Arbeitsgruppe Prof. Hillebrands

Brillouin light scattering (BLS) spectroscopy is a powerful tool for the detection of spin waves and measurement of their characteristics. Nevertheless, it does not allow for spin-wave excitation. Thus, in many of our experiments BLS spectroscopy is combined with microwave techniques which ensure high-efficient generation of spin waves in magnetic structures. Spin waves are emitted by nano- and micro-sized microstrip antennas placed on the surfaces of magnetic thin films and is driven by a microwave signal in the GHz frequency range. Microwave sources in our laboratories generate signals with frequencies of up to 70 GHz providing access to spin waves in a very wide range of frequencies and wavenumbers. Furthermore, large powers provided by the microwave amplifiers allow for the study of strongly nonlinear spin-wave dynamics as well as for quantum effects in parametrically-driven magnon gases and condensates. With the latest upgrades we are able to apply parametric pumping pulses up to 1500 W in peak power. The microwave technique allows for the excitation of both continuous spin waves and short spin-wave packets. Among other advantages, the pulsed technique enables the realization of time resolved (resolution down to 250 ps) BLS spectroscopy shown in Fig. 1. The continuous microwave excitation, by-turn, allows the realization of phase-resolved BLS spectroscopy.

Fig. 1: Scheme of the microwave-assisted time- and space-resolved BLS setup.

Besides the excitation of spin waves, the microwave technique is intensively used for high-sensitive (10-13 W) detection. Using the same antennas the magnetization precession is converted into microwave currents. These currents are amplified by low-noise amplifiers and analyzed using wideband oscilloscopes, vector network analyzers or spectrum analyzers. A vector network analyzer is also used for the ferromagnetic resonance (FMR) measurements allowing determination of such characteristics of the magnetic thin films as the magnetization saturation, the exchange constant, and damping.

Furthermore, we operate a microwave setup for broadband microwave analysis. This setup consists of the following complementary devices: vector network analyzer, spectrum analyzer, microwave signal generator, high-power microwave amplifiers, and a magnet. It has a wide frequency bandwidth of up to 70 GHz which allows for the excitation of spin waves with wavelengths down to 30 nm. This helps us to perform measurements in the frame of modern nanometer-scale magnonics. The broadband microwave analysis can be conducted in both the CW and pulsed regime. The possibility to switch between CW and time-resolved measurements of the vector network analyzer gives us a unique opportunity to discriminate spin-wave signals from much larger electromagnetic leakages.

Fig. 2: Schematic illustration of an angle-resolved spin pumping setup.

Our microwave techniques are amended by electrical detection of spin waves using the recently installed angle-resolved spin pumping setup schematically shown in Fig. 2. It is developed for separation of the inverse spin Hall effect from spin rectification effects using their angular dependence in an in-plane magnetized ferromagnet/normal-metal bilayer system. The spin pumping setup performs fully automated scans in frequency, microwave power, external magnetic field value and angle. The obtained data can be evaluated using a program with automated fits for extracting angular dependent parameters.

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