YBa2Cu3O7/La 2/3 Ca 1/3 MnO3 superconducting/ferromagnetic (SC/FM) multilayers have been studied by neutron reflectometry. Evidence for a characteristic difference between the structural and magnetic depth profiles is obtained from the occurrence of a structurally forbidden Bragg peak in the FM state. The comparison with simulated reflectivity curves allows us to identify two possible magnetization profiles: a sizable magnetic moment within the SC layer antiparallel to the one in the FM layer (inverse proximity effect), or a "dead" region in the FM layer with zero net magnetic moment. The former scenario is supported by an anomalous SC-induced enhancement of the offspecular reflection, which testifies to a strong mutual interaction of SC and FM order parameters.

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The existence of coherent quasiparticles near the Fermi energy in the low-temperature state of hightemperature superconductors has been well established by angle-resolved photoemission spectroscopy ͑ARPES͒. We present a study of La 1.83 Sr 0.17 CuO 4 in the superconducting state and report an abrupt change in the quasiparticle spectral function, as we follow the dispersion of the ARPES signal from the Fermi energy to 0.6 eV. The interruption in the quasiparticle dispersion separates coherent quasiparticle peaks at low energies from broad incoherent excitations at high energies. We find that the boundary between these low-energy and high-energy features exhibits a cosine-shaped momentum dependence, reminiscent of the superconducting d-wave gap. Further intriguing similarities between characteristics of the incoherent excitations and quasiparticle properties suggest a close relation between the electronic response at high and low energies in cuprate superconductors.

Magnetic and structural properties of high quality magnetocaloric MnFe 4 Si 3 single crystals are investigated macroscopically and on an atomic scale. Refinements of combined neutron and x-ray single crystal diffraction data introduce a new structural model in space group P 6 characterized by partial ordering of Mn and Fe into layers perpendicular to c on one of the transition metal sites. A second transition metal site is exclusively occupied by iron. MnFe 4 Si 3 has a phase transition to a ferromagnetically ordered phase at approximately 300 K and displays a strong anisotropy of the 1 magnetization and the magnetocaloric effect with the easy axis of magnetization in the a, b-plane. This is confirmed by a refinement of the magnetic structure in the magnetic spacegroup P m ′ which shows that the spins on the sites with mixed occupancy of Mn and Fe are aligned in the a, b-plane. A significant magnetic moment for site exclusively occupied by iron could not be refined. The thermal evolution of the lattice parameters exhibit an anisotropic behavior and clearly reflects the onset of magnetic ordering. Comparison of the ordered moment and the effective paramagnetic moment hints towards itinerant magnetism in the system.

LaCoO 3 displays two broad anomalies in the DC magnetic susceptibility DC , occurring, respectively, around 50 K and 500 K. We have investigated the first of them within the 10 K Ͻ T Ͻ RT temperature range using Co K␣ 1 x-ray absorption spectroscopy ͑XAS͒ in the partial fluorescence yield mode. In contrast with previous O K-edge XAS reports, our data show the existence of abrupt changes around 50 K which can be nicely correlated with the anomaly in DC. To our knowledge, this is the first time that a clear, quantitative relationship between the temperature dependence of the magnetic susceptibility and that of the XAS spectra is reported. The intensity changes in the preedge region, which are consistent with a transition from a lower to a higher spin state, have been analyzed using a minimal model including the Co 3d and O 2p hybridization in the initial state. The temperature dependence of the Co magnetic moment obtained from the estimated e g and t 2g occupations could be satisfactorily reproduced. Also, the decrease of the Co 3d and O 2p hybridization by increasing temperature obtained from this simple model compares favorably with the values estimated from thermal evolution of the crystallographic structure.