Mechanical-tuning of the cooperativity of SC particles via the matrix crystallization and related size effects

We investigate the composites formed by a volume-changing material with a first-order spin transition [FeIII(3-OMeSalEen)2]PF6, when incorporated into a matrix of 1-butanol with a combination of calorimetric and magnetic measurements. DSC evidences the multiple phase transitions of the composite and the experimental conditions to select for promoting a possibly double phase transition of 1-butanol (crystallization) on both sides of the spin transition. Cycling the compound in the liquid matrix preserves the spin-transition properties of the reference with a 1 K wide hysteresis. In contrast, the crystallization around low-spin (high-spin) particles leads to changes in the heating (cooling) branch resulting in a non-persistent (persistent) hysteresis and a positive (negative) temperature shift, as a consequence of the environmental pressure experienced by the particles. The relaxed hysteretic regime that requires the crystallization of butanol around the high-spin particles of higher volume was studied by first-order reversal curves (FORCs) technique. The particular shape of FORCs shows a cooperative mechanism and the absence of the reversible component that was previously assigned to particles-matrix elastic interactions. The first-order spin transition of a single-crystal was characterized with X-ray diffraction measurements in presence of a thin layer of 1-butanol. The temperature dependence of the volume and unit-cell parameters reproduces the changes characterizing the matrix solidification around an ensemble of low-spin (butanol coating) or high-spin (paratone coating) particles. It is also shown that the increase of cooperativity resulting from the mechanical interactions taking place in the particle-matrix system can partly counterbalance the loss of cooperativity observed when reducing the size of the spin-transition materials