the system. The sinks have been analyzed and interpreted, and critical exponents calculated for the higher order transitions. 相似文献
5.
We present a study, within a mean-field approximation, of the dynamics of a spin-1 metamagnetic Ising system with bilinear and biquadratic interactions in the presence of a time-dependent oscillating external magnetic field. First, we employ the Glauber transition rates to construct the set of mean-field dynamic equations. Then, we study the time variation of the average order parameters to find the phases in the system. We also investigate the thermal behavior of dynamic order parameters to characterize the nature (first- or second-order) of the dynamic transitions. The dynamic phase transitions are obtained and the phase diagrams are constructed in two different the planes. The phase diagrams contain a disordered and ordered phases, and four different mixed phases that strongly depend on interaction parameters. Phase diagrams also display one or two dynamic tricritical points, a dynamic double critical end and dynamic quadruple points. A comparison is made with the results of the other metamagnetic Ising systems. 相似文献
6.
Yu. A. Fridman Ph. N. Klevets D.A. Matunin O.V. Kozhemyako 《Journal of Physics and Chemistry of Solids》2006,67(12):2458-2467
The phase states of a two-dimensional ferromagnet system with anisotropic exchange interaction have been investigated for various relations between the material constants. It is shown that the competition between the exchange and the one-ion anisotropies, and the account of the magnetodipolar and the magnetoelastic interactions lead to the sequence of the phase transitions. The explicit solutions of the dispersion equation have been found in the homogeneous phase states. 相似文献
7.
Z.-S. Liu 《Solid State Communications》2004,131(2):135-139
Perturbation theory and mean-field approach have been employed in the present work to estimate the Heisenberg coupling constants 's for the ferromagnet DyRh4B4 and the antiferromagnet TmRh4B4 from their magnetic transition temperatures TM's in comparison with GdRh4B4. We found that the 's of these substances are different from each other, which may be attributed to their different electronic structures and also the anisotropic nature of the Heisenberg exchange coupling. 相似文献
8.
9.
P.J. von Ranke N.A. de Oliveira V.S.R. de Sousa A. Magnus G. Carvalho 《Journal of magnetism and magnetic materials》2010,322(1):84-87
In this work we present a model to describe the magnetocaloric effect (MCE) in ferrimagnetic arrangements. Our model takes into account the magnetoelastic interactions in the two coupled magnetic sublattices, which can lead to the onset of the first order magnetic phase transition and the giant-MCE. Several profiles of the MCE, such as: the inverse- and giant-MCE were systematically studied. Application of the model to the ferromagnetic compounds GdAl2, Gd5(Ge1.72Si2.28), Gd5(Ge2Si2), and to the ferrimagnetic compound Y3Fe5O12 was performed, showing a good agreement with the experimental data. 相似文献
10.
We discuss the ground state phase transition between an antiferromagnet and a valence-bond solid in a two-dimensional spin-1/2 XY model with a four-spin interaction. This transition has been proposed as a candidate for a deconfined quantum-critical point. We analyze quantum Monte Carlo data in order to accurately characterize the transition. The central question that remains to be answered is whether the transition really is continuous, or whether it is actually weakly first-order. We present the current status of both ground state and finite-temperature calculations. Based on the results, we discuss possible scenarios for the transition, none of which is consistent with deconfined quantum-criticality. However, we argue that a deconfined quantum-critical point may be located nearby in an extended parameter space.We also discuss the staggered Ising phase obtaining in the limit of strong four-spin coupling. 相似文献
11.
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
12.
Investigation on the magnetocaloric effect in DyNi2, DyAl2 and Tb1−nGdnAl2 (n=0, 0.4, 0.6) compounds
V.S.R. de Sousa E.J.R. Plaza B.P. Alho S. Gama P.J. von Ranke 《Journal of magnetism and magnetic materials》2009,321(20):3462-3465
The magnetocaloric effect (MCE) in the DyNi2, DyAl2 and Tb1−nGdnAl2 (n=0, 0.4, 0.6) was theoretically investigated in this work. The DyNi2 and DyAl2 compounds are described considering a model Hamiltonian which includes the crystalline electrical field anisotropy. The anisotropic MCE was calculated changing the magnetic field direction from 〈1 1 1〉 to 〈0 0 1〉 in DyNi2 and from 〈1 0 0〉 to 〈0 1 1〉 in DyAl2. The influence of the second- and first-order spin-reorientation phase transitions on the MCE that occurs in these systems is discussed. For the calculations of the MCE thermodynamic quantities in the Tb1−nGdnAl2 systems we take into account a two sites magnetic model, and good agreement with the available experimental data was obtained. 相似文献
13.
Jeongyong Choi Sungyoul Choi M.H. Sohn Hyoyeol Park Yongsup Park Hyun-Min Park S.C. Hong Sunglae Cho 《Journal of magnetism and magnetic materials》2006
We have grown 500 Å MnP on undoped GaAs(1 0 0) substrate using solid-source molecular beam epitaxy. In order to characterize the crystal structure of MnP, we performed in-situ reflection high energy electron diffraction and θ–2θ XRD X-ray diffraction studies. From the measurements of superconducting quantum interference device, Quantum Design, MnP thin film shows ferromagnetic ordering at around 291.5 K. It shows a metallic resistivity in MnP thin film. 相似文献
14.
V.D. Buchelnikov S.V. Taskaev M.A. Zagrebin D.I. Ermakov V.V. Koledov V.G. Shavrov T. Takagi 《Journal of magnetism and magnetic materials》2007
With the help of the Ginzburg–Landau theory the temperature–magnetic field phase diagrams of Heusler alloys Ni–Mn–Ga are theoretically investigated. The influence of the parameters of the magnetoelastic constants and elastic moduli on the phase diagrams are discussed. It is shown that with the determined combination of the phenomenological parameters, the critical magnetic field for the phase transition appears. The theoretically predicted value of the critical magnetic field is possible to be achieved in the experiment with the help of modern magnetic field sources. 相似文献
15.
For the Nd0.1La0.9Fe11.5Al1.5 compound, the fine structure of the magnetic transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) states has been studied carefully by means of magnetization (M) and heat capacity (Cp) measurements. Although a single phase with the cubic NaZn13-type structure (Fm3c) has been proved by the room temperature X-ray diffraction pattern, the phase transition has been clearly found to be a stepwise process in M(T) and Cp(T) curves under proper fields. Due to the strong competition between the FM order and AFM order, the characteristic is especially evident under low fields, weakens gradually with the increasing applied field and finally vanishes when the field is higher than 2 T. This multi-step magnetic transition results from the inhomogeneity of the sample, probably due to the inhomogeneous distribution of Nd atoms. 相似文献
16.
E.V. Sampathkumaran Niharika Mohapatra Kartik K. Iyer C.D. Cao W. Löser G. Behr 《Journal of magnetism and magnetic materials》2008
Considering certain interesting features in the previously reported 166Er Mössbauer effect, and neutron diffraction data on the polycrystalline form of ErPd2Si2 crystallizing in the ThCr2Si2-type tetragonal structure, we have carried out magnetic measurements (1.8–300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in features due to magnetic ordering in the plot of magnetic susceptibility χ versus temperature T at low temperatures. The χ(T) data reveal that there is a pseudo-low-dimensional magnetic order setting in at 4.8 K, with a three-dimensional antiferromagnetic order setting in at a lower temperature (3.8 K). A new finding in the χ(T) data is that, for H∥〈1 1 0〉 but not for H∥〈0 0 1〉, there is a broad shoulder in the range 8–20 K, indicative of the existence of magnetic correlations above 5 K as well, which could be related to the previously reported slow-relaxation-dominated Mössbauer spectra. Interestingly, the temperature coefficient of electrical resistivity is found to be isotropic; no feature due to magnetic ordering could be detected in the electrical resistivity data at low temperatures, which is attributed to magnetic Brillioun-zone boundary gap effects. The results reveal the complex nature of magnetism of this compound. 相似文献
17.
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
18.
Y.G. Xiao G.H. Rao Q. Zhang G.Y. Liu Y. Zhang J.K. Liang 《Journal of magnetism and magnetic materials》2006
The structural and magnetic properties of Nd1−xTbxFe10.5Mo1.5 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) compounds have been investigated by means of X-ray diffraction and magnetic measurements. All the investigated compounds crystallize in the tetragonal ThMn12-type structure with I4/mmm space group. The lattice parameters a, c and the unit-cell volume V decrease with increasing x. The Curie temperatures TC are almost independent x. There exists a unique spin-reorientation transition for the end compositions of Nd1−xTbxFe10.5Mo1.5 compounds with x=0 and x=1, while two spin-reorientation transitions are observed for x=0.2–0.8. The room-temperature magnetocrystalline anisotropy of Nd1−xTbxFe10.5Mo1.5 compounds changes from uniaxial to planar with increasing x content. Based on magnetic measurements, a magnetic phase diagram of Nd1−xTbxFe10.5Mo1.5 compounds is constructed. By minimizing the magnetocrystalline anisotropy energy, a theoretical magnetic phase diagram for the Nd1−xTbxFe10.5Mo1.5 system is derived, showing a reasonable agreement with the observations. 相似文献
19.
M. Dudzinski J. Sznajd J. Zittartz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(4):575-579
The zero temperature phase diagram of a one-dimensional ferromagnet with cubic single ion anisotropy in an external magnetic
field is studied. The mean-field approximation and the density-matrix renormalization group method are applied. Two phases
at finite magnetic fields are identified: a canted phase with spontaneously broken symmetry and a phase with magnetization
along the magnetic field. Both methods predict that the canted phase exists even for the single-ion anisotropy strong enough
to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density-matrix renormalization
group predicts a reentrant behavior for the model. The character of the phase transition at finite magnetic field has also
been considered and the critical index has been found.
Received 9 May 2000 and Received in final form 5 July 2000 相似文献