Thin film materials with ultralow damping for novel magnonic phenomena

The recently reported successful fabrication of half-metallic \(\text{Co}_2\text{Mn}\)-based Heusler compounds exhibiting excellent properties for the application in magnonic devices has sparked great interest for complementing the widely used insulating YIG due to the otherwise limited choice of a suitable conducting material with comparable properties. Yet, downscaling towards ultrathin films or microstructures is a critical necessity for applications known to impact the properties of magnetic materials. Hence, the influence of decreasing the thin film thickness on the reported ultralow damping associated with the 100% spin polarisation of the \(\text{Co}_2\text{MnSi}\) compound is investigated in this thesis. To this end, a series of epitaxially grown \(\text{Co}_2\text{MnSi}\) thin films with thin film thicknesses in the range of 4 nm to 18 nm is first investigated regarding the structural and chemical properties in order to ensure the comparability of the further results. In particular, the atomic ordering is investigated by means of high resolution transmission electron microscopy and high-angle annular diffraction scanning transmission electron microscopy, where varying ratios of \(\text{L2}_1\) order and \(\text{B2}\) disorder are obtained for the different \(\text{Co}_2\text{MnSi}\) films. This is assessed to be linked to a stoichiometry deviation from the desired atomic composition, which is implied by, inter alia, a systematic shift of the X-ray diffraction peaks as well as energy-dispersive X-ray and electron energy loss spectroscopy results. Performing vibrating sample magnetometry in different geometries reveals a fourfold cubic anisotropy with hard axes along the principal axes, i.e., the [100], [010] and [001] directions, of the Heusler lattice. An additional uniaxial anisotropy contribution is observed and assessed to be linked to the stoichiometry deviation. The thickness and temperature dependence of the saturation magnetisation and the magnetisation reversal is investigated and an exchange stiffness of \(D = ( \text{5.2} \pm \text{0.2} ) \times 10^{-10} \, \text{pJ} \cdot \text{nm}^2\) is extracted. In ferromagnetic resonance measurements, a double resonance is observed associated with two magnetic phases arising from the stoichiometry deviation, the separate analysis of which allows to extract the low Gilbert damping parameter in the range of \( \text{4}\times\text{10}^{-4} \leqslant \alpha < \text{1.7}\times\text{10}^{-3} \) and a high effective magnetisation in the range of \( \text{920}\,\text{kA/m}< M_\mathrm{eff} < \text{990}\,\text{kA/m} \) for thin film thicknesses above 8 nm. In general, all magnetic properties are found to be fairly maintained for a \(\text{Co}_2\text{MnSi}\) film thickness down to 8 nm, confirming the scalability of this compound for magnonic devices. Eventually, the signal intensity obtained from Brillouin light scattering (BLS) spectroscopy of the thermally activated magnons of various \(\text{Co}_2\text{Mn}\)-based Heusler compounds is investigated. With \(\text{Co}_2\text{MnSi}\) exhibiting only poor signal in the obtained spectra, \(\text{Co}_2\text{MnSi}_{x}\text{Al}_{1-x}\) or \(\text{Co}_2\text{MnGe}_{x}\text{Ga}_{1-x}\) might be more suitable candidates for magnonic devices to be characterised using BLS, forming a compromise of comparably better detectability, albeit slightly increased Gilbert damping parameter.