Spin waves as a probe and tool

The tremendous progress seen over the past decades in information processing is the merit of many achievements in the field of nanoengineering and materials research. The empirical description of this improvement is well known as Moore's law. However, the continuous decrease of device size required for further advances results in a disproportionately high rise of cost for chip manufacturing, as well as limitations imposed by thermal dissipation as a consequence of Joule heating. Thus, fundamentally different concepts are required to satisfy ever-expanding demand for more powerful computer technology. A very promising path is the implementation of wave-based logic concepts. Here, information transport is realized by wave propagation, and logic operations are based on interference phenomena and nonlinear interactions. In particular, featuring frequencies in the Gigahertz range and wavelengths down to a few nanometers, the fundamental excitations of magnetic solids - spin waves, or their quanta, magnons - are a promising candidate to implement a wave-based logic technology also referred to as magnonics. Spin waves not only feature wavelengths orders of magnitude smaller than electromagnetic waves of the same frequency, but their properties can also widely be tuned by an adequate choice of sample geometry, interface materials, spin or charge currents. This thesis is devoted to the investigation of spin-wave propagation and dispersion characteristics in novel structures and new material systems. On the one hand, this aims at promoting the application of spin waves as information carriers in future information processing devices, hence, to utilize them as a tool to tailor device functionalities. On the other hand, with their dispersion relation depending on various material and sample parameters, spin waves constitute a versatile probe in the quest for new materials for applications in spintronics and magnonics. In this work, it is shown that a properly designed thin film structure of Yttrium Iron Garnet can be employed to perform a logical majority operation based on the interference of spin waves. Such a majority gate is considered as a basis of beyond-CMOS information processing devices. This result marks an important proof of principle enabling further development. Further progress requires a deep understanding of fundamental mechanisms and the identification of materials tailored to the respective needs by offering desired spin-wave properties. In particular, an increase of operational speed can be achieved by employing materials with an enhanced exchange stiffness resulting in a larger spin-wave group velocity. To that end, the spin-wave dispersion in Gadolinium Iron Garnet has been investigated. This material possesses a magnetic compensation point and its spinwave exchange stiffness is widely tunable by temperature. It can significantly exceed the one of metallic ferromagnets. Even more, this material also represents a potential model system on the route towards the development of antiferromagnetic magnonics. For an advanced functionality of devices involving ultrathin magnetic layers, both the Heisenberg or symmetric exchange, and the antisymmetric exchange or interfacial Dzyaloshinskii-Moriya interaction (iDMI), which can arise in thin magnetic films with broken inversion symmetry, are important properties. These parameters critically govern the characteristics of the spin-wave dispersion relation as well as the stability of spatial configurations of the magnetization of a material. On the one hand, since tuning of the spin-wave dispersion relation is envisaged to enable advanced device functionalities, a detailed understanding of material parameters is decisive for tailoring exchange interactions to the respective needs. On the other hand, topologically stabilized configurations of the magnetization of a material -skyrmions- have gained much interest over the recent years. Their stability and properties are critically governed by the exchange interactions and, hence, an efficient design of material systems for applications requires comprehensive insights into the underlying mechanisms. With the spin-wave dispersion characteristics depending on material parameters, spin waves are, thus, rendered a powerful probe in the field of materials research. In this context, Brillouin light scattering has proven to be a versatile technique for probing the dispersion relation. Within the Collaborative Research Center TRR173 Spin+X, the spin-wave dispersion in single and multilayer thin film samples has been investigated aiming at a study of the exchange interactions. It has been found that a Pt layer adjacent to the magnetic material, as expected, results in a large DMI while an adjacent W layer led to absence of DMI, underlining also the importance of the interfacial characteristics in addition to the phase of the respective material. Also, a thin-film platform system of ultrathin Co and Co/Fe layers has been investigated employing an analysis of the spin-wave dispersion relation. Having been obtained within a collaboration with the A*Star institute in Singapore, these results are discussed along with calculations based on density functional theory. Furthermore, different models for the description of the temperature dependence of the magnetization in ultrathin films are evaluated for the extraction of the Heisenberg exchange. Specifically, the results of the investigation of this material platform suggest a converse relation between DMI and Heisenberg exchange in this system. This allows for improved tailoring of the properties of multilayers composed of the single layer base stacks paving the way towards the development of skyrmion-based devices.