R. Ramírez

Low field magnetoresistance at the metal–insulator transition in epitaxial manganite thin films

We present a study of the magnetotransport behavior around Tc of epitaxial La0.7Ca0.3MnO3 thin films with controlled chemical defects introduced in order to change the electron-lattice coupling (λ). We found unexpected enhancement of the very low field (below 250 Oe) magnetoresistance around Tc and relate it to a reduction of the magnetic domain size. This process is strongly favored by the presence of chemical defects that also promote weak localization of the carriers into polaron clusters that can be overcome by relatively low magnetic fields. This behavior is characteristic of temperatures near Tc where the bandwidth is reduced and localization, due to intrinsic disorder and chemical defects, is more probable. In addition, the increase of λ raises the polaron binding energy and reduces the temperature at which percolation occurs.

Photochromism of vacancy-related defects in thermochemically reduced α‐Al2O3:Mg single crystals

Oxygen vacancies and their aggregates are produced much more readily in Mg-doped α‐Al2O3 than in undoped α‐Al2O3 single crystals during thermochemical reduction at high temperatures. A reversible photochromic effect was discovered in Mg-doped Al2O3 crystals containing large concentrations of oxygen divacancies. Alternate excitation with 5.0 and 3.69 eV light results in brown or yellowish–green coloration, respectively. The yellowish–green coloration can also be restored by thermal anneals at temperatures of about 750 K.

Photochromic effect in magnesium-doped α-Al2O3 single crystals

A reversible photochromic effect was investigated in Mg-doped α-Al2O3 single crystals oxidized at elevated temperatures. Alternate illumination with blue and UV light at T⩽210 K results in reversible disappearance and reappearance of a gray-purple coloration, respectively. The coloration can also be fully rejuvenated by warming the crystal to T⩾215 K.

Local structure around Fe and Cu ions in PrBaFeCuO5+δ

Abstract XANES and EXAFS measurements has been performed in the 10–300 K temperature range in order to investigate the local order around the Fe and Cu ions in two samples with different oxygen stoichiometry: PrBaCuFeO 5.00 and PrBaCuFeO 5.25 . XANES allows determining that extra-oxygen is coordinated to the Fe ions and is not coordinated to the Cu ions. EXAFS data have been analysed in order to obtain the temperature behaviour of the Debye–Waller factor related to the oxygen surrounding the Fe and Cu ions. Additionally, low temperature resistivity data are reported for both samples, they show a variable range hoping mechanism for the conductivity.

Conduction mechanisms in pure and doped polycrystalline orthorhombic manganites

Abstract In this work we present magnetic and transport measurements as a function of temperature of polycrystalline pure and doped RMnO 3 (R=La, Nd and Ho) samples. We have performed AC transport measurement (in the range from DC to 2 MHz) in pellets of La 0.7 Ca 0.3 MnO 3 annealed at several temperatures, which present different connectivity between grains keeping the same mean grain size (about 1 μm). A large increase of resistance is observed at temperatures below the ferromagnetic order temperature where a metallic regime is expected. The behavior with temperature and frequency of the resistance of low temperature annealed pellets can be explained taking into account that two kinds of conduction paths are present in the sample: low resistive metallic ones, where the carriers percolate through well connected grains and highly resistive channels for high intergrain boundaries that behave as capacitors. In order to obtain the conduction mechanisms for non-doped RMnO 3 samples, we have measured the resistance in a large temperature range up to 1400 K. We present a discussion about the deviation of the measured resistance from any kind of polaronic conduction at high and low temperature. The differences in the activation energy with the rare earth ion are also discussed.