Seizo Watarai

Application of the Transverse Ising Model to CsH2PO4 and CsD2PO4

The ferroelectric phase transitions in CDP and DCDP are studied on the basis of the quasi-one-dimensional transverse Ising model. The dielectric susceptibilities are calculated on the assumption that the ratio of the intar-chain interaction to the tunneling energy is very large, and compared with the experimental data. Without the introduction of unnatural large difference in other model parameters between CDP and DCDP, a satisfactory agreement has been obtained for the static as well as dynamic susceptibilities.

Entropy Effect on the Magnetization Process of Hexagonal XY-Like Heisenberg Antiferromagnets.

Because of the frustration the spin structure of hexagonal Heisenberg antiferromagnets, which have weak XY-like anisotropy and ferromagnetic interlayer coupling, is determined by a delicate balance. The Monte Carlo method (for classical spins) is applied to obtain the phase diagram. It is shown that a competition between weak XY-like anisotropy and entropy effect (i.e.,thermal fluctuations) results successively in various phases, which show up in the magnetization process. At high fields a coplanar spin structure is stabilized at finite temperatures, while only one phase with the umbrella-type spin structure is present in the ground state. For a comparison the phase diagram of the Heisenberg model without anisotropy is also studied to find the same field-induced phase at high fields. In the low field region, the spin structure in the Heisenberg model is different from that in the XY-like case. We find entropy-induced phase transitions to occur due to thermal fluctuations, which overcome the weak anisotropy.

Theory of Ferroelectric Phase Transition in AgNa(NO2)2

A theory of ferroelectric phase transition in AgNa(NO 2 ) 2 is proposed on the basis of a two sublattice model. A model calculation is performed to obtain the temperature dependence of spontaneous polarization and the intensity of Raman scattering due to the symmetric bending mode of NO - 2 ions. The theoretical results are shown in a good agreement with the experiments.

Thermodynamic Properties of Ising-Like Heisenberg Antiferromagnets with Anisotropic Coupling on the Hexagonal Lattice.

Nature of ordered phases of Ising-like Heisenberg antiferromagnets on the hexagonal (layered triangular) lattice is investigated by making use of a Monte Carlo method and mean field analysis. Two phase transitions are found, which correspond to the orderings of the z -component and of the x y -components of the spins, respectively. The uniform magnetization along the easy axis is found in ordered phases with a concave curve as a function of the temperature and the spin configuration of the ordered phases is found to be a three-sublattice ferrimagnetic one. In the low temperature phase, the effect of the nontrivial degeneracy (NTD) of the ground state which is inherent to the present model is investigated. NTD causes a nontrivial enhancement of the fluctuation of the uniform magnetization in the hard plane. The magnetization processes in the ground state for fields along the x -axis are also studied.

Theory of temperature dependence of spin-wave impurity modes in an antiferromagnetic perovskite

Abstract The method of two-time Green function is applied to the study of the behaviour of spin-wave impurity modes at low temperatures. Results are compared with two experiments on Ni 2+ and Eu 3+ impurities in KMnF 3 .

The Uniaxial Dipoles on the Square Lattice with the Two-Dimensional Dipolar Interaction

Dipolar interaction in two-dimensional space, ( p i · p j ) r -2 -2( p i · r )( p j · r ) r -4 , is investigated upon uniaxial dipoles on the square lattice. Fourier transform D ( q ) of this interaction is analyzed and expressed in terms of rapidly convergent series. In the uniaxial dipole system, D ( q ) is shown to have a minimum value at q =π/ a (1, 0). An outline of the phase diagram of this uniaxial dipole system, which also has the short-range interaction, is obtained from the mean field approximation and the Monte Carlo simulation.

Monte Carlo Studies of the Square Lattice Ising System with the Two-Dimensional Dipolar Interaction

Monte Carlo simulations are performed on the square lattice Ising system with both the nearest neighbor and the two-dimensional dipolar interactions in order to investigate the critical property of ferromagnetic phase transitions in this 2-dimensional system. Applying the finite size scaling theory to Monte Carlo data just near the transition point, the critical indices of this dipolar system are calculated and α=0.31±0.04, β =0.20±0.02, γ=1.08±0.20 and ν=0.72±0.10 are obtained. However, in the temperature region considerably far from the transition point, different values of α, β and γ seem to be adequate, which are rather close to those of the rigorous result of the 2-dimensional Ising model.

Soft-Magnon Modes in Hematite

The spin-wave theory is developed for the study of the acoustic soft-magnon modes in hematite (α-Fe 2 O 3 ). The energies ω 0 of the soft-modes are approximately given as ω 0 ∞| T M - T | 1/2 near the Morin phase transition temperature T M and the results are compared with the AFMR experiments.