Daniel Chiruta

Lattice architecture effect on the cooperativity of spin transition coordination polymers

We have investigated in the framework of the Ising-like model, by means of Monte Carlo Metropolis method with open boundary condition, the architecture effect on the cooperativity of spin transition coordination polymers. We have analyzed the influence of several physical parameters (size, pressure, and edge effects) on different lattice architectures which were in good agreement with reported experimental data. We show that the cooperativity of a spin crossover system, characterized by the same number of molecules and the same short- and long-range interaction parameters, is progressively enhanced when going from a 1D chain to a 1D ladder type lattice and to a 2D square lattice.

Analysis of long-range interaction effects on phase transitions in two-step spin-crossover chains by using Ising-type systems and Monte Carlo entropic sampling technique

The analysis of two-step spin crossover phenomena in one-dimensional systems is performed in the framework of Ising-type model by using Monte Carlo entropic sampling technique. Both short-range and long-range interactions are considered in the Hamiltonian which also takes into account different degeneracies between the molecular states. The density of states associated to each macrostate is computed by using entropic sampling, via a biased Monte Carlo sampling technique, and the corresponding Ising-type Hamiltonian can thus be solved by using a self-consistent approach. By using this semi-analytical method, the effects of long-range interactions on phase transition driven by temperature variation in spin crossover chains have been investigated. Various types of spin crossover phenomena have been identified from gradual two-step transition to the one accompanied by multiple hysteresis loops, which was also recently observed experimentally. A metastable “plateau” has been observed at a high-spin fraction ap...

Role of open boundary conditions on the hysteretic behaviour of one-dimensional spin crossover nanoparticles

In order to explain clearly the role of the open boundary conditions (OBCs) on phase transition in one dimensional system, we consider an Ising model with both short-range (J) and long-range (G) interactions, which has allowed us to study the cooperative nature of spin-crossover (SCO) materials at the nanometer scale. At this end, we developed a transfer-matrix method for one-dimensional (1D) SCO system with free boundary conditions, and we give numerical evidences for how the thermal spin transition curves vary as a function of the physical parameters (J, G) or an applied pressure. Moreover for OBCs case, we have derived the bulk, surface and finite-size contributions to the free energy and we have investigated the variation of these energies as function of J and system size. We have found that the surface free energy behaves like J⟨σ⟩2, where ⟨σ⟩ is the average magnetization per site. Since the properties of the nanometric scale are dramatically influenced by the systems size (N), our analytical outcom...

Analysis of Architecture Effect on Hysteretic Behavior of 3-D Spin Crossover Nanostructures

We have analyzed the hysteretic properties of thermal and pressure induced low spin (LS)↔high spin (HS) phase transition of a 3-D spin crossover (SCO) nanostructure when the cluster architecture is progressively changed from a cubic to a thin film. For solving the 3-D Ising-like Hamiltonian, the Monte Carlo metropolis technique is used, with the edge molecules trapped in HS state. The role of short- and long-range interactions on the SCO cooperativity has been investigated as well. The obtained results show that the cooperativity of a 3-D SCO system, characterized by the same number of molecules and the same interactions strength are strongly affected when going from a cubic 3-D to thin-film architecture. These results are discussed in terms of surface effects.

Monte Carlo Method Applied to the ABV Model of an Interconnect Alloy

A Monte Carlo (MC) simulation of a 2D microscopic ABV (metal A, metal B and void V) Ising model of an interconnect alloy is performed by taking into account results of Finite Element methods (FEM) calculations on correlated void-thermal effects. The evolution of a homogeneous structure of a binary alloy containing a small percentage of voids is studied with temperature cycling. The diffusion of voids and segregation of A type or B type metals is a function of the relative interaction energy of the different pairs AA, BB, AB, AV and BV, the initial concentrations of A, B and V and local heating effect due to the presence of clusters of voids. Voids segregates in a matrix of A type, of B type or AB type and form large localized clusters or smaller delocalized ones of different shapes.

Analysis of hysteretic spin transition and size effect in 3D spin crossover compounds investigated by Monte Carlo Entropic sampling technique in the framework of the Ising-type model

In spin crossover (SCO) systems, the shape of the hysteresis curves are closely related to the interactions between the molecules, which these play an important role in the response of the system to an external parameter. The effects of short-range interactions on the different shape of the spin transition phenomena were investigated. In this contribution we solve the corresponding Hamiltonian for a three-dimensional SCO system taking into account short-range and long-range interaction using a biased Monte Carlo entropic sampling technique and a semi-analytical method. We discuss the competition between the two interactions which governs the low spin (LS) - high spin (HS) process for a three-dimensional network and the cooperative effects. We demonstrate a strong correlation between the shape of the transition and the strength of short-range interaction between molecules and we identified the role of the size for SCO systems.

Size, shape and temperature effects on Ferro/Antiferro-electric hysteresis loops from Monte Carlo simulations on 2D Ising model

A Monte Carlo (MC) simulation of a 2D microscopic Ising model is performed to study the relation between electric and thermal properties in a 2D lattice which exhibits Ferro-Electric (FE) and Anti-Ferro-Electric (AFE) coupling. The main purpose is to determine how the size and the shape of these crystals affect this relation at different temperatures. The different effects are discussed as a function of the relative strength of two parameters the Ferro and Anti-Ferro interaction constants for different applied cycling external electric field.