The project is devoted to the interaction with the light of the nanoscale thin films, and the time dependence of subsequent processes.

The coupling with phonons will be analysed in the framework of a new model, including the dynamic features of the crystal packing. This work brings the photo-switching of materials into a new perspective, different from the common approach, focused on electron and optical phonon dynamics at ultrafast timescales.

The aim of this project is to understand the switching and controlling the spins in molecular magnets in dynamic phenomena at nanoscale, to better handle and apply the properties of bistability and switching in devices using these materials.

The long-term perspective consists in clarifying the possibilities of their application for the high-density storage of information, or for their use as sensors and actuators. The systematic experimental studies will be correlated and interpreted by the simultaneous use of theoretical and numerical studies. The theoretical studies will be based on the experimental data and will be used not only to explain the observations, but also to propose new experimental procedures.

The central ideas of this scientific project are based on the fact that (i) the behaviour of nanoparticles embedded in various matrices or prepared as thin layers is different when comparing them to isolated micro and nanoparticles; (ii) the physics of ultrafast processes triggers a complex out-of-equilibrium dynamics involving different mechanisms occurring at different time and length scales^, different than the ones observed under nanosecond irradiation. The understanding of the dynamical SC mechanisms is yet in its infancy, in both experimental (e.g. using optical microscopy and by ultrafast experiments), and theoretical details, the current project taking the challenge of these open questions.

 (Objective 1) O1 Study of ultrafast phenomena in spin crossover systems

Experimental activities for objective 1

AE1.1 Synthesis of spin crossover systems of different sizes and shapes

AE1.2 Finding the optimum photoexcitation conditions (temperature, material amount etc.) for photomagnetic and optical measurements

AE1.3 Magnetic and optical measurements of photoexcitation and relaxation after photoexcitation of spin crossover nanoparticles and microparticles with different elastic properties (cooperativity)

AE1.4 Size and shape effects studies including photoexcitation, relaxation and light induced thermal hysteresis

AE1.5 Measurements of minor hysteresis loops during hysteresis induced by the light in order to find experimental FORC distributions followed by distribution analysis for understanding the dynamical change of interactions

AE1.6 Study of the combined roles of the external and environmental pressure for nanoparticles embedded in polymer or surfactant on the photoexcitation and relaxation

Theoretical activities for objective 1

AT1.1 Complex study of ultra-fast photoexcitation and relaxation using the elastic pattern, considering a combination of molecular and stochastic dynamics

AT1.2 Study of domain propagation during elastic stage for 2D and 3D samples, adjusting different relative time scales for spin dynamics and network relaxation

AT1.3 Preliminary studies for anisotropic effects using Ising-like models

AT1.4 Studies of effects to non-volatile transitions beyond transient states via the new developed elastic models considering various crystalline symmetry to predict elastic conversion time

(Objective 2) O2 Study of SC thin nanolayers deposited on substrates

Experimental activities for objective 2

AE2.1. The study of the thermal behavior based on magnetic and optical measurements on spin-crossover monolayers deposited on substrates with a priori fundamentally different interactions with the molecules, i.e. graphite, gold, and copper

AE2.2 Study of the effects of light irradiation as a function of the substrate in order to predict how the light affects the molecular correlations

AE2.3 Apply the FORC method to study interactions distributions as a function of the substrate

AE2.4 Study of the light effects on spin crossover thin films with ferromagnetic substrates

Theoretical activities for objective 2

AT2.1 Modelling the interactions with the substrate by a harmonic potential, using a triangular network of molecules

AT2.2 Elaborate and apply a new elastic model for various crystalline structures; comparison between the new model and the mechanoelastic model; check the dependence on the size and shape of the sample, and find pertinent parameters (spring constants, lattice parameter of the periodic anchoring points on the substrate)

AT2.3 Study how the cooperativity is modified by the molecule-substrate interactions, starting from a free-standing layer and increasing step-by-step the substrate coupling constant.

AT2.4 Quantify the cooperativity by measuring the molecular correlations, as in the experimental case, by calculating the local pressure and probability of switching in different neighboring environments (LS or HS or mixed states) for a direct comparison with experiments