Elastic-frustration-driven unusual magnetoelastic properties in a switchable core-shell spin-crossover nanostructure

Spin-crossover (SCO) solids have been studied for their fascinating properties, exhibiting first-order phase transitions and macroscopic bistabilities, accompanied by significant magnetic, structural, and optical changes. These exceptional properties make these materials promising for applications as high-density information storage and optical switches. Recently, the critical progress made in chemistry allowed the design of spin-crossover nanocomposites, combining the properties of two types of spin-crossover solids having different properties, like different lattice parameters, bulk moduli, transition temperatures, ligand fields, etc. In this paper, we consider a microscopic electroelastic description of a SCO nanostructure made of a SCO active core surrounded by a SCO active shell, for which we impose an unconventional elastic frustration at the core-shell interface. The detailed examination of the thermodynamic properties of such a nanocomposite, as a function of the lattice parameter misfit between the two constituents, revealed that the frustration causes unexpected behaviors on the thermal dependence of the average bond lengths, such as the emergence of two- or three-step spin transitions, with self-organization of the spin states in the plateau regions. These results highlight the nontrivial character of the magnetoelastic properties in switchable SCO nanoparticles.