Structural synergy in a core-shell spin crossover nanoparticle investigated by an electroelastic model

International audienceUnderstanding how surrounding environments act on the functional properties of switchable nano-objects across extended and multiple length scales is of growing interest in many areas of material science. Here, we examine in details, using a microscopic model, the interplay between the structural properties of an inert shell and a spin-active spin-crossover (SCO) core, composed of atoms which can switch thermally between the low-spin (LS) and high-spin (HS) states, a transition which is accompanied with a volume expansion. To come closer to realistic experimental data, we considered a shell having the lattice parameter of the HS state. Intensive Monte Carlo simulations, running on the spin states and atomic positions, are performed on the core-shell spin-crossover nanoparticle using an electroelastic model based on a compressible 2D lattice. A detailed analysis of the effect of the shell's size and rigidity on the magnetostructural properties of the core allows us to address the following issues: (i) the heteroelastic properties of the lattice induce a spatially inhomogeneous pressure (negative in the shell and positive in the core) which strongly distorts the lattice when the core is in the LS state, creating a visible spatial deflection of the shell/core interface; (ii) the thermally-induced first-order SCO transition of the core is significantly affected by the increase of the shell size, which lowers the transition temperature and reduces the thermal hysteresis width; (iii) the shell's rigidity dependence of the thermal hysteresis of the nanoparticle exhibited a resonance behavior when the shell's rigidity equals that of the core, a feature that is analyzed on the basis of acoustic impedance mismatch between the core and the shell. All these outcomes reflect the crucial influence of the surrounding environment on the structural properties of the nanoparticle and provide potentialities in the control of the bistability and cooperativity of the SCO nanoparticles by acting on their shell's rigidity