Toshihiro Idogaki

A new effective field theory for the anisotropic Heisenberg ferromagnet

A new type of effective field theory (NEFT) is developed for the quantum spin system with anisotropic exchange interaction. The calculation is based on a two-atom cluster approximation, and effects from the rest spins are treated by the operator expansion technique assuming Ising character for surrounding spins. The Curie temperature Tc and spontaneous magnetization m are calculated for all Ising-, Heisenberg- and XY-like spin systems, and discussed comparing with results by other traditional pair spin theories. It is practically confirmed that Tc of the NEFT correctly reflects details of the geomtry of the lattice more than coordination number, and that it gives no anti-Curie point even for the XY-like spin system.

The Curie Temperatures of Magnetically Dilute Ferromagnetics

By the method of concentration expansion, the initial gradient I g for a curve of the Curie temperature θ c ( p ) vs the concentration p of magnetic ions in randomly dilute Ising ferromagnet is calculated. The calculation is performed for the eight kinds of crystal lattices and the results are discussed with particular emphasis as to its dependence on the crystal structure and the second neighbor interactions. Various experimental data are compared with the present results of I g together with the other critical properties p c , θ c ( p ) obtained in the previous paper. As a whole, the behaviors of θ c ( p ) determined experimentally are well explained by the theory, and an importance of the second neighbor interactions is indicated for some compounds.

On the Magnetically Dilute Ferromagnetic System

The problems on the magnetic dilution in the ferromagnetic system are discussed by the use of the Ising model of spin 1/2. In addition to the exchange interaction between nearest neighbors, the next nearest neighbor (n.n.n.) interaction has been taken into consideration, and the magnetic properties are calculated by the method of concentration expansion. Particularly the critical concentration and the concentration dependence of the Curie temperature have been calculated for several lattice structures. In some crystal lattices, the n.n.n. interaction concerns to the change of the dimensionality of the magnetic lattice and then the effects of the n.n.n. interaction are quite remarkable in the concentration dependence. The numerical values of the critical concentration and the Curie temperature calculated in the present method are compared with the results previously calculated by various methods and good agreement is obtained.

Concentration Expansion Study of the Randomly Dilute Ising Ferromagnet with Second Neighbor Interaction

The critical properties of the randomly dilute Ising ferromagnet are discussed for the square lattice by the method of concentration expansion. The studies are mostly limited to the investigation of the effects of the second neighbor interaction on the concentration dependence of the Curie temperature T c ( p ) and on the temperature dependence of the critical exponent \barγ(θ) describing the susceptibility divergence. The series obtained is analysed by the root, ratio, and Pade approximant methods, respectively. As for T c ( p ), the previous results are reconfirmed by raising the degree of series expansion. The present calculation gives an apparent temperature dependence of \barγ(θ) and its behavior can be compared with the results previously calculated for some three-dimensional lattices. Furthermore, it is found that the essential feature of \barγ(θ) is not affected by the presence of the second neighbor interaction.

Thermal and percolative properties of a diluted Ising metamagnet in a field

We calculated thermodynamic quantities such as the staggered and direct susceptibilities, internal energy and specific heat of a diluted Ising layered metamagnet in an external field, by the use of an effective field theory (EFT) with correlations, developed for this model in our previous paper. We also extensively investigated how an inter- and intra-layer exchange interaction ratio can influence critical, tricritical and percolative properties of the system.

Magnetization and ground state spin structures of Ising spin system with biquadratic exchange interaction

Abstract The magnetic properties of the spin S (=1, 2 , and 3) Ising system with the bilinear and the biquadratic exchange interactions, J and J1, have been investigated by making use of the four-spin model approximation for the negative value of J1. The ground state spin structures of the Ising spin system are also discussed.

Monte Carlo studies of critical and dynamic phenomena in diluted Ising metamagnet in a field

Abstract Monte Carlo simulations have been used to study critical, tricritical as well as some history-dependent properties of a diluted Ising metamagnet in an external field. Phase boundaries, including location of tricritical points, have been established in a broad concentration range. A first-order transition is estimated to survive down to at least p =0.5, i.e., even in an extreme dilution region. In the region of strong dilution additional shorter simulations on lattices of large linear size have been run in order to study irreversibilities associated with the domain state formation.

Dilute anisotropic Heisenberg ferromagnet with biquadratic exchange interactions and single-ion anisotropy

Abstract The effects of site dilution on the spin-1 anisotropic XXZ Heisenberg ferromagnet with bilinear- and biquadratic- exchange interactions, J and J ′, and single-ion anisotropy, D , have been studied with the use of an improved two-site cluster effective field theory. The dependence of the critical temperature, T c , on the strength of J ′, and/or D , has been calculated for some two- and three- dimensional dilute lattices. With variation of the exchange anisotropy ratio, γ , cross-over from anisotropic Ising to isotropic Heisenberg character is found in the behaviour of the T c versus α (=J′/J) curve. It is suggested that in the vicinity of the critical concentration, c 0 , the value of γ required for the occurrence of this cross-over converges to a unified value, independent of the lattice structure. A spurious reentrant phenomenon appearing in the concentration dependence of T c is discussed in relation to the decoupling approximation adopted in the calculation.

Monte Carlo investigation of the tricritical point stability in a three-dimensional Ising metamagnet

We use Monte Carlo simulations to study multicritical properties of an Ising metamagnet in an external field. According to the mean field theory predictions, a three-dimensional layered metamagnet is expected to display a tricritical point decomposition to a critical and a bicritical end point, when a ratio between intralayer ferromagnetic and interlayer antiferromagnetic couplings becomes sufficiently small. Our simulations show no evidence of such a decomposition and produce a tricritical behavior even for a coupling ratio as small as

Susceptibility analysis of two-dimensional antiferromagnet FeTa2O6

R=0.01.