In this work we present results obtained by the first-order reversal-curve (FORC) method for finding the effect of pressure on a spin transition. The FORC data were recorded by diffuse reflectivity measurements for the spin-transition compounds [Fe(btr)2(NCS)2]\ifmmode\cdot\else\textperiodcentered\fi{}H2O and [Fe0.6Zn0.4(btr)2(NCS)2]\ifmmode\cdot\else\textperiodcentered\fi{}H2O (btr = 4,4$'${}-bis-1,2,4-triazole) under constant pressure in the range 1\char21{}1600 bars. The joint distributions in coercivity-bias coordinates, obtained by the FORC method, were expressed in energy gap $\Delta${} and interaction parameter J coordinates for a complete discussion of the pressure effect in terms of physical parameters, including their average values, distribution widths, and $\Delta${}-J correlation parameter. Pressure increases both $\Delta${} and J, as expected. Pressure has a negligible effect on the distribution widths but sizably decreases the correlation parameter value of the diluted system, an unexpected feature which suggests that the diluted system has multiple-domain behavior with pressure-dependent domain size. This deduction is supported by inspection of the samples using optical microscopy at room temperature. Simulations of the metal distribution in a two-dimensional lattice are performed in order to estimate the pressure-induced change in domain size.

The smallest size of a spin crossover particle which still exhibits hysteric behavior is an open question. It is clear that spin crossover is a cooperative phenomenon, which implies the dependence of the high spin fraction variation on the number of molecules within the system. We use the “atom-phonon coupling” model to explain the different results that have been recently published with regard to the critical dimension of a spin crossover system. In the second part of the paper, the applicability of the model to the pressure induced transition has been developed, and the results are in good agreement with what has been observed experimentally.

We study the spatio-temporal formation and spreading of the low-spin state (LS) during the thermal spin transition and the cooperative relaxation of the photo-induced metastable high spin (HS) state at low temperature, in the presence of a structural defect. The model is made of a two-dimensional rectangular-shaped lattice with discrete spins coupled by springs. The investigations are performed for a perfect lattice and a lattice with a hole (simulating the defect) with a fixed size. We found that the presence of the defect affects the thermal equilibrium by reducing the size of the thermal hysteresis at the transition, although the transition temperature remains unchanged. The study of the low-temperature relaxation of the defect-free lattice from HS to LS state indicated the existence of three different regimes of the growth process: (i) a first regime of growth from one corner of the rectangle along the width, then followed by (ii) a second regime of longitudinal propagation at almost constant velocity, and (iii) a third rapid regime when the system feels the surface or the border of the crystal. When a hole is injected inside the lattice, it results in (i) the deformation of the HS/LS interface’s shape when it approaches the defect position; and (ii) the slowing down of its propagation velocity. These results, which are in good agreement with available experimental data, are discussed in terms of elastic energy stored in the system during the relaxation process.

The recently developed macroscopic model of light-induced equilibrium of spin-crossover solids is extended to the case of two equivalent sublattices coupled `antiferromagnetically'. To do this, we use an Ising-like model involving two coupling parameters of opposite signs for the intra- and inter-sublattice interactions, previously introduced to describe the two-step static spin transition, and we have adapted the macroscopic master equation describing the time evolution of the system accordingly. We mostly report on the quasi-static solutions of the system, under constant light irradiation and for slow temperature variations. Light-induced thermal hysteresis loops are obtained and have typical two-step shapes for suitable sets of parameter values. The model predicts, in analogy with the static transition, a possible light-induced symmetry breaking between the structurally equivalent sublattices. Kinetic effects are briefly described.

In this work we present a detailed study showing the importance of the Kubelka-Munk (KM) correction in the analysis of diffuse reflectivity measurements to characterize spin crossover compounds. Combined reflectance and magnetic susceptibility measurements are carried out as a function of temperature or time to highlight the conditions under which this correction becomes critical. In particular, we investigate the influence of the color contrast between the two spin states on the reflectance measurements. Interestingly, the samples’ contrast seems to play an important role on the spin-like domain structure as suggested by the symmetry of the FORC diagrams. These latest results are discussed within the framework of Classical Preisach model (CPM).