Application and exploration of magnetic nanoparticles as magnetic scatterers in magnonic devices

Spin waves (SW) are collective spin excitations in magnetically ordered materials. SWs oscillation frequencies are in the technological relevant microwave regime. Within this frequency regime SWs feature wavelength λ ≤ 1 μm. Spin waves with sub-100 nm wavelength could become the information carrier of future non charge-based information technology. Integrated microwave antennas on ferri- or ferromagnetic layer are used to excite the underlying magnetization dynamics. Depending on geometry a microwave antenna has a characteristic spatial profile of the microwave field. This provides a fixed set of wavevectors k = 2π/ λ that can be excited thus defining magnons of specific wavelengths. Interfacing short waved SWs and electromagnetic waves (λ ~ 30 cm) at the same frequency is however challenging because of wavelength mismatch. This motivates the research for efficient microwave-to-magnon transduction mechanisms. It has been shown that a periodic lattice of nanomagnets between the microwave antenna and the ferri- or ferromagnetic layer enables generation of short-waved magnons. This so-called magnonic grating coupler effect was so far explored with nanomagnets fabricated by the top-down approach using nanolithography. Here nanomagnets of large spin-precessional frequency were of particular interest. In the course of this thesis we investigate a bottom up approach. To overcome nanolithography limits, we consider ferrimagnetic nanoparticles(NPs) on the surface of 100 nm thick yttrium iron garnet (YIG). YIG is chosen as a prototypical SW guiding medium because it exhibits the lowest SW damping at room temperature. This work performed in this thesis aims at improving coupling between microwaves and magnons. We study the magnetization dynamics of YIG decorated with magnetic nanoparticles (NPs). Colloidal suspensions of NPs are prepared and then deposited by drop-casting on YIG surface. To probe the magnetization dynamics All Electrical Spin Wave Spectroscopy (AESWS) and Brillouin light scattering spectroscopy (BLS) are applied. With both transmission and scanning electron microscopy we characterize our sample after deposition. We observe that nanoparticles tend to form NP agglomerates on the YIG surface and their lateral size is of the order of ~ 10 μm. With AESWS and spatially resolved BLS measurement we observe spin wave resonance changes in YIG. These changes depend on NP distribution. Linewidths of resonance peaks are increased at spatial sites where NPs have been deposited. This effect is attributed to a locally enhanced SW scattering due to the non uniform distribution of NPs and hence the spatially non uniform stray field experienced by SWs in YIG. These results suggest that NP distribution on YIG thin film surface modify spin wave propagation properties. They are promising in the view of the grating coupler.