Institut de Chimie Moléculaire et des Matériaux d'Orsay

Magnetic and photomagnetic hybrid (nano)materials

Anne Bleuzen (Prof.), Amélie Bordage (Researcher), Giulia Fornasieri (Associate Prof.), Eric Rivière (Research engineer, 20%)

Postdocs : Laura Altenschmidt, Adama N'Diaye

PhD student : Gregory Balthazar

The increasing demand for more efficient data storage devices (increased capacity, faster writing/reading processes,...) requires the elaboration of new bi- or multistable materials. In order to integrate them into real applications, we must be able to synthesize them, control their properties at the nanoscale and organize them in a solid phase.

Prussian Blue analogues (PBAs) are coordination polymers built from a tridimensionnal network of metallic ions bonded by a cyanide linkage. These compounds may present a thermo-, piezzo- or photoinduced metal-metal charge transfer between two electronic states and so with different physical properties. They are thus promising materials for high-density data storage applications.

Moreover, thanks to their versatile and adjustable chemical composition as well as the very fine control of the metallic cations organization in the crystalline structure, they are relevant precursors for the preparation of metallic oxides and alloys by a thermal decomposition under controlled atmosphere. We can thus obtain a very wide range of oxides and alloys with exotic compositions that cannot be achieved through other synthesis routes. Such process can allow the preparation of new original phases with improved bi- or multistable properties.

Idealized scheme of the transformation of a PBA into an oxide or an alloy

PBAs are usually obtained as powders. To be able to insert them into applications, it is necessary to control their morphology at the nanoscale during the synthesis process. With the help of a sol-gel process in the presence of structuring agents, it is possible to obtain optically-transparent and chemically-inert matrices with a porosity of well-controlled size and shape. Furthermore, these matrices can also be processed at the macroscale (powders, monoliths, thin films...). We are working on strategies to use this nanostructured oxide porosity as a nanoreactor for i) the synthesis of bi- or multistable nanoparticles and the study of the effects of the processing on the properties (size reduction, confinement effect, interactions with the host matrix...), ii) the synthesis of nanocomposites with new properties originating from the processing (collective behavior of the nanoparticles, anisotropic behaviors caused by their shape and/or their organization, interactive nanocomposites...), and iii) the synthesis of systems enabling the exploration of properties in an application (organization of nanoparticles on a surface or in a matrix).

Each study is performed thanks to the coupling between syntheses, laboratory characterizations and synchrotron experiments.

• Laboratory characterizations : powder XRD, IR spectroscopy, BET, TDA-TGA, SQUID magnetometry, SAXS
• Synchrotron experiments : X-ray absorption spectroscopy at the edges of transition metals and alkali cations (experiments at SOLEIL and at ESRF), transition metal K-edge X-ray magnetic circular dichroism (collaboration with the ODE beamline at SOLEIL)

Axis 1 — Elaboration and fundamental study of bi- and multistable systems

Objective: The elaboration of bi- or multistable systems for data storage applications that can be integrated in a real device (i.e. operating at room temperature, chemically and thermally stable,...)

Some CoFe PBAs present a photomagnetic behaviour. At low temperature, they consist in Co^III-NC-Fe^II diamagnetic pairs. Upon irradiation with visible light, an electron is transferred from the Fe^II cation to the Co^III cation, leading to the formation of ferrimagnetic Co^II-NC-Fe^III pairs with a long lifetime at low temperature (T < 100K). The key parameters for such a photomagnetic behavior are known but a deep understanding and a full control of these properties are still lacking. To address this challenge and so use the PBA in real applications, we aim at chemically controlling the relative stability of the states involved in this property and at transfering their working temperature to room temperature.

We also use the PBAs as precursors of original oxides and alloys with targeted magnetic, ferroelectric or multiferroic properties. The tuning of the oxide or alloy property can be done through the control of its stoichiometry. A promising route is the calcination of PBAs (in oxidizing or reducing atmosphere), since the stoichiometry of the PBA is perfectly controlled and so is the one of the oxide or alloy after thermal treatment. We optimize the calcination conditions to obtain single-crystalline oxides/alloys phases conserving the stoichiometry of the PBA precursor. In order to better understand the formation of the final phase, we investigate in details the calcination mechanism.

Axis 2 — Development of oxide matrices elaborated by sol-gel process for the organization of bi- or multustable nanoparticles

Objective : Organizing the bi- or multistable nanoparticles in a solid matrice (with adjustable properties) thanks to the design flexibility offered by the sol-gel process and combined with the use of structuring agents

The integration of switchable compounds in real applications requires a processing step. Since the switching properties depend on the size, shape and organization of the functional objects, the control of these parameters and the study of their effects on the properties are a crucial prerequisit to their use in devices. The chemistry of nanostructured oxides enables to obtain optically-transparent and chemically-inert matrices with a porosity whose organization, size and shape are perfectly controlled. These matrices can also be processed at the macroscopic scale (powder, monolith, thin films,...)

We thus want to take advantage of this range of control offered by the sol-gel process for the design of switchable materials. To do so, we develop new strategies using the porosity of nanostructured oxides as reactors for the formation of functional nanoparticles.

Axis 3 — (Multi)functional nanomaterials

Objective : Using the porosity of sol-gel matrices to study the processing effects, to develop materials with new (photo)magnetic properties and to organize functional nanoparticles at the solid state to exploit their property in a device.

The oxide matrices synthesized via sol-gel processes offer a priceless modularity (organization and size of the porosity, chemical nature of the walls, nature and quantity of the nanoparticles inside the pores,...), which enable to investigate the effects of the nanocomposite processing on the properties of the nanoparticles synthesized in situ.

This nanoporosity can also be organized for large volumes and surfaces. This offers the possibility to i) organize the functional objects and so make them usable in real applications or ii) induce new properties related to the interaction between the objects and/or the shape and organization anisotropy.

First results showed collective behaviors of the nanoparticles synthesized inside the porosity with original magnetic properties.

Axis 4 — Methodological developments in X-ray absorption spectroscopy

Objective : The development of a new methodology to quantify small structural distortions thanks to the use of PBAs as model compounds

Thanks to their chemical versatility, PBAs offers a wide rage of model compounds particularly interesting for spectroscopic developments. By varying the nature of the transition metals and the inserted alkali cations (number, nature), the electronic and structural parameters can be independently varied. We use this advantage of PBAs to develop a new methodology based on transition metal K-edge X-ray Magnetic Circular Dichroism to quantify small structural distortions. 

This project is developped within a collaboration with the ODE beamline at SOLEIL and is funded by an Young Researcher ANR grant.

Last update on Feb. 25^th 2022