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

Abstract

Understanding 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.

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

“…In addition, even if the concepts of thermodynamics can be still used at small sizes, strictly speaking the notions of “phase equilibrium” and “phase transition” vanish and for further size reduction the thermodynamic description completely fails . Some of these issues have been addressed more deeply for SCO systems using different microscopic models, including the role of particle shape, surface relaxations, and the particle environment . For the dominant role of this latter in the following we discuss matrix/interface effects in more details.…”

“…c) Snapshot of the pressure distribution in an SCO active core@inactive shell particle in the LS state calculated by means of an electro‐elastic model. Reproduced with permission . Copyright 2017, the American Physical Society.…”

“…As a matter of fact, the mechano‐elastic model of Enachescu (vide supra) can be considered as a first variant of the different models, which were developed with the common idea of analyzing the lattice misfit and the resulting changes of elastic energy in order to account for the phase stability and different physical properties of the core and the shell. Boukheddaden and co‐workers used extensive Monte Carlo simulations in combination with the well‐known Ising‐like model and later on using more sophisticated electro‐elastic (spin‐phonon) models as well . This powerful approach allowed them to analyze the different manifestations of the coupling between the core and the shell (shift of the transition temperature, change of the hysteresis width, emergence of residual fractions, acceleration of the kinetics), the role of the particle geometry (core and shell thickness, shape, etc.…”

“…Overall, the most encouraging (and somewhat unexpected) experimental result obtained with these different nanoscale SCO materials is that the hysteresis and a virtually complete spin transition can be preserved even in very small objects (<5 nm) . On the other hand, experiment revealed a range of different behaviors (vide infra), which have not yet been fully rationalized, despite the development of several thermodynamical, continuum mechanics, and statistical physics approaches to this aim. This problem will be addressed in the second part of this review, while the third part will be devoted to examples of successful integration of SCO nanomaterials into devices.…”

Role of Surface Effects in the Vibrational Density of States and the Vibrational Entropy in Spin Crossover Nanomaterials: A Molecular Dynamics Investigation

“…Both experimental and theoretical investigations of size reduction effects have pointed out the strong influence of surface properties, the nature of the interface as well as the impact of the external environment on the switching properties on SCO nanomaterials [18][19][20]. In particular, spin transition phenomena in coreshell nanoparticles suggest a possible control of phase stability and spin transition curves through a modulation of elastic/mechanical stress at the interface [21][22][23][24][25][26][27][28][29]. Therefore, the knowledge of both mechanical and vibrational properties of SCO materials as well as their evolution with the size reductions are of paramount importance in order to understand the spin transition phenomenon at the nanoscale [30].…”