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.
  • Analysis of first order reversal curves in the thermal hysteresis of spin-crossover nanoparticles within the mechanoelastic model
    L. Stoleriu, A. Stancu, P. Chakraborty, A. Hauser and C. Enachescu
    Journal of Applied Physics, 117B (2015), p307
    DOI:10.1063/1.4914953 | unige:55062 | Article HTML | Article PDF
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 is the size of the nanoparticles and another is 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.
  • Thermal hysteresis in spin-crossover compounds studied within the mechanoelastic model and its potential application to nanoparticles
    C. Enachescu, P. Chakraborty, L. Stoleriu, A. Stancu and A. Hauser
    Physical Review B, 84 (13) (2011), p134102
    DOI:10.1103/PhysRevB.84.134102 | unige:17482 | Abstract | Article PDF
 
The recently developed mechanoelastic 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.
  • Study of the relaxation in diluted spin crossover molecular magnets in the framework of the mechano-elastic model
    C. Enachescu, L. Stoleriu, A. Stancu and A. Hauser
    Journal of Applied Physics, 109 (7) (2011), p711
    DOI:10.1063/1.3556702 | unige:17481 | Abstract | Article HTML | Article PDF
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.
  • Competition Between Photo-Excitation and Relaxation in Spin Crossover Complexes in The Frame of a Mechano-Elastic Model
    C. Enachescu, L. Stoleriu, A. Stancu and A. Hauser
    Physical Review B, 82 (2010), p104114
    DOI:10.1103/PhysRevB.82.104114 | unige:14682 | Abstract | Article PDF
In this paper we use a recently proposed elastic model in order to study the competition between linear photoexcitation and cooperative relaxation in spin-crossover molecular magnets. The difference in molecular size between the two possible spin states, that is, the high-spin and the low-spin states, respectively, induces distortions of the crystal lattice. These determine the elastic interactions between molecules, treated here as connecting springs that are either compressed or extended from their equilibrium length, thus modulating the local probability for the high-spinlow-spin relaxation. The crossover of individual molecules within the lattice is checked by a standard Monte Carlo procedure. Using very simple assumptions and a minimum number of parameters, photoexcitation curves and hysteresis loops under continuous irradiation below the thermal transition temperature can thus be simulated. The formation of clusters is analyzed and the presence of inhomogeneities in the system is investigated.
  
  • Cluster evolution in spin crossover systems observed in the frame of a mechano-elastic model
    C. Enachescu, M. Nishino, S. Miyashita, L. Stoleriu, A. Stancu and A. Hauser
    Europhysics Letters, 91 (2) (2010), p27003
    DOI:10.1209/0295-5075/91/27003 | unige:14746
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.
The relaxation in a spin transition compound is modeled on the basis of molecules interacting by theway of connecting springs and situated in a bidimensional open boundary hexagonal lattice. The switch ofindividual molecules is randomly checked using a standard Monte Carlo procedure. The switchingprobability depends on the energy gap between the two states in the absence of interactions and on theelongations of the nearest springs. The main characteristics of the experimental relaxation curves arereproduced and clustering and nucleation phenomena are detected.

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