Iurii Gudyma

Diffusionless phase transition with two order parameters in spin-crossover solids

The quantitative analysis of the interface boundary motion between high-spin and low-spin phases is presented. The nonlinear effect of the switching front rate on the temperature is shown. A compressible model of spin-crossover solid is studied in the framework of the Ising-like model with two-order parameters under statistical approach, where the effect of elastic strain on interaction integral is considered. These considerations led to examination of the relation between the order parameters during temperature changes. Starting from the phenomenological Hamiltonian, entropy has been derived using the mean field approach. Finally, the phase diagram, which characterizes the system, is numerically analyzed.

Study of pressure influence on thermal transition in spin-crossover nanomaterials

The thermal transition accompanied by the variation of the molecular volume in nanoparticles of spin-crossover materials has been studied on the basis of microscopic Ising-like model solved using Monte Carlo methods. For considered model, we examined the spin-crossover phenomenon with applied hydrostatic pressure and thus was shown the possibility to shift transition temperature toward its room value. The obtained results of numerical simulations are in agreement with the experimental ones.

Kinetics of Nonequilibrium Transition in Spin-Crossover Compounds

This chapter is devoted to an analysis of spin-crossover system dynamics in the framework of Ising-like mechanoelastic model and macroscopic phenomenological model. Based on mechanoelastic model, the intermolecular interaction between spin-crossover sites and cooperative effects in materials has been studied. For light-induced spin-transition the relaxation curves has been calculated by Monte Carlo methods. The additive and multiplicative noise influence on transition phenomena following from system contact with environment was found within macroscopic description of spin-crossover system dynamics in Langevin framework and the corresponding Fokker-Planck equation. The mean first passage time from metastable state driven by colored noise was calculated by using the Kramers-like approximation. In addition, it is shown that transitions can be controlled by a parameter that governs relaxation flow, intensity of light and noise intensity.

Optically induced switching in spin-crossover compounds: microscopic and macroscopic models and their relationship.

We present the examination of light induced behavior of spin-crossover compounds. We focus on the switching phenomena between low-spin and high-spin states of the system. The connection between the Ising-like model and the macroscopic evolution equation has been analyzed. By means of numerical simulations, we found the dynamical potential changes, the corresponding light induced hysteresis, and the transient regimes. The statistical properties of the system in contact with a heat bath are shown in a stationary probability distribution.

Surface and Size Effects in Spin-Crossover Nanocrystals

We perform Monte Carlo simulations to analyze the surface and size effects in spin-crossover nanocrystals using an Ising-like model including surface and core intermolecular interactions. The consequences of downsizing effect on the transition temperature and the width of hysteresis as finger of the system cooperativity are discussed. The critical temperature is calculated using the real-space renormalization method. The obtained results are in agreement with the experimental data.

Spin-Crossover Molecular Solids Beyond Rigid Crystal Approximation

The qualitative analysis of the spin-crossover molecular solid with distortion effect is presented. A spin-crossover solid with effect of distortion is studied in the framework of the Ising-like model with two-order parameters under statistical approach, where the effect of elastic strain on inter-ion interaction is considered. These considerations lead to examination of the relation between the primary and secondary order parameters during temperature and pressure changes.

Light-induced hysteresis in spin crossover compounds under noise

We investigate the light induced optical hysteresis of spin-crossover compound driven by Gaussian noise. The stationary states was found from dynamic potential of noiseless system described in terms of Lyapunov functions. Cooperativity causes bistability of the steady state and provokes light-induced thermal and optical hysteresis. We study the influence of noise action on cooperative effects of photoexcited spin-crossover compound, leading to changes of system hysteresis. The stochastic dynamics of spin-crossover system was described following the Langevin formalism for noise driven system.

Cooperative Phenomena in Spin-Crossover Molecular Crystals

In this chapter, we focus on the study of influence of surface on thermal-induced spin-crossover phenomenon which is a subject of a broad and current interest. The modified Ising-like model of spin-crossover solids with the ligand field as function of the molecule positions and random component on the surface has been proposed. By means of the adapted Metropolis Monte Carlo algorithm, the thermal spin transition curves were calculated. The detailed kinetics of switching between high-spin and low-spin molecules have been analyzed from spin configuration at transition temperature, which gives a general idea about contribution of molecules from the surface and inside the lattice into resulting magnetization of the systems. The behavior of hysteresis loop for various surface coupling and fluctuation strengths has been described.

Modeling Problems of Spin Crossover Nanocrystals

The most important approaches which describe the behavior of spin crossover compounds have been reviewed. Thermodynamic consideration of spin crossover phenomenon is one of the most general able to describe theoretically almost any kind of transition in spin crossover system. Microscopic Ising-like model makes it possible to divide the processes that occur in the magnetic molecules and cooperative processes with the aim of highlighting the nature of these phenomena. Ising-like approach is a versatile way to describe analytically and numerically the properties of spin crossover materials despite the simplified consideration of intermolecular interaction. Additionally, discussion of the specific phenomenological model of light-induced transition in spin crossover materials is given.

Phenomenological Models of Photoinduced Transition in Spin-Crossover Materials

The stochastic kinetics of photoinduced transition in spin-crossover systems, as representatives of synthetic molecular magnetic materials, was reviewed. The focus has been done on the macroscopic phenomenological models, which were described by dynamic potential in terms of Lyapunov functions and obeys to Langevin kinetics with white and colored stochastic processes. By corresponding Fokker–Planck equation was studied the evolution of probability of system states. For the system with light-induced transition which is additionally driven by external periodic force it has been shown the possibility of resonance enhancement of magnetization at certain values of the amplitude of periodic signal and noise intensity. The stochastic resonance phenomenon was analyzed by signal-to-noise ratio for Markovian and non-Markovian noise, various amplitudes and frequencies of periodic signal, and also variation of noise intensity.