Magnetic properties of Ni(Co)MnSn and Co2FeGe Heusler alloys and Al-substituted garnets

This thesis is a study of the static and dynamic magnetic properties of Heusler alloys and substituted yttrium iron garnets (YIG) using magnetometry, ferromagnetic resonance (FMR), and Brillouin light scattering (BLS) measurement techniques, in combination with other structural and chemical analysis methods. The following main research results were obtained. Epitaxial films of magnetic shape memory alloy Ni(Co)MnSn were found to undergo at 270 K a martensitic phase transformation from the cubic austenitic phase to a twinned orthorhombic martensitic phase, which is accompanied by a 3-fold splitting of the FMR in the system. The additional resonance lines were explained as due to weak antiferromagnetic coupling of the ferromagnetic twins across twin boundaries, which makes the submicron-twinned martensite phase resemble an artificial antiferromagnetic superlattice. The elastic energy balance of the film/substrate interface was found to result in the formation of a submicron, stripe-like, periodical structure of twins, which is of interest for spintronic and magnonic applications. Epitaxial films of full Heusler alloy Co2FeGe were investigated in terms of their structure and magnetic properties. Improved quality of epitaxy, formation of an atomically ordered L21 phase, increased magnetization, and reduced spin damping were observed for films processed at elevated temperature. BLS and FMR experiments showed that room temperature deposition followed by annealing yields the highest magnetization, exchange stiffness, and lowest spin damping in the material, which are important characteristics for high-speed spintronic devices. Strong hybridization of the spin waves modes in the Damon-Eshbach geometry was found in the BLS dispersion data, which is promising for applications in magnonic waveguides and logic devices. Magnetometry of Al-substituted YIG (Y3AlFe4O12) revealed a sharp rise in magnetic anisotropy below 150 K, while the saturation magnetization, exchange and spin-wave stiffness change only slightly. Using FMR we determined the effective magnetization, anisotropy, resonance field, spin damping, as well as their temperature dependence -- key parameters governing the spin dynamics in the material, important for its applications in high-speed circuits operating at room and cryogenic temperatures.