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Bild des Monats Juli 2022

Hierarchical magnetic materials: A novel strategy for tuning the characteristics of a material by arranging magnetic nanoparticles into mesocrystals

The arrangement of nanometer-sized ferromagnetic entities of well-defined size, shape, and orientation into highly ordered, periodic structures introduces additional degrees of freedom to tune the macroscopic magnetic response of the sample. In such so-called mesocrystals, the response can not only be manipulated on the macroscopic or the atomic scale, but also on the mesoscopic scale. For example, a manipulation of the mutual interaction between the nanoparticles can be achieved by altering their size, shape, and arrangement. Furthermore, the magnetic response depends on the shape of nanoparticle arrangements. This tunability is reflected by the occurring resonant magnetic excitations, which can be studied by ferromagnetic resonance spectroscopy and Brillouin light scattering microscopy.

We study 2D mesocrystals built of spherical magnetite nanoparticles of diameter dM regularly arranged with spacings dS. Depending on the external conditions, such as the external driving microwave field of frequency fext and the applied static magnetic field Bext, multiple resonant excitations occur in the spectra of these 2D mesocrystals of defined shape. Often, the main signal is accompanied by a satellite signal. Typically, the main signal originates from the center of the arrangement, whereas the active areas of the satellite signal are located near its rim.

The manipulation of the interparticle interaction within the mesocrystal, e.g. by varying the spacing dS between the nanoparticles, is reflected in the variation of the resonance positions and linewidths of the excitations. A precise control of the spacing can be achieved by varying the surfactant shell which surrounds the nanoparticles. As the occurrence of multiple signals is crucially affected by the interparticle coupling strength, the relative signal intensities and line widths can be tuned specifically.

Such magnetic mesocrystals are not only ideal model systems for studying the interplay of magnetic interactions, but are also of interest as building blocks of miniaturized devices in the field of magnetotronics.

Dieses Bild wurde eingereicht von Nils Neugebauer, AG Prof. Klar.

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