Nowadays, magnetic nanoparticles may be arranged into highly ordered, crystal-like structures, so-called mesocrystals. Compared to their bulk counterpart, the
characteristics of mesocrystals are not only determined by the specific material employed, but also by the shape of its nanoscale constituents, their ordering on the meso- and on the
macroscale and the resulting mutual interactions between them. We use ferromagnetic resonance (FMR) experiments and Brillouin light scattering (BLS) microscopy in combination with micromagnetic simulations to investigate and characterize
dipolar interactions between magnetic nanoparticles (MNPs) within such mesocrystals. The MNPs investigated in this work consist of iron oxide (magnetite - Fe3O4
) and are coated with
non-magnetic polymers, forming highly ordered hexagonal monolayer crystals as shown in fig. 1 a.
The magnetic response of the regularly arranged hexagonal mesocrystals can be tuned in a controlled way by varying the thickness of the non-magnetic polymer coating of the MNPs and
thus the lattice constant of the mesocrystal. The spectral features reveal that the dipolar coupling strength between the MNPs decreases as the spacing between MNPs increases, as shown in
fig. 2 a.
Structuring the monolayer by employing electron beam lithography, well-defined, circularly shaped MNP assemblies can be fabricated as shown in fig. 1 b. Performing BLS experiments a
main signal accompanied by a satellite signal can be observed, showing distinct dependencies on the externally applied field. 2D-BLS-mapping of the circular assembly reveals the
associated active areas within the assembly for the two resonances. The experimental findings of the FMR and BLS experiments are fully corroborated by micromagnetic simulations.<br>
**References**<br>

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/5a5627a37b171538d2fba65619fefed4.png)
<br>
(a) Self-assembled, hexagonally arranged MNP monolayer. (b) Using electron beam lithography, well-defined MNP assemblies can be fabricated from monolayers as shown in (a). <br>

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/838fcbb80ef8fe957517300f67728c54.png)
<br>
(a) In FMR experiments on MNP monolayers (see fig. 1 (a)) of different spacing between the MNPs, but constant MNP size. (b) Investigations of structured MNP assemblies as shown in
fig. 1 (b) by BLS reveal two signals, both showing distinct dependencies on the externally applied field.<br>

Dynamic properties of magnetic nanoparticles in highly ordered arrangements: distance and frequency dependent properties

Optical frequency combs are well established and powerful tools for synchronization, stabilization and frequency conversion in both fundamental science and technological applications. In this work, we present experimental observations on the generation of spin-wave frequency combs.<br/>Nonlinear four-magnon scattering is known to cause parasitic losses for a driven spin-wave mode with respect to coherence and amplitude. By means of Brillouin light scattering microscopy, we demonstrate that adding a second microwave frequency causes the scattering to populate well-defined states instead of redistributing energy to the entire spin-wave band.<br/>Here, the amplitudes and frequencies of both microwave sources can be utilised in order to tune this behaviour.<br/>When reducing the sizes of the magnetic conduits, lateral confinement causes a discretisation of the eigenmode spectrum. Therefore, the formation of frequency combs can be observed even when driving the conduits into the nonlinear regime by just a single pumping frequency.<br/>The tunability of multimode spin-wave spectra, as demonstrated in our experiments, offers promising technological applications that allow to bridge the frequency gap between CMOS technology and spin waves.

Spin-wave frequency combs



Since the 80’s, when Brillouin light scattering emerged as a powerful tool for investigating magnetization dynamics in thin films and multilayers, it developed into a versatile microscopic probe for studying collective spin excitations. Following a short introduction on studies of millimeter-sized films, we will give examples how to investigate individual magnetic structures down to tens of nanometers in dimension. We will introduce the concepts of time- and phase-resolved Brillouin light scattering which give full access to the spatio-temporal evolution of the optically accessible spin-wave spectrum. During our talk, we will provide hands-on demonstrations how to drive spin waves via spin currents and microwave excitations in magnetic nanostructures using the build in dc/ac probe station in our laboratory and show the capabilities of Brillouin light scattering for quantifying spin-wave phenomena. Furthermore, we will highlight similarities and differences to other optical scanning probe techniques such as time-resolved magneto-optical Kerr microscopy and optically detected magnetic resonance based on vacancy centers. We will outline how those techniques can potentially be combined with Brillouin light scattering to access complementary information.

**References:**



**Images:**





Brillouin Light Scattering Revisited

In the last decade, two revolutionary concepts emerged in nano-magnetism from research for advanced information processing and storage technologies. The first suggests the use of magnetic domain walls (DWs) in ferromagnetic nanowires to permanently store information in DW racetrack memories. The second proposes a hardware realisation of neuromorphic computing in nanomagnets using nonlinear magnetic oscillations in the GHz range. Both ideas originate from the transfer of angular momentum from conduction electrons to localised spins in ferromagnets, either to push data encoded in DWs along nanowires or to sustain magnetic oscillations in artificial neurones. Even though both concepts share a common ground, they live on very different time scales which rendered them incompatible so far. Here, we bridge both ideas by demonstrating the excitation of magnetic auto-oscillations inside nano-scale DWs [1] using pure spin currents originating from the spin-Hall effect. While the auto-oscillations are in a domain wall are shown for a bilayer from a heavy metal (HM) and a ferromagnet (FM) we also demonstrated the realization of a bipolar spin-Hall Nano-oscillator using a FM/HM/FM trilayer system [2].References: [1] N. Sato, K. Schultheiss, L. Körber, N. Puwenberg, T. Mühl, A. A. Awad, S. S. P. K. Arekapudi, O. Hellwig, J. Fassbender, and H. Schultheiss, Phys. Rev. Lett. 123, 057204 (2019). [2] T. Hache, Y. Li, T. Weinhold, B. Scheumann, F. J. T. Gonçalves, O. Hellwig, J. Fassbender, and H. Schultheiss, Appl. Phys. Lett. 116, 192405 (2020).

Domain wall based spin-Hall nano-oscillators INVITED

GOC: Magnetization Dynamics III

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Presentations

Dynamic properties of magnetic nanoparticles in highly ordered arrangements: distance and frequency dependent properties

Spin-wave frequency combs

Brillouin Light Scattering Revisited

Domain wall based spin-Hall nano-oscillators INVITED

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