Magnetic properties of 3d and 4f ferromagnets studied by X-ray absorption spectroscopy

In this thesis, the magnetism of ultrathin ferromagnetic films of Fe, Co, and Ni as well as the magnetism of single crystals of the rare earth elements Gd, Tb, and Dy have been studied by means of X-ray absorption spectroscopy. To support measurements even in an applied magnetic field, a new water-cooled coil system has been developed and installed in the UHV chamber. The electronic structure of ultrathin ferromagnetic films of Fe, Co, and Ni has been investigated by near edge X-ray absorption fine structure (NEXAFS) spectra. Oxygen as surfactant has been used to tailor the structural properties of these ultrathin films. The obtained X-ray magnetic circular dichroism (XMCD) spectra at the oxygen K edge show that the surfactant O acquires an induced orbital moment parallel to the spin and orbital moment of the ferromagnetic film. Ab initio calculations reproduce the experimental spectra and yield the size of spin and orbital moments. Thickness-dependent XMCD investigations at the L_2,3 edges have been carried out in the case of Ni grown with O as surfactant. These experiments show that the total magnetization of the thinnest Ni films is reduced on the Cu(100) surface compared to the bulk value. On the more open Cu(110) surface the magnetization is enhanced, mainly due to the orbital moment. Two-dimensional spin fluctuations and the interlayer exchange coupling (IEC) have been studied on Co/Cu/Ni/Cu(100) trilayers. Using XMCD, the influence of the IEC on the element-specific Ni magnetization has been investigated as a function of temperature. The quasi-critical temperature of Ni in the trilayer is increased compared to the Curie temperature of the single Ni film. The experiments together with calculations show the temperature shift as a function of both the Ni thickness and the Cu thickness. A three-dimensional plot of the full dependence results. Therefore, a relation between the temperature shift and the coupling strength has been derived with the help of a theoretical model including spin fluctuations. Calculations of the temperature shift match the experimental findings. Temperature-dependent XMCD spectra at the L_2,3 edges of the rare earth elements Gd, Tb, and Dy have been measured. In this work high purity single crystals have been used which do not include additional complex interactions as they occur in compounds. The obtained XMCD spectra contain contributions of dipole transitions (E1: 2p -> 5d) and quadrupolar transitions (E2: 2p -> 4f). Therefore, it is possible to investigate the magnetism of 5d and 4f states in a single experiment. In this thesis, it is demonstrated that the contributions of 5d and 4f to the total magnetization can be separated by calculating the spectra for T=0 using the FEFF code. Furthermore, the experiments have revealed that the relative intensities of the different spectroscopic features of the XMCD change as a function of the temperature.