Compensation point (or points) of nanoscaled transverse Ising thin films: effects of surface dilution

The phase diagram (critical temperature and compensation temperature) and the total magnetizations in a nanoscaled transverse Ising thin film with thickness L and a negative interlayer coupling at the surfaces are investigated by the use of both the effective-field theory with correlations and the mean-field theory. Particular emphasis is given to the effects of the surface and its dilution on them. We have found that, for appropriate values of the system parameters, compensation point (or points) may be obtained in the present system with L?=?5 (or L?=?10).     

Magnetic properties of a hexagonal transverse Ising nanoisland

The magnetic properties of a hexagonal transverse Ising nanoisland have been calculated by the effective-field theory with correlations. The effects of surface coupling, interlayer coupling, and the competition between transverse fields at the surface shell and in the inner core on phase diagrams and magnetizations are examined. Meanwhile, the results are also compared with two different nanoislands which consist of two layers with nine atoms or five atoms in each of the layers.     

Frustration in a transverse Ising nanoisland: effects of interlayer coupling

The phase diagram and magnetizations in a transverse Ising nanoisland are examined by using the effective field theory with correlations. The nanoisland is constructed from two layers with nine atoms in each which are coupled by the interlayer coupling. We present some characteristic phenomena in them which come from the frustration induced by an interlayer coupling and two transverse fields.     

The possibility of re-entrant phenomena induced by a transverse field in ultra-thin transverse Ising films

The phase diagrams and temperature dependences of magnetizations in ultra-thin transverse Ising thin films are studied by the use of both the effective-field theory with correlations (EFT) and the mean-field theory (MFA). Novel features, such as the possibility of re-entrant phenomena, are obtained for the magnetic properties in such systems with a zero transverse field at the surfaces, when the EFT is applied to them, although such features could not be found from the use of the MFA. When the transverse field at the surfaces takes a finite value, however, the re-entrant phenomena could not be found from the both formulations of the EFT and the MFA. Similar phenomena are then obtained in the phase diagrams by using the MFA and the EFT.     

Ferrimagnetism in two antiferromagnetic transverse Ising thin films

The phase diagrams and temperature dependences of total magnetization m_T in two antiferromagnetic transverse Ising thin films with same thickness L (L = 4), consisting of two (A and B) layers, are studied by the uses of the effective-field theory with correlations and the mean-field theory. The A and B layers are consisted of spin-1/2 atoms and they have opposite spin directions. Two magnetic structures are discussed and they exhibit rather different and characteristic behaviors for the magnetic properties. Many characteristic behaviors observed in standard ferrimagnetic materials as well as novel phenomena have been obtained for the thermal variations of m_T in the both systems, when the crystallographically equivalent conditions between the A and B layers are broken.     

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.     

Ferrimagnetism and reentrant phenomena in a tetragonal Ising nanoparticle

ABSTRACT

The phase diagrams and magnetizations in a tetragonal Ising nanoparticle have been investigated by using the effective-field theory with correlations and the core–shell model. For the spin-1/2 atoms in the shell, a transverse field is included to cant their spin direction. The results show that they are heavily dependent on the given physical parameters. The ferrimagnetic behaviours, as well as novel type behaviours, have been found in the thermal variation of magnetization. The reentrant phenomena are also obtained.     

Some characteristic properties of initial susceptibility in a Ising nanotube

Initial susceptibility and magnetization of a cylindrical nanotube described by the 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 of Kaneyoshi (2010) [8] using the two theoretical frameworks of the mean field theory and the EFT. The effects of the two exchange couplings at the surface shell and in the core to the initial susceptibility are clarified. Some characteristic phenomena are found in the thermal variations, depending on the ratios of the physical parameters in the surface shell and the core. It is also discussed whether the Neel hyperbola in the paramagnetic region is valid for a nano-scaled ferrimagnetic system.     

Shape dependences of magnetic properties in 2D Ising nano-particles

The shape dependences of magnetic properties (phase diagram and magnetization) in two (circle and square) 2D Ising nano-particles are investigated by the use of both the core-shell model and the effective field theory with correlations, in order to clarify the effects of the shape difference to them. Particular emphasis is given to the effect to the magnetizations when the core-shell exchange coupling takes a different (positive or negative) sing. Some common phenomena are also found in the magnetic properties between the two nanosystems.     

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.     

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.     

Properties of the ground state in a spin-2 transverse Ising model with the presence of a crystal field

The properties of the ground state in the spin-2 transverse Ising model with the presence of a crystal field are studied by using the effective-field theory with correlations. The longitudinal and transverse magnetizations, the phase diagram and the internal energy in the ground state are given numerically for a honeycomb lattice (z=3).     

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 behavior of spin-1 Ising nanotube

We have investigated Blume-Capel model on a cylindrical Ising nanotube by using the effective field theory with correlations. We have found first- and second-order phase transition. Magnetization, magnetic susceptibility, internal energy, specific heat and free energy expressions have been found and calculated numerically for different crystal field parameter. Moreover, we have obtained that tricritical point depends on the coupling constant between core and surface shell.     

横向随机晶场Ising模型的相图和磁化行为研究

在有效场理论和切断近似框架内，选择自旋S=1的二维方格子，研究横向随机晶场Ising模型的相图和磁化行为，重点是横向随机晶场浓度和晶场比率对相图和磁化的影响.给出了i>T-D_x空间的相图和m-T空间的磁化图.在晶场稀疏情况下，负晶场方向存在临界温度的峰值，正方向可出现重入现象.晶场比率取+0.5和-0.5时，磁有序相范围缩小，特别是晶场比率取-0.5时，随晶场浓度的降低，临界温度峰值从横向晶场负方向渡越到正方向.固定某一负晶场值，不同晶场比率的磁化行为有明显差异.同时与纵向稀疏晶场Ising模型结果进行有意义的比较. 关键词： 横向随机晶场Ising模型 相图 磁化行为     

Magnetism in an antiferromagnetic Ising nano-ladder under an applied transverse field

ABSTRACT

The phase diagram and magnetization curves of an antiferromagnetic Ising nano-ladder in an applied transverse field are investigated by using the effective-field theory with correlations. They are compared with those in the antiferromagnetic Ising ladder (or bulk). Some characteristic features, such as the reentrant phenomena, are found in these properties, when a finite transverse field h is applied or even when h?=?0.0.     

Unconventional magnetic properties in transverse Ising nanoislands: Effects of interlayer coupling

The phase diagram and magnetizations in two transverse Ising nanoislands with different (hexagonal and square) structures which are consisted of the two layers are examined by using the effective-field theory with correlations. The effects of interlayer coupling and two transverse fields at the center and the perimeter atoms on them are examined. We present some characteristic phenomena in the magnetic properties which come from the frustration induced by them.     

Phase transitions in the Ising model with transverse field

We show the existence of a phase transition in the Ising model with transverse field for dimensionsv 2 provided the transverse term is sufficiently small. This is done by proving long-range order occurs using the reflection positivity of the Hamiltonian and localization of eigenvectors.     

Phase Diagram and Tricritical Behavior of a Spin-2 Transverse Ising Model in a Random Field

The phase diagrams of a spin-2 transverse Ising model with a random field on honeycomb, square, and simple-cubic lattices, respectively, are investigated within the framework of an effective-field theory with correlations.We find the behavior of the tricritical point and the reentrant phenomenon for the system with any coordination number z, when the applied random field is bimodal. The behavior of the tricritical point is also examined as a function of applied transverse field. The reentrant phenomenon comes from the competition between the transverse field and the random field.     

Magnetic Properties of Transverse Ising Model under a Time Oscillating Longitudinal Field

A transverse Ising spin system, in the presence of time-dependentlongitudinal field, is studied by the effective-field theory (EFT). Theeffective-field equations of motion of the average magnetization are givenfor the simple cubic lattice (Z = 6) and the honeycomb lattice (Z = 3).The Liapunov exponent λ is calculated for discussing the stability of the magnetization and it is used to determine the phase boundary. Thedynamic phase transition diagrams in h₀/ ZJ -Γ/ZJ plane and in h₀/ZJ-T/ZJ plane have been drawn, and there is no dynamical tricritical point on the dynamic phase transition boundary. The effect of the thermal fluctuations upon the dynamic phase boundary has been discussed.