Free-standing and positionable microwave antenna device for magneto-optical spectroscopy experiments

Modern spectroscopic techniques for the investigation of magnetization dynamics in micro and nanostructures or thin films typically use microwave antennas. They are directly fabricated on top of the sample by means of electron-beam lithography (EBL). Following this approach, every magnetic structure on the sample needs its own antenna, resulting in additional EBL steps and layer-deposition processes. Here, we demonstrate an approach for magnetization excitation that is suitable for optical and nonoptical spectroscopy techniques. By patterning the antenna on a separate flexible glass cantilever and insulating it electrically, we solve the mentioned issues. Since we use flexible transparent glass as the antenna substrate, optical spectroscopy techniques like microfocused Brillouin-light-scattering microscopy ($\ensuremath{\mu}\mathrm{BLS}$), time-resolved magneto-optical Kerr-effect measurements, or optically detected magnetic resonance measurements can be carried out at visible laser wavelengths. As the antenna is detached from the sample it can be freely positioned in all three dimensions to address only the desired magnetic structures and to achieve an effective excitation. We demonstrate the functionality of these antennas using $\ensuremath{\mu}\mathrm{BLS}$ and compare coherently and thermally excited magnon spectra to reveal an enhancement of the signal by a factor of about 400 due to the strong excitation by the antenna. Moreover, we succeed in characterizing yttrium-iron-garnet thin films with spatial resolution using optical ferromagnetic resonance experiments. We analyze the spatial excitation profile of the antenna by measuring the magnetization dynamics in two dimensions. The technique is furthermore applied to investigate injection locking of spin Hall nano-oscillators in the most favourable geometry with the highest spin-torque efficiency.