A. Serrano

Room-temperature ferromagnetism in the mixtures of the TiO2 and Co3O4 powders

We report here the observation of ferromagnetism (FM) at 300 K in mixtures of TiO₂ and Co₃O₄ powders despite the antiferromagnetic and diamagnetic characters of both oxides, respectively. The ferromagnetic behavior is found in the early stages of reaction and only for TiO₂ in anatase structure; no FM is found for identical samples prepared with rutile-TiO². Optical spectroscopy and x-ray absorption spectra confirm a surface reduction of octahedral Co^(+3) -> Co^(+2) in the mixtures which is in the origin of the observed magnetism.

Effect of photodiode angular response on surface plasmon resonance measurements in the Kretschmann-Raether configuration.

We study the effect of photodiode angular response on the measurement of surface plasmon resonance (SPR) in metallic thin films using the Kretschmann-Raether configuration. The photodiode signal depends not only on the light intensity but also on the incidence angle. This implies that the photodiode sensitivity changes along the SPR curve. Consequently, the measured SPR spectrum is distorted, thus affecting fits and numerical analyses of SPR curves. We analyze the magnitude of this change, determine when it is significant, and develop a calibration method of the experimental setup which corrects for this type of spectral shape distortions.

Study of Co-phthalocyanine films by surface plasmon resonance spectroscopy

We present a Surface Plasmon Resonance spectroscopy study of Co-Phthalocyanine (CoPc) thin films grown on Au layers at different substrate temperatures. We demonstrate that for quantitative analysis, fitting of the resonance angle alone is insufficient and Whole Curve Analysis (WCA) needs to be performed. This is because CoPc thin film dielectric constant and thickness are strongly affected by substrate temperature, even when the total deposited mass remains fixed. Using WCA, we are able to uniquely fit both the dielectric constants and the thicknesses of the films without making a priori assumptions.

Origin of the magnetic transition at 100 K in ε-Fe2O3 nanoparticles studied by x-ray absorption fine structure spectroscopy

We present a study of the correlation between the magnetic phase transition and the structural distortion observed at 100 K in uf065ε-Fe2O3. For this purpose, we have designed a novel one-pot sol-gel method assisted by glycerol, which reproducibly provides samples with a nominal 100% concentration of ε-Fe2O3 nanoparticles embedded in a SiO2 matrix. The high crystallinity of the samples and the absence of other iron oxide polymorphs has allowed us to perform, for the first time, temperature-dependent X-ray absorption fine structure spectroscopy experiments, with the aim of investigating the origin of the magnetic quenching anomaly observed at 100 K. The deformation of the structure at a local scale, where the tetrahedral and octahedral Fe sites undergo distortions of different intensities, has been simulated to fulfill the long-range order. Our results point to a local structure distortion accompanied by the magnetism quenching through a magneto-elastic coupling.The current study unveils the structural origin of the magnetic transition of the ε-Fe2O3 polymorph from an incommensurate magnetic order to a collinear ferrimagnetic state at low temperature. The high crystallinity of the samples and the absence of other iron oxide polymorphs have allowed us to carry out temperature-dependent x-ray absorption fine structure spectroscopy experiments out. The deformation of the structure is followed by the Debye-Waller factor for each selected Fe-O and Fe-Fe sub-shell. For nanoparticle sizes between 7 and 15u2009nm, the structural distortions between the Fete and Fe-D1oc sites are localized in a temperature range before the magnetic transition starts. On the contrary, the inherent interaction between the other sub-shells (named Fe-O1,2 and Fe-Fe1) provokes cooperative magneto-structural changes in the same temperature range. This means that the Fete with Fe-D1oc polyhedron interaction seems to be uncoupled with temperature dealing with these nanoparticle sizes wherein the structural distortions are likely moderate due to surface effects.

Formation of a magnetite/hematite epitaxial bilayer generated with low energy ion bombardment

We have used a low-energy ion bombardment to fabricate an epitaxial single-crystalline magnetite/hematite bilayer grown on Au(111). This non-conventional fabrication method involves the transformation of the upper layers of a single-crystalline hematite thin film to single-crystalline magnetite, a process driven by the preferential sputtering of oxygen atoms and favoured by the good structural matching of both phases. We show the reversibility of the transformation between hematite and magnetite, always keeping the epitaxial and single-crystalline character of the films. The magnetic characterization of the bilayer grown using this method shows that the magnetic response is mainly determined by the magnetite thin film, exhibiting a high coercivity.

X-ray irradiation of soda-lime glasses studied in situ with surface plasmon resonance spectroscopy

We present here a study of hard X-ray irradiation of soda-lime glasses performed in situ and in real time. For this purpose, we have used a Au thin film grown on glass and studied the excitation of its surface plasmon resonance (SPR) while irradiating the sample with X-rays, using a recently developed experimental setup at a synchrotron beamline [Serrano et al., Rev. Sci. Instrum. 83, 083101 (2012)]. The extreme sensitivity of the SPR to the features of the glass substrate allows probing the modifications caused by the X-rays. Irradiation induces color centers in the soda-lime glass, modifying its refractive index. Comparison of the experimental results with simulated data shows that both, the real and the imaginary parts of the refractive index of soda-lime glasses, change upon irradiation in time intervals of a few minutes. After X-ray irradiation, the effects are partially reversible. The defects responsible for these modifications are identified as non-bridging oxygen hole centers, which fade by recombination with electrons after irradiation. The kinetics of the defect formation and fading process are also studied in real time.