J. Linares

Numerical Investigations of the Thermal, Pressure and Size Effects on 2D Spin Crossover Nanoparticles

In the framework of the Ising-like model, the thermal and pressure effects on the spin crossover systems are evaluated through two-states fictitious spin operators σ with eigenvalues and respectively associated with the low-spin (LS) and high-spin (HS) states of each spin-crossover (SCO) molecule. Based on each configurational state, the macroscopic SCO system, is described by the following variables: m=Σ σi , s=Σ σi σj and c=Σ σk standing respectively for the total magnetization, the short-range correlations and surface magnetization. To solve this problem, we first determine the density of macrostates d[m][s][c], giving the number of microscopic configurations with the same m, s and c values. In this contribution, two different ways have been performed to calculate this important quantity: (i) the entropic sampling method, based on Monte Carlo simulations and (ii) a new algorithm based on specific dynamic programming. These two methods were tested on the 2D SCO nanoparticles for which, we calculated the average magnetization taking into account for short-, long-range interactions as well as for the interaction between surface molecules with their surrounding matrix. We monitored the effect of the pressure, temperature and size on the properties of the SCO nanoparticles

2D Spin Crossover Nanoparticles described by the Ising-like model solved in Local Mean-Field Approximation

Some six-coordinate iron (II) coordination compounds exhibit thermal-, optical-, electrical-, magnetic- and pressure-induced switching between the diamagnetic low-spin (LS, S=0) and the paramagnetic high-spin (HS; S=2) states [1]. This may lead to potential application of these complexes in molecular devices such as temperature and pressure sensors [2]. An Ising-like model has been proposed to explain the occurrence of the thermal hysteresis behaviour [3,4] of this switchable solids. In this contribution, the local mean field approximation is applied to solve the Hamiltonian modelling interactions pertaining to 2D nanoparticles embedded in a magnetically-inactive matrix

Simulations of Edge Effect in 1D Spin Crossover Compounds by Atom-Phonon Coupling Model

We used the atom-phonon coupling model to explain and illustrate the behavior of a linear nano-chain of molecules. The analysis of the system’s behavior was performed using Free Energy method, and by applying Monte Carlo Metropolis (MCM) method which take into account the phonon contribution. In particular we tested both the MCM algorithm and the dynamic-matrix method and we expose how the thermal behavior of a 1D spin crossover system varies as a function of different factors. Furthermore we blocked the edge atoms of the chain in its high spin state to study the effect on the system’s behavior

Monte Carlo - Metropolis Investigations of Shape and Matrix Effects in 2D and 3D Spin-Crossover Nanoparticles

The Ising like model, taking into account short-, long-range interaction as well as surface effects is used to investigate size and shape effects on the thermal behaviour of 2D and 3D spin crossover (SCO) nanoparticles embedded in a matrix. We analyze the role of the parametert, representing the ratio between the number of surface and volume molecules, on the unusual thermal hysteresis behaviour (appearance of the hysteresis and a re-entrance phase transition) at small scales.

Monte Carlo entropic sampling applied to Ising-like model for 2D and 3D systems

In this paper we present the Monte Carlo entropic sampling (MCES) applied to an Ising-like model for 2D and 3D system in order to show the interaction influence of the edge molecules of the system with their local environment. We show that, as for the 1D and the 2D spin crossover (SCO) systems, the origin of multi steps transition in 3D SCO is the effect of the edge interaction molecules with its local environment together with short and long range interactions. Another important result worth noting is the co-existence of step transitions with hysteresis and without hysteresis. By increasing the value of the edge interaction, L, the transition is shifted to the lower temperatures: it means that the role of edge interaction is equivalent to an applied negative pressure because the edge interaction favours the HS state while the applied pressure favours the LS state. We also analyse, in this contribution, the role of the short- and long-range interaction, J respectively G, with respect to the environment interaction, L.

Non-Destructive Characterization by Spectroscopic Ellipsometry of Interfaces in Mechatronic Devices

Abstract Failures often originate at the interfaces in the packaging of mechatronic devices because of the differences in the thermo-mechanical properties of the materials in contact. To enhance the reliability of these devices, it is necessary to have nondestructive analysis techniques such as spectroscopic ellipsometry (SE) to probe the quality of the surfaces and interfaces when the environmental constraints vary. In the field of characterization spectroscopy, SE has become indispensable in microelectronics, as well as in the study of semiconductors, protective coatings based on polymers, metals or other types of metamaterials. It is used to characterize thin films, mono- or multi-layers and bulk materials from a structural and optical point of view by probing transitions whether electronic, vibrational or rotational. In the ultraviolet (UV)- visible and near- to mid-infrared (IR) ranges, which correspond, respectively, to the electronic and vibrational absorption, SE reveals the composition, structure (amorphous or crystalline), porosity and morphology (density, roughness, etc.) of materials as a function of the light wavelength. In order to be used, SE parameters generally require an inverse method based on the simplex, the Levenberg–Marquardt or the Broyden– Fletcher–Goldfarb–Shanno algorithm to determine the dielectric function or complex optical constants. This chapter describes the SE technique and illustrates its application with two examples of characterization, that of sintered silver and polymers present in a mechatronic device. A study of the effects of temperature in dry and wet conditions is also presented and discussed in terms of optical properties.

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.