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Item Open Access Resonant X-ray scattering studies of ruthenium oxides and ruthenocuprates(2009) Bohnenbuck, Britta; Keimer, Bernhard (Prof. Dr.)The magnetic and orbital properties of the ruthenium oxides Ca3Ru2O7 and Mn doped Sr3Ru2O7 and the ruthenocuprate RuSr2GdCu2O8 were investigated using resonant and high-energy x-ray diffraction. Bilayered Ca3Ru2O7 is a paramagnetic metal at high temperatures and orders antiferromagnetically at T_N=56K. A second phase transition to a less conductive state is observed at T_MI=48K. This transition is accompanied by abrupt structural changes and a reorientation of the magnetic moment. In addition, there is experimental evidence for the existence of orbital order below T_MI. Our resonant x-ray diffraction studies at the Ru L-absorption edges were focused on the investigation of the magnetic reflections (001) and (110). The observation of a magnetic signal at these reciprocal space positions is in full agreement with an A-type antiferromagnetic structure, consisting of ferromagnetic bilayers coupled antiferromagnetically along the c-axis. Based on the azimuthal angle dependence of the signals, the direction of the magnetic moment was determined to lie along the b-axis below T_MI and along the $a$-axis between T_MI and T_N. The origin of the reorientation of the magnetic moment at T_MI is not yet completely understood. However, it might result from the strong spin-orbit coupling which presumably causes an unquenched orbital magnetization. The latter might then induce additional terms in the spin Hamiltonian that are responsible for the reorientation of the magnetic moment. Although various experiments have given indirect evidence of orbital order below T_MI, we did not detect any orbital signal within the experimental sensitivity. This indicates that the orbital ordering parameter is significantly weaker than in the single layered counterpart Ca2RuO4, which is presumably due to residual charge or orbital fluctuations in the insulating state. RuSr2GdCu2O8 exhibits long range magnetic order and superconductivity within a broad coexistence range. Only limited information about the magnetic structure has been available so far, as most studies were performed on powder samples due to the small size of available crystals. In this situation, resonant x-ray diffraction at the Ru L-absorption edges has turned out to be the ideal tool for the investigation of RuSr2GdCu2O8 since it is sensitive to magnetism, but does not depend on a large crystal mass. Our single crystal studies of the magnetic reflections (1/2 1/2 1/2) and (1/2 1/2 3/2) indicate a G-type antiferromagnetic structure, characterized by a doubling of the unit cell along all three crystallographic directions. From the azimuthal angle dependence of the magnetic signal, we deduced a magnetic moment direction along a low symmetry axis with substantial components parallel and perpendicular to the RuO2 planes. These findings are consistent with previous neutron powder diffraction results and magnetization data. A symmetry analysis in conjunction with a recent crystallographic study revealed that the experimentally observed G-type antiferromagnetic structure needs to be accompanied by an additional ferromagnetic in-plane component, which alternates between neighboring RuO2 layers. This ferromagnetic mode corresponds exactly to the one deduced from nuclear and ferromagnetic resonance experiments. Therefore, our resonant x-ray diffraction data reconcile a variety of apparently contradictory results on the magnetic structure of RuSr2GdCu2O8 and thus resolve a big controversy in the experimental literature. Bilayered Sr3Ru2O7 has attracted a lot of interest in the past years due to the observation of the quantum critical behavior which is related to a metamagnetic transition. In the ground state, the material is a paramagnetic metal and shows Fermi liquid behavior below 10K. Upon substituting Mn for Ru, an insulating antiferromagnetic state is induced; its transition temperature varies with the Mn concentration. Using resonant x-ray diffraction at the Ru L-absorption edges, we investigated the antiferromagnetic structure of 10 Mn substituted Sr3Ru2O7. Our studies of the superstructure reflections (1/4 1/4 0) and (3/4 3/4 0) indicate that the magnetic order is essentially two dimensional and that the magnetic moments are aligned along the c-axis. In combination with a previous neutron powder diffraction study, which was carried out on 5% Mn substituted Sr3Ru2O7, our results suggest an up-up-down-down spin arrangement in the RuO2 planes, which is independent of the Mn concentration. This implies that an antiferromagnetic instability is already present in the parent compound Sr3Ru2O7. Interestingly, the anisotropic resistivity behavior, observed in the nematic phase of Sr3Ru2O7, could be explained assuming the same up-up-down-down spin arrangement as in Mn substituted Sr3Ru2O7. If the two phases are in fact identical, has to be checked by a detailed single crystal neutron diffraction study including a complete structure refinement.