Analysis of First Order Reversal Curves in the Thermal Hysteresis of Spin-crossover Nanoparticles within the Mechanoelastic Model

The recently obtained spin-crossover nanoparticles are possible candidates for applications in the recording media industry as materials for data storage, or as pressure and temperature sensors. For these applications the intermolecular interactions and interactions between spin-crossover nanoparticles are extremely important, as they may be essential factors in triggering the transition between the two stable phases: the high-spin and low-spin ones. In order to find correlations between the distributions in size and interactions and the transition temperatures distribution, we apply the FORC (First Order Reversal Curves) method, using simulations based on a mechanoelastic model applied to 2D triangular lattices composed of molecules linked by springs and embedded in a surfactant. We consider two Gaussian distributions: one of the size of the nanoparticles and one of the elastic interactions between edge spin-crossover molecules and the surfactant molecules. In order to disentangle the kinetic and non-kinetic parts of the FORC distributions, we compare the results obtained for different temperature sweeping rates. We also show that the presence of few larger particles in a distribution centered around much smaller particles dramatically increases the hysteresis width.Comment: 14 pages, 5 figures, 2014 59-th MMM conferenc

PROTECTED SAPROXYLIC COLEOPTERA IN "THE FORESTS IN THE SOUTHERN PART OF THE CÂNDEŞTI PIEDMONT", A ROMANIAN NATURA 2000 PROTECTED AREA

The observations conducted between May and October 2014 in the protected area "The Forests in the Southern part of Cândeşti Piedmont" clearly show three species of protected saproxylic beetles: Lucanus cervus, Cerambyx cerdo and Morimus asper funereus. The Quercus forests, which are dominant in that area, ensure optimal living conditions for the species L. cervus and M. asper funereus, which are common species in this site. Several aspects are presented that concern the period of activity of the individuals, sex ratio, the presence of predators and the distribution map of the species. The species C. cerdo was only found on Quercus sp, and the small number of the individuals counted in the area show that the species does not benefit from favourable development conditions. A number of pressures identified make the rational management of this protected area to be extremely important

Study of the relaxation in diluted spin crossover molecular magnets in the framework of the mechano-elastic model

We model here the behavior of spin transition compounds, considering molecules arranged in a 2D hexagonal lattice and interacting via springs. The role of impurities in the clustering and nucleation phenomena is analyzed, as well as the manner in which the impurities affect the relaxation curves. The switching of the individual molecules is checked using a Monte Carlo procedure. When a molecule changes its state, it also modifies its volume, and the new equilibrium positions of all the molecules are calculated. As in previously reported experiments, it is found here that bigger impurities slow down the relaxation from the metastable high-spin state to the low-spin state, while smaller impurities act in an opposite way. It is shown that if the concentration of the impurities is higher than a certain threshold, then they act as a barrier, trammeling the fast evolution of domains developing from the edges

First order reversal curves diagrams for describing ferroelectric switching characteristics

First Order Reversal Curves (FORC) are polarization-field dependences described between saturation field Esat and a variable reversal field Er∈(-Esat, Esat). The FORC diagrams were proposed to describe some characteristics of the switching process in ferroelectrics. The approach is related to the Preisach model which considers the distribution of the elementary switchable units over their coercive and bias fields. The influence of the anisotropic porosity in Pb(Zr,Ti)O3 bulk ceramics on the FORC distributions demonstrated the existence of a positive/negative bias as a result of the confinement induced by anisotropy. The reducing of grain size in Ba(Zr,Ti)O3 ceramics causes an increase of the ratio of the reversible/irreversible components of the polarization on the FORC distribution indicating the tendency of system towards the superparaelectric state. The FORC method demonstrates to provide a kind of ‘fingerprinting’ of various types of switching characteristics in ferroic systems

Thermal hysteresis in spin-crossover compounds studied within the mechanoelastic model and its potential application to nanoparticles

The recently developedmechanoelastic model is applied to characterize the thermal transition in spin-crossover complexes, with special attention given to the case of spin-crossover nanoparticles. In a two-dimensional system, hexagonal-shaped samples with open boundary conditions are composed of individual molecules that are linked by springs and can switch between two states, namely, the high-spin (HS) and the low-spin (LS) states. The switching of an individual molecule during the spin transition is decided by way of a Monte Carlo standard procedure, using transition probabilities depending on the temperature, the energy gap between the two states, the enthalpy difference, the degeneracy ratio, and the local pressure determined by the elongation or compression of its closest springs. The influence of external parameters, such as temperature sweeping rate and pressure, or intrinsic features of the system, such as the value of its spring constant, on the width of the thermal hysteresis, its shape, and its position are discussed. The particular case of spin-crossover nanoparticles is treated by considering them embedded into a polymer environment, which essentially affects the molecules situated at the edges and faces by decreasing their transition probabilities from HS to LS. Finally, the pressure hysteresis, obtained by varying the external pressure at constant temperature is discussed

Cluster evolution in molecular three-dimensional spin-crossover systems

The nucleation and growth properties of domains of molecules of the same state in open boundary three-dimensional (3D) spin-crossover systems of various shapes are discussed within the framework of the mechanoelastic model. The molecules are situated on face-centered-cubic lattices and are linked by springs through which they interact. Monte Carlo simulations imply that clusters nucleate from corners in the case of systems having well-developed faces and from kinks in the case of spherical samples, in accordance with available experimental data. In addition, a method to characterize the cooperativity in these systems is proposed, which by scanning the fluctuations in the 3D samples can be related directly to powder x-ray-diffraction experiments

Multiscale out-of-equilibrium dynamics driven by pulsed laser excitation in spin-crossover materials: A combined thermoelastic and mechanoelastic study

International audienceIn this paper, we present an elastic model coupled with a heat propagation process in order to reproduce the out-of-equilibrium dynamics of spin crossover materials driven by femtosecond laser excitation: a multiscale out-of-equilibrium dynamics driven by pulsed laser excitation in spin-crossover materials (thermoelastic step), the thermal switching (thermal step), and the subsequent relaxation to the initial state on cooling. The simulations were performed for open boundaries two and three-dimensional samples, composed of individual molecules linked by springs, which stand for elastic interactions. This building-up of the samples allows the propagation of elastic waves, which leads to accumulation of high spin molecules towards edges at the maximum of the thermoelastic step. We first show that a simple model with a single “temperature” reproduces the thermoelastic, the thermal step and the relaxation to the original equilibrium state. However, the too slow thermalization of the lattice obtained in this model does not correspond to the experimental data. Therefore, to overcome this drawback, we consider either an inhomogeneous photoexcitation or different “temperatures” for the lattice and the spin states. The effect of the sample size, which prevents the existence of a thermal step in the case of nanoparticles is also discussed, as well as the three-dimensional model. ©2023 American Physical Society