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We report on the study of magnetic properties of the La 1.5 Ca 0.5 CoIrO 6 double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature, the material exhibits exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of Ir 4+ ions located between the antiferromagnetically coupled Co ions.

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.

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Core-level and valence-band optical spectra provide important information on highly correlated systems. The former corresponds to transitions from the core level to the conduction band, and it is usually related to the unoccupied electronic structure. The later corresponds to transitions from the valence band to the conduction band, and it is sometimes described in terms of the joint density of states. These spectra are usually treated separately due to the differences in the experimental and theoretical methods. We present here a combined description of the core-level and valence-level optical spectra of Mott-Hubbard compounds. In particular, we studied the O 1s x-ray absorption and the optical-conductivity spectra of SrVO 3 -CaVO 3 -LaVO 3 -YVO 3 . The experimental data were analyzed using an extended p-d cluster model solved by an exact diagonalization method. The results show that correlation effects alone cannot account for the experimental structures, and that crystal-field effects and exchange interactions are necessary to explain the spectra. We also show that there is a correspondence between the features in the charge-transfer region of both spectra.