[en] 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

[en] Effects of ferroquadrupolar exchange interaction J₂ on critical behavior of a frustrated classical XY model with antiferromagnetic exchange interaction J₁ are investigated on stacked triangular lattice by means of histogram Monte Carlo simulations. We identify four long-range order phases separated by the boundaries which can be either of second or first order, depending on the kind of transition and magnitude of J₁/J₂. Characteristics of the respective phases, mechanisms of their creation, and natures of the transitions between the respective phases are elaborated on

[en] Critical properties of a classical plane rotator spin system with ferromagnetic and antiferroquadrupolar interactions on stacked triangular lattice are investigated by histogram Monte Carlo simulations. The competing bilinear and biquadratic interactions, J₁ and J₂, respectively, and the frustrated triangular geometry bring about rather complicated critical behavior. In the range of J₁/|J₂|<2, as temperature is lowered, the system is found to pass through four phase transitions, ending in the ground state that shows no conventional LRO. We draw phase diagram in J₁/|J₂|-k_BT_c/|J₂| space, classify the respective phase transitions and elaborate on reasons of the low-temperature disorder

[en] The three-dimensional XY model with bilinear-biquadratic exchange interactions J and J', respectively, has been studied by Monte Carlo simulations. From the detailed analysis of the thermal variation of various physical quantities, as well as the order parameter and energy histogram analysis, the phase diagram including two different ordered phases has been determined. There is a single phase boundary from a paramagnetic to a dipole-quadrupole ordered phase, which is of second order in a high J/J' ratio region, changing to a first-order one for 0.35≤J/J'≤0.5. Below J/J'=0.35 there are two separate transitions: the first one to the quadrupole long-range order (QLRO) phase at higher temperatures, followed by another one to the dipole-quadrupole long-range order (DLRO) phase at lower temperatures. The finite-size scaling analysis yields values of the critical exponents for both the DLRO and QLRO transitions close to the values for the conventional XY model which includes no biquadratic exchange

[en] 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

[en] 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

[en] The magnetic susceptibility (χ) of the tetrameric complex compound of ruthenium pentafluoride (RuF₅)₄ is investigated theoretically by the aid of the exchange Hamiltonian taking into account higher-order exchange interactions and the covalency effect of magnetic ions. The theoretical curve is χ as a function of temperature shows a good agreement with the experiment at temperatures below and above the maximum point around T = 40 K (corresponding to an antiferromagnetic intra-cluster ordering)

[en] Combining the Shastry inequality and infrared bound based on reflection positivity, we prove the absence of spontaneous symmetry breaking associated with antiferro-spin, antiferro-orbital, and ferro-spin-orbital ordering in the ground state of the one-dimensional spin-orbital model in the region satisfying reflection positivity

[en] The spin-1 ±J Ising model with uniform biquadratic couplings on a simple cubic lattice is studied by the Monte Carlo simulation using the non-equilibrium relaxation method. The reentrant phase transition induced by competition between the bilinear and biquadratic couplings is eliminated gradually with increasing randomness of bilinear couplings and disappears entirely in the strong random system. The dynamic exponent of ferromagnetic transition shows non-universal behavior with changing randomness, while this behavior is not observed in the case of staggered quadrupolar transition

[en] The three-dimensional ±J Ising spin model with uniform biquadratic exchange interaction, K, is studied by Monte Carlo simulations. In the spin glass region, the exponent ν_s is invariable for the strength of K, and consistent with that of the spin-((1)/(2)) glass