An antiferromagnetic transverse Ising nanoisland; unconventional surface effects

The phase diagrams and temperature dependences of magnetizations in a transverse Ising nanosisland with an antiferromagnetic spin configuration are studied by the use of the effective-field theory with correlations (EFT). Some novel features, such as the re-entrant phenomena with two compensation points being free from disorder induced frustration, are obtained for the magnetic properties in the system.     

Magnetizations of a transverse Ising nanowire

Magnetizations of a cylindrical nanowire described by the transverse Ising model are investigated by the use of the effective field theory with correlations (EFT), since the phase diagrams of the system have been examined in the previous work (J. Magn. Magn. Mater. (2010), in press) by using the two theoretical frameworks of the mean field theory and the EFT. The temperature dependences of longitudinal and transverse magnetizations in the system are strongly affected by the surface situations. Many characteristic phenomena are found in the thermal variations, depending on the ratio of the physical parameters in the surface shell and the core. In particular, the effects of the two transverse fields at the surface shell and in the core to these magnetizations have been firstly clarified.     

Magnetic properties of Ising metamagnet in both external longitudinal and transverse fields

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−h_x 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 h_x decreases.     

Ferrimagnetic magnetizations of transverse Ising thin films with diluted surfaces

The temperature dependences of longitudinal and transverse magnetizations in transverse Ising thin films with diluted surfaces which are coupled antiferromagnetically to the bulk are studied by the use of the effective-field theory (EFT) with correlations. Novel features are obtained for the thermal variations of longitudinal magnetization, being different from those of the bulk ferrimagnetic materials, such as the possibility of two compensation points. They are depending on the thickness of a film and the surface dilution. These characteristic phenomena come from the competition between the surfaces and the bulk when the ratios of the physical parameters (transverse field and exchange interaction) between the surfaces and the bulk are selected as some large values.     

Ground-state magnetic properties of the spin-2 transverse Ising model

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.     

The phase diagrams of a spin-1 transverse Ising model

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).     

Characteristic phenomena in nanoscaled transverse Ising thin films with diluted surfaces

The phase diagrams and magnetizations of two nanoscaled thin films with a negative interlayer interaction and dilution at the surfaces, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations. A lot of characteristic phenomena which heavily depend on the surface parameters can be found in them. In particular, the behaviors of a compensation point (or points) in these nanosystems with a negative interlayer interaction at the surfaces are examined by changing the transverse field and the physical parameters at the surfaces.     

The random transverse Ising thin film with decorated ferrimagnetic surfaces

The phase diagrams and magnetization curves of the random transverse Ising film with decorated spin-1 atoms on the surface are investigated by the use of an effective field method within the framework of a single cluster theory. In particular, the case in which decorated atoms at the surfaces are ordered ferrimagnetically is examined. The effects of the interactions, random transverse field at the surface and in the bulk, and film thickness are studied. A number of characteristic phenomena, such as the possibility of compensation points on the surfaces and in the film are found.     

Phase transition features of the bond-dilution transverse ferromagnetic Ising spin system with random crystal field

The bond dilution spin-1 transverse ferromagnetic Ising model with random crystal field is investigated in the framework of effective field theory (EFT). The general expressions of magnetization are derived for lattice with any coordination number z. In particular, we calculate the phase diagrams of a square lattice. The second-order phase transition lines and the tricritical points are presented in different conditions for the square lattice. Special emphasis is placed on the influence of bond dilution, random crystal field and the transverse field on the phase diagrams. We discover some unusual phenomena. New results obtained in this paper are discussed in detail.     

A quadrangular transverse Ising nanowire with an antiferromagnetic spin configuration

The phase diagrams and the temperature dependences of magnetizations in a transverse Ising nanowire with an antiferromagnetic spin configuration are investigated by the use of the effective-field theory with correlations (EFT) and the core–shell concept. Many characteristic and unexpected behaviors are found for them, especially for thermal variation of total magnetization m_T. The reentrant phenomenon induced by a transverse field in the core, the appearance of a compensation point, the non-monotonic variation with a compensation point, the reentrant phenomena with a compensation point and the existence of both a broad maximum and a compensation point have been found in the thermal variations of m_T.     

Magnetic properties in a transverse Ising nanowire with an antiferromagnetic spin configuration at the surface

The phase diagrams and temperature dependences of magnetizations in a transverse Ising nanowire with an antiferromagnetic spin configuration at the surface are studied by the use of the effective-field theory with correlations. A number of characteristic features, such as the possibility of reentrant phenomena being free from disorder-induced frustration, are obtained for the magnetic properties in the system.     

随机横场与晶场作用混合自旋系统的热力学性质

利用有效场理论和切断近似，在伊辛模型的框架内考虑随机横场与晶场作用的混合自旋1/2 和自旋1系统的热力学性质。重点研究了晶场、横场和随机浓度对混合自旋系统相变的影响 ，研究发现在随机横场条件下，较小的晶场存在并不能改变临界横场阈值；取较大横场值时 在某些随机浓度的范围内出现重入相变现象，而取较小横场值时则没有出现重入相变现象。 给出了有关相图并进行了讨论。 关键词： 混合自旋系统 伊辛模型 热力学性质 随机横场     

Critical properties of a random transverse crystal field Ising model with bond dilution

Within effective field theory (EFT), the critical properties of a random transverse crystal field Ising model with bond dilution are studied on a square lattice. Under both weak and strong bond dilution conditions, we consider three cases (α = 0,±0.5) of a transverse crystal field ratio, obtaining global phase diagrams in T−D _x space for changes in the random transverse crystal field concentration. The phase diagrams obtained for a weak bond dilution are very similar in shape to those of pure bond but with decreases in corresponding ordered phases and critical values. However, the phase diagrams for a strong bond dilution exhibit varieties, including a change in reentrant phenomenon, the occurrence of transverse crystal field degeneration, and the opposite direction crossover of temperature peak value.     

Two compensation points induced by surface parameters in nanoscaled transverse Ising ferrimagnetic thin films

The phase diagrams and magnetizations of two nanoscaled thin films with a negative interlayer interaction at the surfaces, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations. It is proved that two compensation points induced by the surface parameters (exchange interaction and transverse field at the surfaces) can be found in these nanosystems.     

Amorphization in an Ising nanowire; unconventional effects of a uniformly applied transverse field

The phase diagrams and magnetizations of an Ising nanowire with an amorphization are investigated by the use of the effective field theory with correlations, when a uniformly transverse field is applied perpendicularly to the main axis of the nanowire. Some characteristic behaviors have been obtained in them, such as the two types of reentrant phenomena induced by an applied transverse field.     

Phase diagrams of a transverse Ising nanowire

Phase diagrams of a cylindrical nanowire described by the transverse Ising model are investigated by the use of two theoretical frameworks, namely the mean field theory and the effective field theory with correlations. The phase diagram of the system is strongly affected by the surface situations. Some characteristic phenomena are found in the phase diagram, depending on the ratio of the physical parameters in the surface shell and the core.     

Some characteristic phenomena in a transverse Ising nanotube

The phase diagram and magnetizations of a cylindrical nanotube described by the transverse Ising model are investigated by the use of the effective field theory with correlations. Some comparisons between the nanotube and the nanowire have been given for the phase diagrams. In particular, the temperature dependences of longitudinal magnetization in the system with a negative shell–core interaction are investigated. Some characteristic phenomena (new types in ferrimagnetism) which have not been observed in the nanowire as well as similar phenomena are found in the thermal variations, depending on the ratio of the physical parameters in the surface shell and the core. The possibilities of two compensation points and a field induced compensation point in the nanotube are also discussed.     

An effective-field study of Ising metamagnet in an external field

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).     

Phase diagrams in an ultra-thin spin-1 transverse Ising film with bond or site dilution at surfaces

The phase diagrams of two nanoscaled thin films with bond and site dilutions at the surfaces, described by the spin-1 transverse Ising model, are investigated by the use of an effective field theory with correlations. A number of characteristic phenomena have been found in them, which are heavily dependent on the ratios (r = J₁/J and p = Ω_S/Ω, where J is the exchange interaction in the inner layer, J₁ is the exchange interaction between the surface and the next inner layer, Ω_S is the transverse field at the surfaces and Ω is the transverse field in the inner layer). Some of them have exhibited very similar behaviors found in the two spin-1/2 nanoscaled thin films with bond and site dilutions at the surfaces.     

Dynamic phase transition of ferroelectric nanotube described by a spin-1/2 transverse Ising model

The dynamic phase transition properties for ferroelectric nanotube under a spin-1/2 transverse Ising model are studied under the effective field theory(EFT)with correlations.The temperature effects on the pseudo-spin systems are unveiled in three-dimensional(3-D)and two-dimensional(2-D)phase diagrams.Moreover,the dynamic behaviors of exchange interactions on the 3-D and 2-D phase transitions under high temperature are exhibited.The results present that it is hard to obtain pure ferroelectric phase under high temperature;that is,the vibration of orderly pseudo-spins cannot be eliminated completely.