Akiyoshi Kuroda

Existence of Phase Transition in Ising-like Heisenberg Antiferromagnets on the Kagome Lattice

A phase transition is found in Ising-like Heisenberg antiferromagnets on the kagome lattice. Below the critical temperature, an exotic ferromagnetic order exists. Namely, a twofold symmetry of the system is broken and the system has a nonzero total magnetization, but no a sublattice long-range order.

Peculiar `from-Edge-to-Interior' Spin Freezing in a Magnetic Dipolar Cube

By molecular dynamics simulation, we have investigated classical Heisenberg spins, which are arrayed on a finite simple cubic lattice and interact with each other only by the dipole–dipole interaction, and have found its peculiar from-edge-to-interior freezing process. As the temperature is decreased, spins on each edge predominantly start to freeze in a ferromagnetic alignment parallel to the edge around the corresponding bulk transition temperature, then from each edge grow domains with short-ranged orders similar to the corresponding bulk orders, and finally the system is expected to end up with a unique multidomain ground state at the lowest temperature. We infer that these freezing characteristics are attributed to the anisotropic and long-range nature of the dipole–dipole interaction combined with a finite-size effect.

Magnetic Properties of Two-Dimensional Dipolar Squares: Boundary Geometry Dependence

By means of the molecular dynamics simulation of gradual cooling processes, we investigate magnetic properties of classical spin systems only with the magnetic dipole–dipole interaction, which we call dipolar systems. Focusing on their finite-size effect, particularly their boundary geometry dependence, we study two finite dipolar squares cut out from a square lattice with Φ=0 and π/4, where Φ is the angle between the direction of the lattice axis and that of the square boundary. Distinctly different results are obtained in the two dipolar squares. In the Φ=0 square, the “from-edge-to-interior freezing” of spins is observed. Its ground state has a multidomain structure whose domains consist of two among infinitely (continuously) degenerated Luttinger–Tisza (LT) ground-state orders on a bulk square lattice, i.e., the two antiferromagnetically aligned ferromagnetic chains (af-FMC) orders directed parallel to the two lattice axes. In the Φ=π/4 square, on the other hand, the freezing starts from the interior ...

Finite dipolar hexagonal columns on piled layers of triangular lattice

Abstract We have investigated, by the Monte Carlo simulation, spin systems which represent moments of arrayed magnetic nanoparticles interacting with each other only by the dipole–dipole interaction. In the present paper we aim the understanding of finite size effects on the magnetic nanoparticles arrayed in hexagonal columns cut out from the close-packing structures or from those with uniaxial compression. In columns with the genuine close-packing structures, we observe a single vortex state which is also observed previously in finite two-dimensional systems. On the other hand in the system with the inter-layer distance set 1 / 2 times of the close-packing one, we found ground states which depend on the number of layers. The dependence is induced by a finite size effect and is related to a orientation transition in the corresponding bulk system.

A Note on Stable States of Dipolar Systems at Low Temperatures

To summarize, we showed “antiferromagnetic structure” is stable for the square lattice and the SC lattice as LT predicted. On the other hand, for the triangular lattice and the BCC lattice, magnetic domain structure is stable. Theoretical approach which assumes Γ 2 symmetry fails in the triangular lattice and the BCC lattice. For future works, examination of the face centered cubic lattice and estimation of critical temperatures and exponents for several lattices are remained.

Spin melting and refreezing driven by uniaxial compression on a dipolar hexagonal plate

We investigate the freezing characteristics of a finite dipolar hexagonal plate by Monte Carlo simulation. The hexagonal plate is cut out from a piled triangular lattice of three layers with FCC-like (ABCABC) stacking structure. In the present study an annealing simulation is performed for a dipolar plate uniaxially compressed in the direction of layer-piling. We find spin melting and refreezing driven by the uniaxial compression. Each of the melting and refreezing corresponds one-to-one with a change of the ground states induced by compression. The freezing temperatures of the ground-state orders differ significantly from each other, which gives rise to the spin melting and refreezing of the present interest. We argue that these phenomena are originated by a finite size effect combined with a peculiar anisotropic nature of the dipole–dipole interaction.