Functional Materials: Magnetic Nanoparticles and Hybrid Materials
Soft materials that respond to external stimuli have attracted great interest in the fabrication of functional materials and smart systems. In particular, the hybridization of magnetic nanoparticles (MNPs) with isotropic and anisotropic organic matrices (technical oils and liquid crystals (LCs), respectively) opens new perspectives for the development of novel magnetoresponsive materials (e.g., ferrofluids, ferronematics) for various applications.
Figure 1. Examples of magnetic hybrid materials: Ferrofluid with typical Rosensweig-instabilities and magnetic catalyst separation.
We develop nanoparticles of various inorganic magnetic materials (e.g. ferrites with spinel or magnetoplumbite structure, magnetic metals/alloys) with adjustable size and shape (spheres, rods, platelets etc.) and tailored magnetic properties by liquid phase syntheses (e.g. via solvothermal, decomposition, reduction or precipitation processes).
Figure 2. Synthesis of nanoparticles in liquid phase: Autoclave for solvothermal synthesis, hexagonal, Sc-doped BaFe12O19 platelets and typical magnetisation curves of a blocked ferromagnet, a paramagnet and a superparamagnet.
To prepare functional hybrid materials, these particles are successively stabilized in isotropic or anisotropic organic matrices (e.g. technical oils, LCs). LCs are characterized by combining the fluidity of ordinary liquids with anisotropic, optical properties of crystalline materials. The introduction of inorganic magnetic particles into LC matrices leads to elastic distortions of the director and/or perturbation of the local order parameter in the vicinity of the magnetic nanoparticles, resulting in LC-mediated interparticle interactions. Therefore, the tendency of the magnetic nanoparticles to agglomerate in the anisotropic LC phase up to the complete phase separation is much more pronounced than in the isotropic phase. Therefore, the surface properties of the magnetic particles must be specifically tailored to stabilize the nanoparticles in the LC matrix and to obtain ferronematics.
Figure 3. Stabilization of magnetic nanoparticles in LCs and preparation of ferronematics require specific surface functionalization of the particles by (pro)mesogenic ligands. These ligands typically consist of three structural entities, i.e., a mesogenic unit and a functional group for binding to the magnetic particles, which are connected by a flexible linker. Shown is a ferronematic in 4-cyano-4‘-pentylbiphenyl (5CB) and modulation of the optical properties of a ferronematic in the magnetic field.
These hybrid materials (ferronematics) reveal interesting optomagnetic and optoelectric properties.
Selected review articles related to the topic:
- M. Hähsler, I. Appel, S. Behrens, Magnetic Hybrid Materials in Liquid Crystals, Phys. Sci. Rev. (2021), https://doi.org/10.1515/psr-2019-0090
- S. Behrens, I. Appel, Magnetic Nanocomposites, Curr. Op. Biotechnol. (2016), 39, 89 – 96
- S. Behrens, Preparation of Functional Magnetic Nanocomposites and Hybrid Materials: Recent Progress and Future Directions, Nanoscale (2011), 3, 877 – 892
For further information see publications.