T. Kaneyoshi

Differential operator technique for higher spin problems

A new effective-field theory for the Blume-Capel model with a high spin value S is developed by making use of exact spin identities and taking advantage of the differential operator technique. The general formulation for evaluating the transition line and the tricritical point is derived. In particular, the phase diagrams are examined for S = 32 and S = 2. Our results show that the tricritical behavior does not exist in the spin-32 Blume-Capel model but does exist in the spin-2 Blume-Capel model. The tricritical point in the S = 2 system is found at D/zJ≅−0.498, where z is the coordination number, D the crystal-field constant and J the exchange interaction.

Phase transition of the mixed spin system with a random crystal field

Abstract The effect of a random crystal field on the transition temperature of the spin- 1 2 and spin-1 mixed Ising spin system is investigated with the use of an effective-field theory with correlations. We find a variety of interesting phenomena resulting from the fluctuation of the crystal field interaction.

Correlated-effective-field treatment of spin-one Ising models

A new version of correlated effective-field theory is applied to a study of the spin-one Ising models, namely the Blume-Capel and Blume-Emery-Griffiths modesl, using the Honmura-Kaneyoshi differential operator technique. This method is illustrated in a honeycomb lattice by investigating the phase diagram, the magnetization, the correlated effective-field parameter and the short-range order parameter. The theory correctly accounts for many physical quantities and leads to results that are quite superior, and in some aspects different, to those of the previous works.

Surface magnetism in a random surface field

Surface phase diagrams of a semi-infinite simple cubic spin 12 Ising system in a random surface field are investigated by the use of effective-field theory with correlations. Surface tricritical behavior is found, when the enhanced surface exchange interaction takes a large value.

The Curie temperature of thin films; the role of anisotropy at the surfaces

Abstract The ferromagnetic phase transition in thin films consisting of spin- 1 2 Ising layers with spin- 3 2 overlayers is studied by the use of the effective-field theory superior to the standard mean-field theory. The model calculation shows that the Curie temperature severely depends on the value of the single-ion anisotropy D at the surfaces, namely inplane (D 0), as well as the thickness and the interactions at and with the surfaces.

Surface tricritical behavior of a semi-infinite Ising model with a spin-one free surface

Surface magnetic properties of a semi-infinite ferromagnetic Ising model with a spin-one free surface are investigated within the framework of effective field theory with correlations. Magnetic behavior associated with the surface single-ion anisotropy is discussed. In particular, a new type of tricritical behavior for the surface magnetization is found depending on the strength of the perpendicular exchange interaction between the spins at the surface and their neighbors in the next layer.

A general theory of spin-one Ising models in the correlated effective-field approximation

A new correlated effective-field theory of spin-one Ising systems, namely the Blume-Capel and Blume-Emery-Griffiths models, is developed, using exact Ising spin identities, the Honmura-Kaneyoshi differential operator technique and the concept of correlated effective field. In particular, the general expressions for evaluating the second-order phase transition and the tricritical point of both models are obtained. The present method can be applied to a wide class of spin-one Ising systems.

Second-order phase transitions and tricritical points in the random-field Ising models

A method that applies to a wide class of the random-field Ising models is used to investigate the influence of random-field on the critical phenomena. The 3D random-field Ising models with nearest-neighbour interactions exhibit tricritical behavior. The obtained results express rather large deviations from the mean-field theory. For particular cases, the formulation gives some well-known exact results.

Magnetic properties of the mixed spin system with a random crystal field

The effects of a random crystal field on the magnetic properties (phase diagram, magnetization and initial susceptibility) of a spin-12 and spin-1 mixed Ising spin system are investigated by using effective-field theory with correlations. We find a variety of interesting phenomena resulting from a fluctuation of the crystal field interaction. In particular, the reentrant phenomenon induced by the fluctuation of the crystal field is found.

Surface phase diagrams of a spin-one monolayer on a semi-infinite spin-12 Ising ferromagnet

Surface phase diagrams of a spin-one monolayer on a semi-infinite spin-12 ferromagnet are investigated by using a version of the effective-field theory. In particular, the effects of the surface biquadratic exchange interaction on the surface phase diagram are discussed. Some characteristic features of the surface magnetism are found, such as the surface tricritical behavior and the surface reentrant phenomena.