Mesocrystals based on Magnetic- and Semiconducting Nanoparticle Assemblies: From Structure-Properties Relation to Applications

The development of green nanomaterials and (spin-)electronic devices with reduced power consumption and multifunctional properties is one of the greatest challenges in current nanoscience and nanotechnology. An interesting materials concept is to employ mesocrystalline assemblies of magnetic nanocrystals, hosting only exactly one magnetic domain. Here, we explore the properties of magnetic mesocrystals composed of magnetite (Fe₃O₄) nanocubes.[1] The static and dynamic magnetic response of these single-domain nanoparticles are governed by an individual macrospin. However, interactions between superspins in regular assemblies give rise to exciting magnetic ordering phenomena on mesoscopic length scales. From a fundamental point of view, magnetic mesocrystals therefore constitute a unique model system to study (frustrated) magnetic interactions in confined 2D and 3D systems, where magnetostatic, dipole and exchange interactions among superspins in the assembly result in a wealth of magnetically ordered states, including superparamagnetism with modified Néel–Brown relaxation, frustrated spin glass behavior and superferromagnetism. Interestingly, magnetic mesocrystals also provide means to precisely tune the complex magnetic behavior, e.g. through the temperature, the size-, morphology and order of the nanoparticles as well as inter-particle interactions, which depend on the superstructure and the electronic coupling of the nanocrystals. Together, the fundamental understanding and the possibility to modify the magnetic superspin textures in magnetic mesocrystals enables the design of tailored magnetic nanomaterials for applications in magnetic field sensing elements and spintronics devices. In this project, we explore the rich interplay between composition, structure, interactions and physical properties and emerging magnetic order in 2D and 3D mesocrystals.
A thermal fusion process is used to cross-link nanoparticles, which reduces the tunnel barrier and induces an electronic exchange coupling among nanoparticles in the mesocrystalline assembly without perturbing the long-range crystal structure. In this isolated-but-interconnected state, we explore the emergence of unusual magnetically ordered states, which are reflected in the macroscopic magnetic behavior and the (spin-)transport properties of the material.

Publications

1. Brunner, J.; Baburin, I. A.; Sturm, S.; Kvashnina, K.; Rossberg, A.; Pietsch, T.; Andreev, S.; Sturm, E.; Cölfen, H. Self-Assembled Magnetite Mesocrystalline Films: Toward Structural Evolution from 2D to 3D Superlattices. Advanced Materials Interfaces 2016, 1600431