Nanocomposites and nanomaterials

The bistability in spin-crossover nanocrystals with (anti)ferromagnetic surface
Gudyma Iu.V.1, Maksymov A. Iu.2 
1 Department of General Physics, Yuriy Fedkovych Chernivtsi National University, Kotsyubynsky 2, 58012 Chernivtsi, Ukraine. 
E-mail: yugudyma@gmail.com
2 Marian Smoluchowski Institute of Physics, Jagiellonian University, Stanislawa Lojasiewicza 11, 30-348 Krakow, Poland.

The pseudo-octahedral transition metal coordination complexes with ligand environment may undergo spin-crossover (SCO) transitions from a diamagnetic low-spin (LS) to a paramagnetic high-spin (HS) states under the effect of a weak external perturbation. The molecules in HS state are characterized by larger volume and higher effective degeneracy than the ones in the LS state. The size difference between the HS and LS lead to an effective long-range elastic interaction, responsible for inducing the first-order phase transition. 
The microscopic processes in SCO compounds, that occur at the inter-molecular level are studied by the magnetization dynamics that describe the behavior of HS molecules. The mesoscopic model under consideration is based on Ising-like Hamiltonian of the following form:
,			(1)
where h0 is external effective field and  is an intersite coupling of ferromagnetic or antiferromagnetic nature describing the interactions of bulk sites (), surface sites () and bulk-surface sites () [1].
Basing on the model (1) the properties of size effects for ferromagnetic and antiferromagnetic surface has been analyzed in detail. It was found that the nature of cooperativity for sites on the surface drastically impacts the completeness of phase transition. In the system with antiferromagnetic coupling of the sites on the surface the temperature transition curves show a premature saturation with smaller magnetization value for HS state and greater one for LS configuration.
The obtained numerical results show very good agreement with the experimental studies.

1.	Gudyma Iu., Maksymov A. The cooperativity in 3D spin-crossover nanocrystals with ferromagnetic and antiferromagnetic surface // Appl Surf Sci.-2019.-483, N 5.-P. 779-784.