Spin crossover micro and nanoparticles embedded in various environments: an experimental and theoretical study
(Micro și nanoparticule cu tranzitie de spin încorporate in diverse medii: studiu experimental si teoretic)
Financed by the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project PN-II-RU-TE-2014, no. 151 / 2015
Project Director: Conf. univ. dr. Cristian ENACHESCU
(Faculty of Physics, Alexandru Ioan Cuza University, Iași)
The project is devoted to the study of fundamental properties of nano and micro structured spin-crossover (SC) materials embedded in various media. These compounds, having two spin states with different magnetic and optical properties, are intensely studied due to their potential use in technological applications, as data storage or sensors and actuators. As the molecules also have different volumes in the two states, the SC materials are dominated by elastic interactions which trigger multi-hysteretic behavior induced by temperature, pressure or light. In addition, some micro and nanoparticles of SC materials embedded in polymers or surfactants show a huge hysteresis of mechanical nature. Their behaviour is not only influenced by the environment, but equally, in core-shell structures, the transition of shell SC materials could influence the piezoelectric properties of core materials. Therefore, the SC nanoparticles embedded in various media are interesting for potential applications due to their intrinsic properties but also due to the mechanical influence on environment. The present research will have two main directions: one experimental and the other of modeling. The experimental study of the mentioned compounds will include magnetometric and optical measurements. The experiments will be closely accompanied by a modeling effort using Monte Carlo approaches which will cover the domain from the molecular level up to and nano and microstructures.
The aim of the project is to understand the behaviour of spin crossover micro and nano particles interacting with the environment, in order to handle and apply the properties of the bistability and switching of these materials at meso and nanoscale. The long term perspective consists in clarifying the possibilities of their application for the storage of high density 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 for correcting the experiments and to propose new experimental procedures.
The idea of this scientific project is centred on the study of spin crossover micro and nanoparticles dispersed in various environments, either polymeric for the beginning or piezoelectric in a second stage. This idea is based on the fact that the behaviour of micro and nanoparticles embedded in various matrices is totally different comparing to free micro and nanoparticles. Due to the change of volume during the transition, not only the spin crossover compound is influenced, but also the environment. Therefore, its eventual piezoelectric properties could be controlled by the state of spin crossover compound. The hypotheses suggested by some of our preliminary results show that the effects could depend on the volume change of the particles, on elastic interactions between the particle and the molecules composing the matrices (as van der Waals forces) and on the properties of the matrix (rigidity, density, fragility, viscosity, relaxation dynamics etc)
The experiments will concentrate on the following objectives and activities, oriented towards the study of the thermal, optical and pressure induced bistability:
(OE1) The study of thermal spin transition in various environments
(AE1.1) Magnetic measurements of the hysteresis cycle during thermal transition and comparison of data with those in the case of free nanoparticles (these data are available for some compounds).
(AE1.2) Optical measurements of the thermal transition; finding differences from magnetic data.
(AE1.3) Measurements of the minor cycles and FORCs (First Order Reversal Curves): finding experimental FORC distributions
(OE2) The study of light on the spin transition particles
(AE2.1) Finding the best photoexcitation conditions (temperature, material amount) for photomagnetic and optical measurements
(AE2.2) Relaxation and photoexcitation measurements and comparison of data with free particles
(AE2.3) Study of the light induced thermal hysteresis and comparing the FORC distributions with those in the thermal hysteresis
(OE3) Study of combined effects of external and environmental pressure on spin transition micro and nano particles and the study of pressure hysteresis
(AE3.1) Measuring the thermal hysteresis as a function of pressure using pressure cell in SQUID
(OE4) Study of influence of spin crossover microparticles on the piezoelectric environment
(AE4.1) Conductivity measurements as a function of the spin state using specific SQUID module
Theoretical objectives will consist in modelling experimental data for samples embedded in polymers or disposed in core-shell structures.
(OT1) Analysis using Ising-like model and mean field model
(AT1.1) Use of the mean field model for finding the activation energy, enthalpy and entropy difference between the two states
(AT1.2) Application of the Ising-like model for finding the interaction parameters (both activities will be cyclically realized a function of experiments (see GAND diagram from Methodology).
(OT2) Application of the mechanoelastic model (using parameters from OT1)
(AT2.1) The study of environmental effects on nano and microstructures
(AT2.2) The study of interaction propagation as a function of elastic constants of the environment
(AT2.3) The study of the influence of spin transition particles from the shell on the inside piezoelectric material in core-shell structures.
(OM) Besides scientific objectives, a permanent objective will be the project management (acquiring new devices and consumables, opportune stages and conferences etc.).
Papers (click on the title for the paper)
Enachescu, C; Hauser, A Study of switching in spin transition compounds within the mechanoelastic model with realistic parameters PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Volume: 18, Issue: 30, Pages: 20591-20599, DOI: 10.1039/c6cp02806c, AUG 14 2016
Enachescu, C; Tanasa, R; Stancu, A; Tissot, A; Laisney, J; Boillot, ML Matrix-assisted relaxation in Fe(phen)(2)(NCS)(2) spin-crossover microparticles, experimental and theoretical investigations APPLIED PHYSICS LETTERS, Volume: 109, Issue: 3, Article Number: 031908, DOI: 10.1063/1.4959262, JUL 18 2016
Bertoni, R; Lorenc, M; Cailleau, H; Tissot, A; Laisney, J; Boillot, ML; Stoleriu, L; Stancu, A; Enachescu, C; Collet, E Elastically driven cooperative response of a molecular material impacted by a laser pulse NATURE MATERIALS, Volume: 15, Pages: 606, DOI: 10.1038/NMAT4606, JUN 2016
Cristian Enachescu The mechanoelastic model for spin transition molecular magnets – Euro Asian Conference on magnetism, Krasnoyarsk, Rusia, august 2016 (oral)
Cristian Enachescu Spin crossover molecular magnets: premises for fully optical memories – International Conference of magnetic materials and new technologies, Listvyanka, Rusia, august 2016 (oral)
Cristian Enachescu, Radu Tanasa, Laurentiu Stoleriu, Alexandru Stancu, Antoine Tissot, Jérôme Laisney, Marie-Laure Boillot High spin- low spin relaxation of spin-crossover microparticles modulated by the environment. Theoretical and experimental investigations – GDR Magnetisme et commutation moleculaire, Dourdan Franța, noiembrie 2016 (oral)
Cristian Enachescu, Andreas Hauser, Marie-Laure Boillot, Maciej Lorenc, Eric Collet Understanding experimental data in spin crossover systems using elastic models – Phase Transition and Dynamical properties of Spin Transition Materials, Gandia, Spania, 2016 (Invitat)