FMR investigations of half metallic ferromagnets

Full text: The spin polarization of the conduction electrons is the most important parameter determining the performance of various magnetoelectronic devices. For that reason, half-metallic ferromagnets, which have highly spin-polarized electrons at the Fermi level, appear very promising for various applications in magnetoelectronics. It is well known that ferromagnetic resonance (FMR) is a very efficient technique to study the nanoscale magnetic properties of various ferromagnetic materials. In this work thin films of various half metallic ferromagnets, such as chromium dioxide (CrO2) and Heusler alloys (Co2Cr0.6Fe0.4Al, Co2MnSi) have been investigated by FMR technique in X-band (9.5 GHz). Epitaxial CrO2 thin films of various thicknesses (25- 535 nm) were grown on TiO2 (100) single-crystalline substrates by chemical vapour deposition process. The magnetic anisotropies of the epitaxial chromium-dioxide films have been probed by FMR. Analysis of the FMR spectra shows that the magnetic behaviour of the CrO2 films results from a competition between magnetocrystalline and strain anisotropies. The strain anisotropy appears due to lattice mismatch of the CrO2 epitaxial film with the TiO2 single-crystalline substrate. For the ultra-thin CrO2 film (25 nm) the magnetic easy axis switches from the (c) direction to the (b) direction of the structure. Thin film Co2Cr0.6Fe0.4Al samples (25 nm or 100 nm) were grown by DC magnetron sputtering either on un-buffered SiO2 (100) substrates or on the substrates capped by a 50 nm thick V buffer layer. The effects of the vanadium buffer layer and of the film thickness were revealed by FMR studies of the Co2Cr0.6Fe0.4Al samples. Well-resolved multiple spin-wave modes were observed in the un-buffered Co2Cr0.6Fe0.4Al sample with thickness of 100 nm and the exchange stiffness constant was estimated. Thin film samples of Co2MnSi (4-100 nm) were grown by DC sputtering on silicon substrates on a 42 nm thick V seed layer and capped either by Al2O3 or by Co and V layers. A set of the 80 nm thick films were annealed in the range of 425-550 degree Celsius. FMR studies of the Co2MnSi samples shows that the highest magnetization is observed in the sample with thickness of 61 nm, while the thicker samples (100 nm) reveal not only a lower magnetization but higher magnetic inhomogeneity as well. The optimal annealing temperature, which provides the highest magnetization of the Co2MnSi samples, was also established