Masamichi Nishino

Monte Carlo simulation of pressure-induced phase transitions in spin-crossover materials.

Pressure-induced phase transitions of spin-crossover materials are studied in a microscopic model taking into account the elastic interaction among distortions of lattice due to the difference of the molecular sizes between the high-spin state and the low-spin state. We perform Monte Carlo simulations in the constant pressure ensemble and reproduce several important properties of the pressure effect in a unified way with a microscopic mechanism for the first time. The simulation newly reveals how the temperature dependence of the ordering process changes with the pressure.

Realization of the mean-field universality class in spin-crossover materials

temperature, respectively. The model does not exhibit clusters, even near the critical point. We also found that cluster growth is suppressed in the present model and that there is no critical opalescence in the coexistence region. During the relaxation process from a metastable state at the end of a hysteresis loop, nucleation phenomena are not observed, and spatially uniform configurations are maintained during the change of the fraction of HS and LS. These characteristics of the mean-field model are expected to be found not only in spin-crossover materials, but also generally in systems where elastic distortion mediates the interaction among local states.

Cluster evolution in spin crossover systems observed in the frame of a mechano-elastic model

In this paper we study the cluster formation and evolution in spin crossover systems during the thermal transition in the frame of a mechano-elastic model applied to open boundary hexagonal lattices. The switching processes between the high-spin (HS) and low-spin (LS) state are studied by a method combining a Monte Carlo standard procedure on the spin state and the lattice relaxation. In the present study, we adopt the transition probabilities of the spin state taking into account the energy gap between the two states, the degeneracy ratio and the local pressure determined by the elongations of the closest springs. It is found that clusters of molecules in the same state tend to grow starting from corners, as in available experimental data. Some qualitative differences between the processes of cluster formation for the two hysteresis branches, i.e., HS to LS and LS to HS are pointed out. Moreover, we have studied the dependence of cluster formation on the strength of the elastic interactions, and also on the system size. The size dependence of the ratio between the system size and the maximum cluster length is very weak, which indicates the appearance of macroscopic domains.

Intrinsic effects of the boundary condition on switching processes in effective long-range interactions originating from local structural change

Masamichi Nishino, Cristian Enachescu, Seiji Miyashita, Kamel Boukheddaden, and François Varret Computational Materials Science Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Japan Department of Physics, Alexandru Ioan Cuza University, Iasi, Romania Department of Physics, Graduate School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo, Japan Groupe d’Etudes de la Matière Condensée, CNRS-Université de Versailles/St. Quentin en Yvelines, 45 Avenue des Etats Unis, F78035 Versailles Cedex, France CREST, JST, 4-1-8 Honcho Kawaguchi, Saitama, 332-0012, Japan (Dated: December 29, 2009)

Dynamical process for switching between the metastable ordered magnetic state and the nonmagnetic ground state in a photoinduced phase transition

We propose a dynamical mechanism for the two-way switching between the metastable state and the stable state, which has been found in experiments of photoinduced reversible magnetization and photoinduced structural phase transitions. We find that the two-way switching by a nonsymmetry-breaking perturbation such as illumination is possible only in systems with appropriate parameters. We investigate the relaxation dynamics with two kinds of transition probabilities by a mean-field theory and a Monte Carlo method. We make it clear that the existence of two time scales in the dynamical process is important for the two-way switching and we actually demonstrate stable two-way switching.

Threshold phenomena under photoexcitation of spin-crossover materials with cooperativity due to elastic interactions

Photo-induced switching from the low-spin state to the high-spin state is studied in a model of spin-crossover materials, in which long-range interactions are induced by elastic distortions due to different molecular sizes the two spin states. At a threshold value of the light intensity we observe nonequilibrium critical behavior corresponding to a mean-field spinodal point. Finite-size scaling of the divergence of the relaxation time is revealed by analysis of kinetic Monte Carlo simulations.

Effective interaction range in the spin crossover phenomenon: Wajnflasz and domain models

The importance of cooperative interaction has been pointed out in the spin crossover phenomena. Sorai and co-workers have proposed a so-called domain model, in which the degree of correlation is expressed by a phenomenological parameter characterizing the sharpness of the transition. Wajnflasz has introduced a pioneering statistical model, in which the microscopic interaction and the entropy effect are considered. In the present work, we study the relation between these two models. In particular, we derive the analytical relation between the effective interaction range of the domain model and the microscopic parameters of the Wajnflasz model.

Magnetic Properties and Metastable States in Spin-Crossover Transition of Co–Fe Prussian Blue Analogues

The combination of spin transitions and magnetic ordering provides an interesting structure of phase transitions in Prussian blue analogues (PBAs). To understand the structure of stable and metastable states of Co–Fe PBA, it is necessary to clarify free energy as a function of magnetization and the fraction of the high-temperature component. Including the magnetic interaction between high-temperature states, we study the magnetic phase transition of Co–Fe PBA in addition to spin transitions. Here, we take into account the degeneracy changes due to charge transfer between Co and Fe atoms accompanying the spin transition. In this study, the charge transfer between Co and Fe atoms is explicitly taken into account and also the ferrimagnetic structure of Co–Fe PBAs is expressed in the proper way. First, we found systematic changes in the structures of stable and metastable states as functions of system parameters using mean field theory. In particular, the existence of a metastable magnetic-ordered high-temper...

Local magnetic structures induced by inhomogeneities of the lattice in S = 1 2 bond-alternating chains and their response to a time-dependent magnetic field with noise

We study the effect of inhomogeneities of the lattice in the

New type of ordering process with volume change of molecules in the spin-crossover transition, and its new aspects of dynamical processes

S=1/2