T. Kasama

Magnetization and ground state spin structures of Ising spin system with four-spin interaction

Abstract The magnetization of spin S(= 1 2 and 1) Ising systems with bilinear and four-site four-spin interactions has been investigated by making use of the Monte Carlo simulation. In low-temperature region, the magnetization curves show an anomalous inverse inclination and decrease (increase) with the decrease (increase) of temperature within a certain exchange ratio range. The relation between the value of the magnetization at zero-temperature and the spin structure expected for the ground state is discussed.

Partially disordered states of the three-dimensional ANNNI model

Abstract We analyze the three-dimensional axial next-nearest-neighbor Ising model composed of two kinds of alternately stacked ferromagnetic layers with different intralayer interactions by using Monte Carlo (MC) simulation. The detailed analyses of the magnetization and the spin correlations perpendicular to the stacked direction, assure and clarify the existence and the nature of the paramagnetic layers in some modulated phases with wave numbers, 1 4 , 1 8 , 3 16 . This is the first MC confirmation of the partially disordered states which have been predicted by previous molecular field calculation and/or observed in materials such as CsCoCl3, CuFeO2 and CeSb.

Partially Disordered Phases of the Axial Next‐Nearest‐Neighbor Ising Model with Alternating Intra‐ and Inter‐layer Interactions

We study a new axial next‐nearest‐neighbor Ising (ANNNI) model in which both the strength of intralayer and interlayer interactions alternate along the c‐axis, respectively. The results of a site‐dependent mean‐field approximation show that the alternation of these interactions causes the partially disordered phase (PDP) and a crossing behavior between two PDPs of ↑ ∘ ↓ ∘ and ∘ ↑ ∘ ↓ where ↑ (or ↓) means a layer magnetized along ↑ (or ↓) direction and ∘ means a paramagnetic layer. In an extremely low temperature region around the multi‐phase point, we found the reentrant phase transition producing the successive phase transitions ↑↑↑↑⇒↑↑↑↓↓↓⇒↑↑↑↑ with increasing temperature.

Magnetic Properties of an Ising Spin System with Two‐Spin and Four‐Spin Interactions

The magnetic properties of the Ising spin (S=1/2 and 1) system with a bilinear exchange interaction J0SizSjz, and a four‐site four‐spin interaction JSizSjzSkzSlz, are discussed by making use of Monte Carlo (MC) simulations of the magnetization and magnetic specific heat. Especially at a certain interaction ratio J/J0, e.g. for the square lattice at the condition |J/J0| = 2 and 1/2 for S = 1/2 and 1, respectively, the thermal average 〈SizSjzSkzSlz〉 shows a characteristic temperature dependence in the low temperature region and becomes an essential parameter to detect the phase change of the present system.

Monte Carlo Study of the Partially Disordered States of Mixed-spin Axial Next-nearest-neighbor Ising Model

The three-dimensional axial next-nearest-neighbor Ising model, which is composed of alternately stacked ferromagnetic spin-1/2 and spin-1 layers with competing interlayer interactions along the c-axis, is studied by using the Monte Carlo (MC) calculation. The partially disordered phases predicted in the previous molecular field calculation is numerically confirmed by the investigation of the temperature and lattice-size dependencies of magnetization together with those of intra- and inter-layer spin-correlations by the MC calculation.