Dipolar Magnetism in Self-assembled nanoparticle Systems

Marco Beleggia
External collaborators: Cathrine Frandsen (DTU Physics), Victor Puntes and Miriam Varon (Barcelona), Richard Harrison (Cambridge), Rafal Dunin-Borkowski (Jülich FZ)
Staff at DTU Cen: Takeshi Kasama
Funding: DTU Cen
Meta materials are artificial structures where atoms are replaced by larger building blocks such as nanoparticles. When the nanoparticles are magnetic, the resulting meta-magnet is characterized by the absence of inter-particle exchange interaction. Dipolar interactions come into play, and govern the properties and functional response of these assemblies. Yet, experimental data on the magnitude and orientation of each macro spin as a function of geometry, material composition, nanoparticle shape, and other relevant parameters is lacking, and therefore the physics of these systems is not fully clarified. With electron holography, we have access to
the nanoscale electric (scalar) and magnetic (vector) potential topography, which gives us the opportunity to map and quantify the local field sources, in this case the magnitude and orientation of the macro spins. We are studying self-assembled Cobalt meta-magnets with electron holography, aiming at developing a comprehensive physical picture of these systems, capable of explaining in what conditions dipolar ordering occurs, whether it is ferro-, ferri-, or antiferro-magnetic in character, and what the resulting emerging properties are. Ultimately, magnetic metamaterials may represent a new class of materials with functionalities that are not available in conventional magnets; for example, the possibility of fine-tuning the effective transition temperature, coercive field and saturation magnetization, by controlling the meta-lattice spacing and the magnitude of each macro-spin.


Electron holography data on a chain of self-assembled Co nanoparticles arranged over a closepacked meta-lattice. The figure shows evidence of dipolar ferromagnetic order: the particle moments are well aligned in spite of underlying meta-lattice disorder induced by the size distribution. 


Marco Beleggia
DTU Nanolab
+45 45 25 31 47
26 FEBRUARY 2020